Analytical chemistryAnalytical chemistry is the study of the separation, identification, and quantification of

the chemical components of natural and artificialmaterials. Qualitative analysis gives an

indication of the identity of the chemical species in the sample and quantitative

analysis determines the amount of one or more of these components. The separation of

components is often performed prior to analysis.

Analytical methods can be separated into classical and instrumental. Classical methods

(also known as wet chemistry methods) use separations such

as precipitation, extraction, and distillation and qualitative analysis by color, odor, or

melting point. Quantitative analysis is achieved by measurement of weight or volume.

Instrumental methods use an apparatus to measure physical quantities of the analyte

such aslight absorption, fluorescence, or conductivity. The separation of materials is

accomplished using chromatography or electrophoresismethods.

Analytical chemistry is also focused on improvements in experimental

design, chemometrics, and the creation of new measurement tools to provide better

chemical information. Analytical chemistry has applications

in forensics, bioanalysis, clinical analysis, environmental analysis, and materials

analysis.

History

Gustav Kirchhoff (left) and Robert Bunsen (right)

Analytical chemistry has been important since the early days of chemistry, providing

methods for determining which elements and chemicals are present in the world around

us. During this period significant analytical contributions to chemistry include the

development of systematic elemental analysis by Justus von Liebig and systematized

organic analysis based on the specific reactions of functional groups.

The first instrumental analysis was flame emissive spectrometry developed by Robert

Bunsen andGustav Kirchhoff who discovered rubidium (Rb) and caesium (Cs) in 1860.

Most of the major developments in analytical chemistry take place after 1900. During

this period instrumental analysis becomes progressively dominant in the field. In

particular many of the basic spectroscopic and spectrometric techniques were

discovered in the early 20th century and refined in the late 20th century.

The separation sciences follow a similar time line of development and also become

increasingly transformed into high performance instruments. In the 1970s many of these

techniques began to be used together to achieve a complete characterization of

samples.

Starting in approximately the 1970s into the present day analytical chemistry has

progressively become more inclusive of biological questions (bioanalytical chemistry),

whereas it had previously been largely focused on inorganic or small organic molecules.

Lasers have been increasingly used in chemistry as probes and even to start and

influence a wide variety of reactions. The late 20th century also saw an expansion of the

application of analytical chemistry from somewhat academic chemical questions

to forensic, environmental, industrial and medical questions, such as in histology.

Modern analytical chemistry is dominated by instrumental analysis. Many analytical

chemists focus on a single type of instrument. Academics tend to either focus on new

applications and discoveries or on new methods of analysis. The discovery of a

chemical present in blood that increases the risk of cancer would be a discovery that an

analytical chemist might be involved in. An effort to develop a new method might involve

the use of a tunable laser to increase the specificity and sensitivity of a spectrometric

method. Many methods, once developed, are kept purposely static so that data can be

compared over long periods of time. This is particularly true in industrial quality

assurance (QA), forensic and environmental applications. Analytical chemistry plays an

increasingly important role in the pharmaceutical industry where, aside from QA, it is

used in discovery of new drug candidates and in clinical applications where

understanding the interactions between the drug and the patient are critical.

Classical methods

The presence of copper in this qualitative analysis is indicated by the bluish-green color of the flame.

Although modern analytical chemistry is dominated by sophisticated instrumentation,

the roots of analytical chemistry and some of the principles used in modern instruments

are from traditional techniques many of which are still used today. These techniques

also tend to form the backbone of most undergraduate analytical chemistry educational

labs.

Qualitative analysis

A qualitative analysis determines the presence or absence of a particular compound,

but not the mass or concentration. That is it is not related to quantity.

Chemical tests

There are numerous qualitative chemical tests, for example, the acid test for gold and

the Kastle-Meyer test for the presence of blood.

Flame test

Inorganic qualitative analysis generally refers to a systematic scheme to confirm the

presence of certain, usually aqueous, ions or elements by performing a series of

reactions that eliminate ranges of possibilities and then confirms suspected ions with a

confirming test. Sometimes small carbon containing ions are included in such schemes.

With modern instrumentation these tests are rarely used but can be useful for

educational purposes and in field work or other situations where access to state-of-the-

art instruments are not available or expedient.

Gravimetric analysis

Gravimetric analysis involves determining the amount of material present by weighing

the sample before and/or after some transformation. A common example used in

undergraduate education is the determination of the amount of water in a hydrate by

heating the sample to remove the water such that the difference in weight is due to the

water loss of water.

Volumetric analysis

Titration involves the addition of a reactant to a solution being analyzed until some

equivalence point is reached. Often the amount of material in the solution being

analyzed may be determined. Most familiar to those who have taken college chemistry

is the acid-base titration involving a color changing indicator. There are many other

types of titrations, for example potentiometric titrations. These titrations may use

different types of indicators to reach some equivalence point.

Instrumental methods

Block diagram of an analytical instrument showing the stimulus and measurement of response

Spectroscopy

Spectroscopy measures the interaction of the molecules with electromagnetic radiation.

Spectroscopy consists of many different applications such as atomic absorption

spectroscopy, atomic emission spectroscopy, ultraviolet-visible spectroscopy, x-ray

fluorescence spectroscopy, infrared spectroscopy, Raman spectroscopy, dual

polarisation interferometry, nuclear magnetic resonance spectroscopy, photoemission

spectroscopy, Mössbauer spectroscopy and so on.

Mass spectrometry

.

An accelerator mass spectrometerused for radiocarbon dating and other analysis.

Mass spectrometry measures mass-to-charge ratio of molecules

using electric and magnetic fields. There are several ionization methods: electron

impact, chemical ionization, electrospray, fast atom bombardment, matrix assisted laser

desorption ionization, and others. Also, mass spectrometry is categorized by

approaches of mass analyzers: magnetic-sector, quadrupole mass analyzer,quadrupole

ion trap, time-of-flight, Fourier transform ion cyclotron resonance, and so on.

Crystallography

Crystallography is a technique that characterizes the chemical structure of materials at

the atomic level by analyzing the diffraction patterns of usually x-rays that have been

deflected by atoms in the material. From the raw data the relative placement of atoms in

space may be determined.

Electrochemical analysis

Electroanalytical methods measure the potential (volts) and/or current (amps) in

an electrochemical cell containing the analyte. These methods can be categorized

according to which aspects of the cell are controlled and which are measured. The three

main categories arepotentiometry (the difference in electrode potentials is

measured), coulometry (the cell's current is measured over time), and voltammetry (the

cell's current is measured while actively altering the cell's potential).

Thermal analysis

Calorimetry and thermogravimetric analysis measure the interaction of a material

and heat.

Separation

Separation of black ink on a thin layer chromatography plate.

Separation processes are used to decrease the complexity of material

mixtures. Chromatography andelectrophoresis are representative of this field.

Hybrid techniques

Combinations of the above techniques produce a "hybrid" or "hyphenated" technique.[9]

[10][11][12][13]Several examples are in popular use today and new hybrid techniques are

under development. For example, gas chromatography-mass spectrometry, gas

chromatography-infrared spectroscopy, liquid chromatography-mass spectrometry,

liquid chromatography-NMR spectroscopy. liquid chromagraphy-infrared spectroscopy

and capillary electrophoresis-mass spectrometry.

Hyphenated separation techniques refers to a combination of two (or more) techniques

to detect and separate chemicals from solutions. Most often the other technique is some

form of chromatography. Hyphenated techniques are widely used

in chemistry and biochemistry. A slash is sometimes used instead of hyphen, especially

if the name of one of the methods contains a hyphen itself.

Microscopy

Fluorescence microscope image of two mouse cell nuclei in prophase(scale bar is 5 µm).

The visualization of single molecules, single cells, biological tissues

and nanomaterials is an important and attractive approach in analytical science. Also,

hybridization with other traditional analytical tools is revolutionizing analytical

science. Microscopy can be categorized into three different fields: optical

microscopy, electron microscopy, and scanning probe microscopy. Recently, this field is

rapidly progressing because of the rapid development of the computer and camera

industries.

Lab-on-a-chip

A glass microreactor

Devices that integrate (multiple) laboratory functions on a single chip of only millimeters

to a few square centimeters in size and that are capable of handling extremely small

fluid volumes down to less than pico liters.

Standards

Standard curve

A calibration curve plot showing limit of detection (LOD), limit of quantification(LOQ), dynamic range,

and limit of linearity (LOL).

A general method for analysis of concentration involves the creation of a calibration

curve. This allows for determination of the amount of a chemical in a material by

comparing the results of unknown sample to those of a series known standards. If the

concentration of element or compound in a sample is too high for the detection range of

the technique, it can simply be diluted in a pure solvent. If the amount in the sample is

below an instrument's range of measurement, the method of addition can be used. In

this method a known quantity of the element or compound under study is added, and

the difference between the concentration added, and the concentration observed is the

amount actually in the sample.

Internal standards

Sometimes an internal standard is added at a known concentration directly to an

analytical sample to aid in quantitation. The amount of analyte present is then

determined relative to the internal standard as a calibrant.

Standard addition

The method of standard addition is used in instrumental analysis to determine

concentration of a substance (analyte) in an unknown sample by comparison to a set of

samples of known concentration, similar to using a calibration curve. Standard addition

can be applied to most analytical techniques and is used instead of a calibration

curve to solve the matrix effect problem.

Signals and noise

One of the most important components of analytical chemistry is maximizing the desired

signal while minimizing the associated noise.[15]The analytical figure of merit is known as

the signal-to-noise ratio (S/N or SNR).

Noise can arise from environmental factors as well as from fundamental physical

processes.

Thermal noise

Thermal noise results from the motion of charge carriers (usually electrons) in an

electrical circuit generated by their thermal motion. Thermal noise is white

noise meaning that the power spectral density is constant throughout the frequency

spectrum.

The root mean square value of the thermal noise in a resistor is given by[15]

where kB is Boltzmann's constant, T is the temperature, R is the resistance,

and Δf is the bandwidth of the frequency f.

Shot noise

Main article: Shot noise

Shot noise is a type of electronic noise that occurs when the finite number of

particles (such as electrons in an electronic circuit or photonsin an optical device) is

small enough to give rise to statistical fluctuations in a signal.

Shot noise is a Poisson process and the charge carriers that make up the current

follow a Poisson distribution. The root mean square current fluctuation is given by[15]

where e is the elementary charge and I is the average current. Shot noise is

white noise.

Flicker noise

Flicker noise is electronic noise with a 1/ƒ frequency spectrum; as f increases,

the noise decreases. Flicker noise arises from a variety of sources, such as

impurities in a conductive channel, generation and recombination noise in

a transistor due to base current, and so on. This noise can be avoided

by modulation of the signal at a higher frequency, for example through the use

of a lock-in amplifier.

Environmental noise

Noise in a thermogravimetric analysis; lower noise in the middle of the plot results from

less human activity (and environmental noise) at night.

Environmental noise arises from the surroundings of the analytical instrument.

Sources of electromagnetic noise are power lines, radio and television

stations, wireless devices and electric motors. Many of these noise sources

are narrow bandwidth and therefore can be avoided. Temperature

and vibration isolation may be required for some instruments.

Noise reduction

Noise reduction can be accomplished either in hardware or software.

Examples of hardware noise reduction are the use of shielded cable, analog

filtering, and signal modulation. Examples of software noise reduction

are digital filtering, ensemble average, boxcar average,

and correlation methods.

Applications

Analytical chemistry research is largely driven by performance (sensitivity,

selectivity, robustness, linear range, accuracy, precision, and speed), and cost

(purchase, operation, training, time, and space). Among the main branches of

contemporary analytical atomic spectrometry, the most widespread and

universal are optical and mass spectrometry.  In the direct elemental analysis

of solid samples, the new leaders are laser-induced breakdown and laser

ablation mass spectrometry, and the related techniques with transfer of the

laser ablation products into inductively coupled plasma. Advances in design of

diode lasers and optical parametric oscillators promote developments in

fluorescence and ionization spectrometry and also in absorption techniques

where uses of optical cavities for increased effective absorption pathlength are

expected to expand. Steady progress and growth in applications of plasma-

and laser-based methods are noticeable. An interest towards the absolute

(standardless) analysis has revived, particularly in the emission spectrometry.

A lot of effort is put in shrinking the analysis techniques to chip size. Although

there are few examples of such systems competitive with traditional analysis

techniques, potential advantages include size/portability, speed, and cost.

(micro Total Analysis System (µTAS) or Lab-on-a-chip). Microscale

chemistry reduces the amounts of chemicals used.

Much effort is also put into analyzing biological systems. Examples of rapidly

expanding fields in this area are:

Genomics  - DNA sequencing and its related research. Genetic

fingerprinting and DNA microarray are very popular tools and research

fields.

Proteomics  - the analysis of protein concentrations and modifications,

especially in response to various stressors, at various developmental

stages, or in various parts of the body.

Metabolomics  - similar to proteomics, but dealing with metabolites.

Transcriptomics  - mRNA and its associated field

Lipidomics  - lipids and its associated field

Peptidomics - peptides and its associated field

Metalomics - similar to proteomics and metabolomics, but dealing with

metal concentrations and especially with their binding to proteins and other

molecules.

Analytical chemistry has played critical roles in the understanding of basic

science to a variety of practical applications, such as biomedical applications,

environmental monitoring, quality control of industrial manufacturing, forensic

science and so on.

The recent developments of computer automation and information

technologies have innervated analytical chemistry to initiate a number of new

biological fields. For example, automated DNA sequencing machines were the

basis to complete human genome projects leading to the birth of genomics.

Protein identification and peptide sequencing by mass spectrometry opened a

new field of proteomics. Furthermore, a number of ~omics based on analytical

chemistry have become important areas in modern biology.

Also, analytical chemistry has been an indispensable area in the development

of nanotechnology. Surface characterization instruments,electron

microscopes and scanning probe microscopes enables scientists to visualize

atomic structures with chemical characterizations.

Among active contemporary analytical chemistry research fields, micro total

analysis system is considered as a great promise of revolutionary technology.

In this approach, integrated and miniaturized analytical systems are being

developed to control and analyze single cells and single molecules. This

cutting-edge technology has a promising potential of leading a new revolution

in science as integrated circuits did in computer developments.

Sensory Analysis And Instruments

Sensory analysis is a method used to describe the sensations that humans perceive with their 5 senses when in contact with a product. Among the 5 senses, we distinguish physical (hearing, touch, sight) and chemical (olfaction, taste) senses. Alpha MOS has developed solutions linked to chemical senses.

The sense of smell, also called olfaction, is the perception of odorant molecules either by direct inhalation or during mastication (retro nasal way).

The sense of taste consists of the perception of savors that are the sensations perceived by the tongue during tasting. Commonly, savors are classified in 5 categories: salt, sour, bitter, sweet, umami

In sensory analysis, there exist 2 types of tests: hedonic tests (I like it / I don't like it) which consist of consumer tests

analytical tests (descriptive or discriminative) that are conducted by a trained sensory panel.

 

For analytical tests, Electronic nose and tongue analyzers can also be used. These instruments have the specificity to analyze odor / taste in the same way as human nose and tongue: instead of separating and identifying the various chemical compounds, they measure a global odor / taste.

LIST OF ELECTRONIC DEVICES USED IN CHEMICAL ANALYSIS AND MEASUREMENTS

BreathalyzerFrom Wikipedia, the free encyclopedia

Jump to: navigation, search

A law enforcement grade breathalyzer

A breathalyzer (U.S.A.) or breathalyser (U.K.) (a portmanteau of breath and analyzer/analyser) is a device for estimating blood alcohol content (BAC) from a breath sample. Breathalyzer is the brand name of a series of models made by one manufacturer of these instruments (originally Smith and Wesson [1] , later sold to National Draeger), but has become a genericized trademark for all such instruments.[citation needed] In Canada, a preliminary non-evidentiary screening device can be approved by Parliament as an approved screening device, and an evidentiary breath instrument can be similarly designated as an approved instrument. The U.S. National Highway Traffic Safety Administration maintains a Conforming Products List of breath alcohol devices approved for evidentiary use,[2] as well as for preliminary screening use.[3]

Contents

[hide]

1 Origins 2 Chemistry

3 Law enforcement

4 Consumer use

5 Breath test evidence in the United States

6 Common sources of error

o 6.1 Calibration

o 6.2 Non-specific analysis

o 6.3 Interfering compounds

o 6.4 Homeostatic variables

o 6.5 Mouth alcohol

o 6.6 Testing during absorptive phase

o 6.7 Retrograde extrapolation

7 Photovoltaic assay

8 Breathalyzer myths

9 Products that interfere with testing

10 References

11 External links

[edit] Origins

A 1927 paper produced by Emil Bogen[4], who collected air in a football and then tested this air for traces of alcohol, discovered that the alcohol content of 2 litres of expired air was a little greater than that of 1 cc of urine. However, research into the possibilities of using breath to test for alcohol in a person's body dates as far back as 1874, when Anstie made the observation that small amounts of alcohol were excreted in breath[5].

The first practical roadside breath-testing device intended for use by the police was the drunkometer. The drunkometer was developed by Professor Harger in 1938. The drunkometer collected a motorist's breath sample directly into a balloon inside the machine. The breath sample was then pumped through an acidified potassium permanganate solution. If there was alcohol in the breath sample, the solution changed colour. The greater the colour change, the more alcohol there was present in the breath.

The drunkometer was quite cumbersome and was approximately the size of a shoe box. It was more reminiscent of a portable laboratory.

In late 1927, in a case in Marlborough, England, a Dr. Gorsky, Police Surgeon, asked a suspect to inflate a football bladder with his breath. Since the 2 liters of the man's breath contained 1.5 ml of ethanol[dubious – discuss], Dr. Gorsky testified before the court that the defendant was "50% drunk".[6] Though technologies for detecting alcohol vary, it is widely accepted that Dr. Robert Borkenstein (1912–2002), a captain with the Indiana State Police and later a professor at Indiana University at Bloomington, is regarded as the first to create a device that measures a subject's blood alcohol level based on a breath sample. In 1954, Borkenstein invented his breathalyzer, which used chemical oxidation and photometry to determine alcohol concentration. Subsequent breathalyzers have converted primarily to infrared spectroscopy. The invention of the breathalyzer provided law enforcement with a non-invasive test providing immediate results to determine an individual's breath alcohol concentration at the time of testing. Also, the breath alcohol concentration test result itself can vary between individuals consuming identical amounts of alcohol due to gender, weight, and genetic pre-disposition.

[edit] Chemistry

When the user exhales into the breathalyzer, any ethanol present in their breath is oxidized to acetic acid at the anode:

CH3CH2OH(g) + H2O(l) -> CH3CO2H(l) + 4H+(aq) + 4e-

At the cathode, atmospheric oxygen is reduced:

O2(g) + 4H+(aq) + 4e- -> 2H2O(l)

The overall reaction, then, is the oxidation of ethanol to acetic acid and water.

CH3CH2OH(l) + O2(g) -> CH3COOH(l) + H2O(l)

The electrical current produced by this reaction is measured, processed, and displayed as an approximation of overall blood alcohol content by the breathalyzer.

[edit] Law enforcement

Breath analyzers do not directly measure blood alcohol content or concentration, which requires the analysis of a blood sample. Instead, they estimate BAC indirectly by measuring the amount of alcohol in one's breath. Two breathalyzer technologies are most prevalent. Desktop analyzers generally use infrared spectrophotometer technology, electrochemical fuel cell technology, or a combination of the two. Hand-held field testing devices are generally based on electrochemical platinum fuel cell analysis and, depending upon jurisdiction, may be used by officers in the field as a form of "field sobriety test" commonly called PBT (preliminary breath test) or PAS (preliminary alcohol screening) or as evidential devices in POA (point of arrest) testing.

[edit] Consumer use

There are many models of consumer or personal breath alcohol testers on the market. These hand-held devices are generally less expensive than the devices used by law enforcement. Most retail consumer breath testers use semiconductor-based sensing technology, which is less expensive, less accurate, and less reliable than fuel cell and infrared devices. While semiconductor devices can sense the presence of breath alcohol, they do not provide as reliable measures of BAC.

All breath alcohol testers sold to consumers in the United States are required to be certified by the Food and Drug Administration [7] , while those used by law enforcement must be approved by the Department of Transportation's National Highway Traffic Safety Administration.[8]

Manufacturers of over-the-counter consumer breathalyzers must submit an FDA 510(k) Premarket Clearance to demonstrate that the device to be marketed is at least as safe and effective, that is, substantially equivalent, to a legally marketed device (21 CFR 807.92(a) (3))

that is not subject to Premarket Approval (PMA). Submitters must compare their device to one or more similar legally marketed devices and make and support their substantial equivalency claims.[9] The devices are cleared as "screeners" which means they have met the requirements used by the FDA for detecting the presence of alcohol in the breath. Screener certification does not mean that the device can measure breath alcohol content accurately. Many breathalyzers cleared by the FDA are very innaccurate when it comes to BAC measurement. No semiconductor device has ever been approved for evidential use (to stand-up in a court of law) by any State Law Enforcement Agencies or the U.S. Department of Transportation.

[edit] Breath test evidence in the United States

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A breathalyzer in action

The breath alcohol content reading is used in criminal prosecutions in two ways. The operator of a vehicle whose reading indicates a BrAC over the legal limit for driving will be charged with having committed an illegal per se offense: that is, it is automatically illegal throughout the United States to drive a vehicle with a BrAC of 0.08 or higher. One exception is the State of Wisconsin, where a first time drunk driving offense is normally a civil ordinance violation.[10] The breathalyzer reading will be offered as evidence of that crime, although the issue is what the BrAC was at the time of driving rather than at the time of the test. Some jurisdictions now allow the use of breathalyzer test results without regard as to how much time passed between operation of the vehicle and the time the test was administered.[citation needed] The suspect will also be charged with driving under the influence of alcohol (sometimes referred to as driving or operating while intoxicated). While BrAC tests are not necessary to prove a defendant was under the influence, laws in most states require the jury to presume that he was under the influence if his BrAC is found and believed to be over 0.08 (grams of alcohol/210 liters breath) when driving.[citation needed] This is a rebuttable presumption, however: the jury can disregard the test if they find it unreliable or if other evidence establishes a reasonable doubt.

Infrared instruments are also known as "evidentiary breath testers" and generally produce court-admissible results.[citation needed] Other instruments, usually hand held in design, are known as "preliminary breath testers" (PBT), and their results, while valuable to an officer attempting to establish probable cause for a drunk driving arrest, are generally not admissible in court. Some states, such as Idaho, permit data or "readings" from hand-held PBTs to be presented as evidence in court. If at all, they are generally only admissible to show the presence of alcohol or as a pass-fail field sobriety test to help determine probable cause to arrest. South Dakota does not permit data from any type of breath tester, and relies entirely on blood tests to ensure accuracy.

[edit] Common sources of error

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Breath testers can be very sensitive to temperature, for example, and will give false readings if not adjusted or recalibrated to account for ambient or surrounding air temperatures. The temperature of the subject is also very important.[citation needed]

Breathing pattern can also significantly affect breath test results. One study found that the BAC readings of subjects decreased 11–14% after running up one flight of stairs and 22–25% after doing so twice. Another study found a 15% decrease in BAC readings after vigorous exercise or hyperventilation. Hyperventilation for 20 seconds has been shown to lower the reading by approximately 32%. On the other hand, holding one's breath for 30 seconds can increase the breath test result by about 28%.[citation needed]

Some breath analysis machines assume a hematocrit (cell volume of blood) of 47%. However, hematocrit values range from 42 to 52% in men and from 37 to 47% in women. A person with a lower hematocrit will have a falsely high BAC reading.

Research indicates that breath tests can vary at least 15% from actual blood alcohol concentration. An estimated 23% of individuals tested will have a BAC reading higher than their true BAC. Police in Victoria, Australia, use breathalyzers that give a recognized 20% tolerance on readings. Noel Ashby, former Victoria Police Assistant Commissioner (Traffic & Transport), claims that this tolerance is to allow for different body types.[11]

[edit] Calibration

Many handheld breathalyzers sold to consumers use a silicon oxide sensor (also called a semiconductor sensor) to determine the blood alcohol concentration. These sensors are far more prone to contamination and interference from other substances besides breath alcohol. The sensors require recalibration or replacement every six months. Higher end personal breathalyzers

and professional-use breath alcohol testers use platinum fuel cell sensors.These too require recalibration but at less frequent intervals than semiconductor devices, usually once a year.

Calibration is the process of checking and adjusting the internal settings of a breathalyzer by comparing and adjusting its test results to a known alcohol standard. Law enforcement breathalyzers are meticulously maintained and re-calibrated frequently to ensure accuracy.

There are two methods of calibrating a precision fuel cell breathalyzer, the Wet Bath and the Dry Gas method. Each method requires specialized equipment and factory trained technicians. It is not a procedure that can be conducted by untrained users or without the proper equipment.

The Dry-Gas Method utilizes a portable calibration standard which is a precise mixture of alcohol and inert nitrogen available in a pressurized canister. Initial equipment costs are less than alternative methods and the steps required are fewer. The equipment is also portable allowing calibrations to be done when and where required.

The Wet Bath Method utilizes an alcohol/water standard in a precise specialized alcohol concentration, contained and delivered in specialized simulator equipment. Wet bath apparatus has a higher initial cost and is not intended to be portable. The standard must be fresh and replaced regularly.

Some semiconductor models are designed to allow the sensor module to be replaced without the need to send the unit to a calibration lab.[citation needed]

[edit] Non-specific analysis

One major problem with older breathalyzers is non-specificity: the machines not only identify the ethyl alcohol (or ethanol) found in alcoholic beverages, but also other substances similar in molecular structure or reactivity.

The oldest breathalyzer models pass breath through a solution of potassium dichromate, which oxidizes ethanol into acetic acid, changing color in the process. A monochromatic light beam is passed through this sample, and a detector records the change in intensity and, hence, the change in color, which is used to calculate the percent alcohol in the breath. However, since potassium dichromate is a strong oxidizer, numerous alcohol groups can be oxidized by it, producing false positives. This source of false positives is unlikely as very few other substances found in exhaled air is oxidisable.

Infrared-based breathalyzers project an infrared beam of radiation through the captured breath in the sample chamber and detect the absorbance of the compound as a function of the wavelength of the beam, producing an absorbance spectrum that can be used to identify the compound, as the absorbance is due to the harmonic vibration and stretching of specific bonds in the molecule at specific wavelengths (see infrared spectroscopy). The characteristic bond of alcohols in infrared is the O-H bond, which gives a strong absorbance at a short wavelength. The more light is absorbed by compounds containing the alcohol group, the less reaches the detector on the other

side—and the higher the reading. Other groups, most notably aromatic rings and carboxylic acids can give similar absorbance readings.[12] Even water vapour does.

[edit] Interfering compounds

Some natural and volatile interfering compounds do exist, however. For example, the National Highway Traffic Safety Administration (NHTSA) has found that dieters and diabetics may have acetone levels hundreds or even thousand of times higher than those in others. Acetone is one of the many substances that can be falsely identified as ethyl alcohol by some breath machines. However, fuel cell based systems are non-responsive to substances like acetone.

A study in Spain showed that metered-dose inhalers (MDIs) used in asthma treatment are also a cause of false positives in breath machines.

Substances in the environment can also lead to false BAC readings. For example, methyl tert-butyl ether (MTBE), a common gasoline additive, has been alleged anecdotally to cause false positives in persons exposed to it. Tests have shown this to be true for older machines; however, newer machines detect this interference and compensate for it.[13] Any number of other products found in the environment or workplace can also cause erroneous BAC results. These include compounds found in lacquer, paint remover, celluloid, gasoline, and cleaning fluids, especially ethers, alcohols, and other volatile compounds.

[edit] Homeostatic variables

Breathalyzers assume that the subject being tested has a 2100-to-1 partition ratio[14] in converting alcohol measured in the breath to estimates of alcohol in the blood. If the instrument estimates the BAC, then it measures weight of alcohol to volume of breath, so it will effectively measure grams of alcohol per 2100 ml of breath given. This measure is in direct proportion to the amount of grams of alcohol to every 100 ml of blood. Therefore, there is a 2100-to-1 ratio of alcohol in blood to alcohol in breath. However, this assumed partition ratio varies from 1300:1 to 3100:1 or wider among individuals and within a given individual over time. Assuming a true (and US legal) blood-alcohol concentration of .07%, for example, a person with a partition ratio of 1500:1 would have a breath test reading of .10%—over the legal limit.

Most individuals do, in fact, have a 2100-to-1 partition ratio in accordance with William Henry's law, which states that when the water solution of a volatile compound is brought into equilibrium with air, there is a fixed ratio between the concentration of the compound in air and its concentration in water. This ratio is constant at a given temperature. The human body is 37 degrees Celsius on average. Breath leaves the mouth at a temperature of 34 degrees Celsius. Alcohol in the body obeys Henry's Law as it is a volatile compound and diffuses in body water. To ensure that variables such as fever and hypothermia could not be pointed out to influence the results in a way that was harmful to the accused, the instrument is calibrated at a ratio of 2100:1, underestimating by 9 percent. In order for a person running a fever to significantly overestimate, he would have to have a fever that would likely see the subject in the hospital rather than driving in the first place. Studies suggest that about 1.8% of the population have a partition ratio below 2100:1. Thus, a machine using a 2100-to-1 ratio could actually overestimate the BAC. As much

as 14% of the population has a partition ratio above 2100, thus causing the machine to under-report the BAC.

Further, the assumption that the test subject's partition ratio will be average—that there will be 2100 parts in the blood for every part in the breath—means that accurate analysis of a given individual's blood alcohol by measuring breath alcohol is difficult, as the ratio varies considerably.

Variance in how much one breathes out can also give false readings, usually low.[15] This is due to biological variance in breath alcohol concentration as a function of the volume of air in the lungs, an example of a factor which interferes with the liquid-gas equilibrium assumed by the devices. The presence of volatile components is another example of this; mixtures of volatile compounds can be more volatile than their components, which can create artificially high levels of ethanol (or other) vapors relative to the normal biological blood/breath alcohol equilibrium.

[edit] Mouth alcohol

One of the most common causes of falsely high breathalyzer readings is the existence of mouth alcohol. In analyzing a subject's breath sample, the breathalyzer's internal computer is making the assumption that the alcohol in the breath sample came from alveolar air—that is, air exhaled from deep within the lungs. However, alcohol may have come from the mouth, throat or stomach for a number of reasons. To help guard against mouth-alcohol contamination, certified breath-test operators are trained to observe a test subject carefully for at least 15–20 minutes before administering the test.

The problem with mouth alcohol being analyzed by the breathalyzer is that it was not absorbed through the stomach and intestines and passed through the blood to the lungs. In other words, the machine's computer is mistakenly applying the partition ratio (see above) and multiplying the result. Consequently, a very tiny amount of alcohol from the mouth, throat or stomach can have a significant impact on the breath-alcohol reading.

Other than recent drinking, the most common source of mouth alcohol is from belching or burping. This causes the liquids and/or gases from the stomach—including any alcohol—to rise up into the soft tissue of the esophagus and oral cavity, where it will stay until it has dissipated. The American Medical Association concludes in its Manual for Chemical Tests for Intoxication (1959): "True reactions with alcohol in expired breath from sources other than the alveolar air (eructation, regurgitation, vomiting) will, of course, vitiate the breath alcohol results." For this reason, police officers are supposed to keep a DUI suspect under observation for at least 15 minutes prior to administering a breath test. Instruments such as the Intoxilyzer 5000 also feature a "slope" parameter. This parameter detects any decrease in alcohol concentration of 0.006 g per 210 L of breath in 0.6 second, a condition indicative of residual mouth alcohol, and will result in an "invalid sample" warning to the operator, notifying the operator of the presence of the residual mouth alcohol. PBT's, however, feature no such safeguard.

Acid reflux, or gastroesophageal reflux disease, can greatly exacerbate the mouth-alcohol problem. The stomach is normally separated from the throat by a valve, but when this valve

becomes herniated, there is nothing to stop the liquid contents in the stomach from rising and permeating the esophagus and mouth. The contents—including any alcohol—are then later exhaled into the breathalyzer.[16]

Mouth alcohol can also be created in other ways. Dentures, for example, will trap alcohol. Periodental disease can also create pockets in the gums which will contain the alcohol for longer periods. Also known to produce false results due to residual alcohol in the mouth is passionate kissing with an intoxicated person. Recent use of mouthwash or breath freshener—possibly to disguise the smell of alcohol when being pulled over by police—contain fairly high levels of alcohol.

[edit] Testing during absorptive phase

Absorption of alcohol continues for anywhere from 20 minutes (on an empty stomach) to two-and-one-half hours (on a full stomach) after the last consumption. Peak absorption generally occurs within an hour. During the initial absorptive phase, the distribution of alcohol throughout the body is not uniform. Uniformity of distribution, called equilibrium, occurs just as absorption completes. In other words, some parts of the body will have a higher blood alcohol content (BAC) than others. One aspect of the non-uniformity before absorption is complete is that the BAC in arterial blood will be higher than in venous blood. Laws generally require blood samples to be venous.

During the initial absorption phase, arterial blood alcohol concentrations are higher than venous. After absorption, venous blood is higher. This is especially true with bolus dosing. With additional doses of alcohol, the body can reach a sustained equilibrium when absorption and elimination are proportional, calculating a general absorption rate of 0.02/drink and a general elimination rate of 0.015/hour. (One drink is equal to 1.5 ounces of liquor, 12 ounces of beer, or 5 ounces of wine [1].)

Breath alcohol is a representation of the equilibrium of alcohol concentration as the blood gases (alcohol) pass from the (arterial) blood into the lungs to be expired in the breath. The venous blood picks up oxygen for distribution throughout the body. Breath alcohol concentrations are generally lower than blood alcohol concentrations, because a true representation of blood alcohol concentration is only possible if the lungs were able to completely deflate. Vitreous (eye) fluid provides the most accurate account of blood alcohol concentrations.

[edit] Retrograde extrapolation

The breathalyzer test is usually administered at a police station, commonly an hour or more after the arrest. Although this gives the BrAC at the time of the test, it does not by itself answer the question of what it was at the time of driving. The prosecution typically provides an estimated alcohol concentration at the time of driving utilizing retrograde extrapolation, presented by expert opinion. This involves projecting back in time to estimate the BrAC level at the time of driving, by applying the physiological properties of absorption and elimination rates in the human body.

Extrapolation is calculated using five factors and a general elimination rate of 0.015/hour.

For example: Time of breath test-10:00pm...Result of breath test-0.080...Time of driving-9:00pm (stopped by officer)...Time of last drink-8:00pm...Last food-12:00pm

Using these facts, an expert can say the person's last drink was consumed on an empty stomach, which means absorption of the last drink (at 8:00) was complete within one hour-9:00. At the time of the stop, the driver is fully absorbed. The test result of 0.080 was at 10:00. So the one hour of elimination that has occurred since the stop is added in, making 0.080+0.015=0.095 the approximate breath alcohol concentration at the time of the stop.

[edit] Photovoltaic assay

The photovoltaic assay, used only in the dated Photo Electric Intoximeter (PEI), is a form of breath testing rarely encountered today. The process works by using photocells to analyze the color change of a redox (oxidation-reduction) reaction. A breath sample is bubbled through an aqueous solution of sulfuric acid, potassium dichromate, and silver nitrate. The silver nitrate acts as a catalyst, allowing the alcohol to be oxidized at an appreciable rate. The requisite acidic condition needed for the reaction might also be provided by the sulfuric acid. In solution, ethanol reacts with the potassium dichromate, reducing the dichromate ion to the chromium (III) ion. This reduction results in a change of the solution's colour from red-orange to green. The reacted solution is compared to a vial of non-reacted solution by a photocell, which creates an electric current proportional to the degree of the colour change; this current moves the needle that indicates BAC.

Like other methods, breath testing devices using chemical analysis are somewhat prone to false readings. Compounds that have compositions similar to ethanol, for example, could also act as reducing agents, creating the necessary color change to indicate increased BAC.

[edit] Breathalyzer myths

There are a number of substances or techniques that can supposedly "fool" a breathalyzer (i.e., generate a lower blood alcohol content).

A 2003 episode of the popular science television show MythBusters tested a number of methods that supposedly allow a person to fool a breathalyzer test. The methods tested included breath mints, onions, denture cream, mouthwash, pennies and batteries; all of these methods proved ineffective. The show noted that using items such as breath mints, onions, denture cream and mouthwash to cover the smell of alcohol may fool a person, but, since they will not actually reduce a person's BAC, there will be no effect on a breathalyzer test regardless of the quantity used. Pennies supposedly produce a chemical reaction, while batteries supposedly create an electrical charge, yet neither of these methods affected the breathalyzer results.[17]

The Mythbusters episode also pointed out another complication: It would be necessary to insert the item into one's mouth (e.g. eat an onion, rinse with mouthwash, conceal a battery), take the

breath test, and then possibly remove the item — all of which would have to be accomplished discreetly enough to avoid alerting the police officers administering the test (who would obviously become very suspicious if they noticed that a person was inserting items into their mouth prior to taking a breath test). It would likely be very difficult, especially for someone in an intoxicated state, to be able to accomplish such a feat.[17]

In addition, the show noted that breath tests are often verified with blood tests (which are more accurate) and that even if a person somehow managed to fool a breath test, a blood test would certainly confirm a person's guilt.[17]

Other substances that might reduce the BAC reading include a bag of activated charcoal concealed in the mouth (to absorb alcohol vapor), an oxidizing gas (such as N2O, Cl2, O3, etc.) that would fool a fuel cell type detector, or an organic interferent to fool an infrared absorption detector. The infrared absorption detector is more vulnerable to interference than a laboratory instrument measuring a continuous absorption spectrum since it only makes measurements at particular discrete wavelengths. However, due to the fact that any interference can only cause higher absorption, not lower, the estimated blood alcohol content will be overestimated.[citation

needed] Additionally, Cl2 is rather toxic and corrosive.

A 2007 episode of the Spike network's show Manswers showed some of the more common and not-so-common ways of attempts to beat the breathalyzer, none of which work. Test 1 was to suck on a copper coin. (Actually, copper coins are now generally often only copper-coated and mostly zinc or steel.[citation needed]) Test 2 was to hold a battery on the tongue. Test 3 was to chew gum. None of these tests showed a "pass" reading if the subject had consumed alcohol.

[edit] Products that interfere with testing

On the other hand, products such as mouthwash or breath spray can "fool" breath machines by significantly raising test results. Listerine mouthwash, for example, contains 27% alcohol. The breath machine is calibrated with the assumption that the alcohol is coming from alcohol in the blood diffusing into the lung rather than directly from the mouth, so it applies a partition ratio of 2100:1 in computing blood alcohol concentration—resulting in a false high test reading. To counter this, officers are not supposed to administer a PBT for 15 minutes after the subject eats, vomits, or puts anything in their mouth. In addition, most instruments require that the individual be tested twice at least two minutes apart. Mouthwash or other mouth alcohol will have somewhat dissipated after two minutes and cause the second reading to disagree with the first, requiring a retest. (Also see the discussion of the "slope parameter" of the Intoxilyzer 5000 in the "Mouth Alcohol" section above.)

This was clearly illustrated in a study conducted with Listerine mouthwash on a breath machine and reported in an article entitled "Field Sobriety Testing: Intoxilyzers and Listerine Antiseptic" published in the July 1985 issue of The Police Chief (p. 70). Seven individuals were tested at a police station, with readings of 0.00%. Each then rinsed his mouth with 20 milliliters of Listerine mouthwash for 30 seconds in accordance with directions on the label. All seven were then tested on the machine at intervals of one, three, five and ten minutes. The results indicated an average reading of 0.43 blood-alcohol concentration, indicating a level that, if accurate, approaches lethal

proportions. After three minutes, the average level was still 0.020, despite the absence of any alcohol in the system. Even after five minutes, the average level was 0.011.

In another study, reported in 8(22) Drinking/Driving Law Letter 1, a scientist tested the effects of Binaca breath spray on an Intoxilyzer 5000. He performed 23 tests with subjects who sprayed their throats and obtained readings as high as 0.81—far beyond lethal levels. The scientist also noted that the effects of the spray did not fall below detectable levels until after 18 minutes.

Carbon dioxide sensorFrom Wikipedia, the free encyclopedia

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A carbon dioxide sensor or CO2 sensor is an instrument for the measurement of carbon dioxide gas. The most common principles for CO2 sensors are infrared gas sensors (NDIR) and chemical gas sensors. Measuring carbon dioxide is important in monitoring Indoor air quality and many industrial processes.

Contents

[hide]

1 Nondispersive Infrared (NDIR) CO 2 Sensors 2 Chemical CO 2 Sensors

3 Applications

4 See also

5 References

[edit] Nondispersive Infrared (NDIR) CO2 Sensors

NDIR sensors are spectroscopic sensors to detect CO2 in a gaseous environment by its characteristic absorption. The key components are an infrared source, a light tube, an interference (wavelength) filter, and an infrared detector. The gas is pumped or diffuses into the light tube, and the electronics measures the absorption of the characteristic wavelength of light. NDIR sensors are most often used for measuring carbon dioxide.[1] The best of these have sensitivities of 20-50 PPM.[1] Typical NDIR sensors are still in the (US) $100 to $1000 range. New developments include using Microelectromechanical systems to bring down the costs of this sensor and to create smaller devices (for example for use in air conditioning applications). NDIR CO2 sensors are also used for dissolved CO2 for applications such as beverage carbonation, pharmeceutical fermentation and CO2 sequestration applications. In this care they are mated to an ATR (attenuated total reflection) optic and measure the gas insitu.

[edit] Chemical CO2 Sensors

Chemical CO2 gas sensors with sensitive layers based on polymer- or heteropolysiloxane have the principal advantage of a very low energy consumption and can be reduced in size to fit into microelectronic-based systems. On the downside, short- and long term drift effects as well as a rather low overall lifetime are major obstacles when compared with the NDIR measurement principle[2].

[edit] Applications

For air conditioning applications these kind of sensors can be used to monitor the quality of air and the tailored need of fresh air, respectively.

In applications where direct temperature measurement is not applicable NDIR sensors can be used. The sensors absorb ambient infrared radiation (IR) given off by a heated surface.

Carbon monoxide detectorFrom Wikipedia, the free encyclopedia

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‹ The template below (Globalize/North America) is being considered for deletion. See templates for discussion to help reach a consensus.›

The examples and perspective in this article deal primarily with North America and do not represent a worldwide view of the subject. Please improve this article and discuss the issue on the talk page.

Carbon Monoxide detector connected to a North American power outlet

A carbon monoxide detector or CO detector is a device that detects the presence of the carbon monoxide (CO) gas in order to prevent carbon monoxide poisoning. CO is a colorless and odorless compound produced by incomplete combustion. It is often referred to as the "silent killer" because it is virtually undetectable without using detection technology.[1] Elevated levels of CO can be dangerous to humans depending on the amount present and length of exposure. Smaller concentrations can be harmful over longer periods of time while increasing concentrations require diminishing exposure times to be harmful.[2]

CO detectors are designed to measure CO levels over time and sound an alarm before dangerous levels of CO accumulate in an environment, giving people adequate warning to safely ventilate the area or evacuate. Some system-connected detectors also alert a monitoring service that can dispatch emergency services if necessary.

While CO detectors do not serve as smoke detectors and vice versa, dual smoke/CO detectors are also sold. Smoke detectors detect the smoke generated by flaming or smoldering fires, whereas CO detectors go into alarm and warn people about dangerous CO buildup caused, for example, by a malfunctioning fuel-burning device. In the home, some common sources of CO include open flames, space heaters, water heaters, blocked chimneys or running a car inside a garage.[3]

Contents

[hide]

1 Installation 2 Sensors

o 2.1 Opto-Chemical

o 2.2 Biomimetic

o 2.3 Electrochemical

o 2.4 Semiconductor

3 Digital

4 Wireless

5 Legislation

6 Manufacturers

7 References

8 External links

[edit] Installation

The devices, which retail for $20-$60USD and are widely available, can either be battery-operated or AC powered (with or without a battery backup). Battery lifetimes have been increasing as the technology has developed and certain battery powered devices now advertise a battery lifetime of over 6 years. All CO detectors have "test" buttons like smoke detectors.

CO detectors can be placed near the ceiling or near the floor because CO is very close to the same density as air[4][5].

Since CO is colorless, tasteless and odorless (unlike smoke from a fire), detection in a home environment is impossible without such a warning device. It is a highly toxic inhalant and attracts to the hemoglobin(in the blood stream) 200x faster than oxygen, producing inadequate amounts of oxygen traveling through the body. In North America, some state, provincial and municipal governments have statutes requiring installation of CO detectors in construction - among them, the U.S. states of Alaska, Colorado, Connecticut, Florida, Georgia, Illinois, Maryland, Massachusetts, Minnesota, New Jersey, New York, Rhode Island, Texas, Vermont, Virginia, Wisconsin and West Virginia, as well as New York City and the Canadian province of Ontario.[6]

When carbon monoxide detectors were introduced into the market, they had a limited lifespan of 2 years. However technology developments have increased this and many now advertise up to 7 years. [7] Newer models are designed to signal a need to be replaced after that time-span although there are many instances of detectors operating far beyond this point.

According to the 2005 edition of the carbon monoxide guidelines, NFPA 720 [8], published by the National Fire Protection Association, sections 5.1.1.1 and 5.1.1.2, all CO detectors “shall be centrally located outside of each separate sleeping area in the immediate vicinity of the bedrooms,” and each detector “shall be located on the wall, ceiling or other location as specified in the installation instructions that accompany the unit.”

Installation locations vary by manufacturer. Manufacturers’ recommendations differ to a certain degree based on research conducted with each one’s specific detector. Therefore, make sure to read the provided installation manual for each detector before installing.

CO detectors are available as stand-alone models or system-connected, monitored devices[9]. System-connected detectors, which can be wired to either a security or fire panel, are monitored by a central station. In case the residence is empty, the residents are sleeping or occupants are already suffering from the effects of CO, the central station can be alerted to the high concentrations of CO gas and can send the proper authorities to investigate.

The gas sensors in CO alarms have a limited and indeterminable life span, typically two to five years. The test button on a CO alarm only tests the battery and circuitry not the sensor. CO alarms should be tested with an external source of calibrated test gas, as recommended by the latest version of NFPA 720. Alarms over five years old should be replaced but they should be checked on installation and at least annually during the manufacturers warranty period.

[edit] Sensors

Early designs were basically a white pad which would fade to a brownish or blackish colour if carbon monoxide were present. Such chemical detectors are cheap and widely available, but only give a visual warning of a problem. As carbon monoxide related deaths increased during the 1990s, audible alarms became standard.

The alarm points on carbon monoxide detectors are not a simple alarm level (as in smoke detectors) but are a concentration-time function. At lower concentrations (eg 100 parts per million) the detector will not sound an alarm for many tens of minutes. At 400 parts per million (PPM), the alarm will sound within a few minutes. This concentration-time function is intended to mimic the uptake of carbon monoxide in the body while also preventing false alarms due to relatively common sources of carbon monoxide such as cigarette smoke.

There are four types of sensors available and they vary in cost, accuracy and speed of response.[10] The latter three types include sensor elements that typically last up to 10 years. At least one CO detector is available which includes a battery and sensor in a replaceable module. Most CO detectors do not have replaceable sensors.

[edit] Opto-Chemical

The detector consists of a pad of a coloured chemical which changes colour upon reaction with carbon monoxide. They only provide a qualitative warning of the gas however. The main advantage of these detectors is that they are the lowest cost but are also offer the lowest level of protection.

[edit] Biomimetic

A biomimetic (chem-optical or gel cell) sensor works with a form of synthetic hemoglobin which darkens in the presence of CO, and lightens without it. This can either be seen directly or connected to a light sensor and alarm. Battery lifespan usually lasts 2-3 years. Device lasts on the average of about 10 years. These products were the first to enter the mass market but have now largely fallen out of favour.

[edit] Electrochemical

This is a type of fuel cell that instead of being designed to produce power, is designed to produce a current that is precisely related to the amount of the target gas (in this case carbon monoxide) in the atmosphere. Measurement of the current gives a measure of the concentration of carbon monoxide in the atmosphere. Essentially the electrochemical cell consists of a container, 2 electrodes, connection wires and an electrolyte - typically sulfuric acid. Carbon monoxide is oxidized at one electrode to carbon dioxide while oxygen is consumed at the other electrode. For carbon monoxide detection, the electrochemical cell has advantages over other technologies in that it has a highly accurate and linear output to carbon monoxide concentration, requires minimal power as it is operated at room temperature, and has a long lifetime (typically commercial available cells now have lifetimes of 5 years or greater). Until recently, the cost of these cells and concerns about their long term reliability had limited uptake of this technology in the marketplace, although these concerns are now largely overcome. This technology is now the dominant technology in USA and Europe.

[edit] Semiconductor

Thin wires of the semiconductor tin dioxide on an insulating ceramic base provide a sensor monitored by an integrated circuit. This sensing element needs to be heated to approximately 400 deg C in order to operate. Oxygen increases resistance of the tin dioxide, but carbon monoxide reduces resistance therefore by measurement of the resistance of the sensing element means a monitor can be made to trigger an alarm. The power demands of this sensor means that these devices can only be mains powered although a pulsed sensor is now available that has a limited lifetime (months) as a battery powered detector. Device usually lasts on the average of 5-10 years. This technology has traditionally found high utility in Japan and the far east with some market penetration in USA. However the superior performance of electrochemical cell technology is beginning to displace this technology

[edit] Digital

Although all home detectors use an audible alarm signal as the primary indicator, some versions also offer a digital readout of the CO concentration, in parts per million. Typically, they can display both the current reading and a peak reading from memory of the highest level measured over a period of time. These advanced models cost somewhat more but are otherwise similar to the basic models.

The digital models offer the advantage of being able to observe levels that are below the alarm threshold, learn about levels that may have occurred during an absence, and assess the degree of hazard if the alarm sounds. They may also aid emergency responders in evaluating the level of past or ongoing exposure or danger.

[edit] Wireless

Wireless home safety solutions are available that link carbon monoxide detectors to vibrating pillow pads, strobes or a remote warning handset. This allows those with impediments such as hard of hearing, partially sighted, heavy sleepers or the infirm the precious minutes to wake up and get out in the event of carbon monoxide in their property.

[edit] Legislation

House builders in Colorado will be required to install carbon monoxide detectors in new homes in a bill signed into law in March 2009 by the state legislature.

House Bill 1091 requires installation of the detectors in new and resold homes near bedrooms as well as rented apartments and homes. It takes effect from July 1, 2009. The legislation was introduced after the death of Denver investment banker Parker Lofgren and his family. Lofgren, 39; his wife Caroline, 42; and their children, Owen, 10, and Sophie, 8, were found dead in a multimillion-dollar home near Aspen, Colorado on Nov. 27, 2008, victims of carbon-monoxide poisoning.

In New York State, Amanda's Law,” (A6093A/C.367) requires one- and two-family residences which have fuel burning appliances to have at least one carbon monoxide alarm installed on the lowest story having a sleeping area, effective February 22, 2010. Although homes built before Jan. 1, 2008 will be allowed to have battery-powered alarms, homes built after that date will need to have hard-wired alarms. In addition, New York State contractors will have to install a carbon monoxide detector when replacing a fuel burning water heater or furnace if the home is without an alarm. The law is named for Amanda Hansen, a teenager who died from carbon monoxide poisoning from a defective boiler while at a sleepover at a friend's house.[11]

[edit] Manufacturers

System Sensor

DuPont

First Alert

Kidde

KWJ Engineering, Inc.

Sprue Aegis

Duomo (UK) Ltd.

Ei Electronics (Ireland) www.eielectronics.com

Gas Safe Europe Ltd (www.detectagas.com)

Catalytic bead sensorFrom Wikipedia, the free encyclopedia

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A catalytic bead sensor is a type of sensor that is used for gas detection.

Contents

[hide]

1 Principle 2 Issues

3 See also

4 References

[edit] Principle

The catalytic bead sensor MSA 94150

The catalytic bead sensor consist of two coils of fine platinum wire each embedded in a bead of alumina, connected electrically in a Wheatstone bridge circuit. One of the pellistors is impregnated with a special catalyst which promotes oxidation whilst the other is treated to inhibit oxidation. Current is passed through the coils so that they reach a temperature at which oxidation of a gas readily occurs at the catalysed bead (500-550°C). Passing combustible gas raises the temperature further which increases the resistance of the platinum coil in the catalysed bead, leading to an imbalance of the bridge. This output change is linear, for most gases, up to and beyond 100% LEL, response time is a few seconds to detect alarm levels (around 20% LEL)[1], at least 12% oxygen by volume is needed for the oxidation.

[edit] Issues

Catalyst poisoning - because of the direct contact of the gas with the catalytic surface it may be deactived in some circumstances.

Sensor drift - Decreased sensitivity may occur depending on operating and ambient conditions.

Modes of failure - which include poisoning and sinter blockage, they become apparent during routine maintenance checking.

Chemical field-effect transistorFrom Wikipedia, the free encyclopedia

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This article does not cite any references or sources.Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (December 2009)

ChemFET, or chemical field-effect transistor, is a type of a field-effect transistor acting as a chemical sensor. It is a structural analog of a MOSFET transistor, where the charge on the gate electrode is applied by a chemical process. It may be used to detect atoms, molecules, and ions in liquids and gases.

ISFET, an ion-sensitive field-effect transistor, is the best known subtype of ChemFET devices. It is used to detect ions in electrolytes.

ENFET is a CHEMFET specialized for detection of specific enzymes.

Electrochemical gas sensorFrom Wikipedia, the free encyclopedia

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Electrochemical gas sensors are gas detectors that measure the concentration of a target gas by oxidizing or reducing the target gas at an electrode and measuring the resulting current.

Contents

[hide]

1 Construction 2 Theory of operation

3 Diffusion controlled response

4 Cross sensitivity

5 See also

6 References

[edit] Construction

The sensors contain two or three electrodes, occasionally four, in contact with an electrolyte. The electrodes are typically fabricated by fixing a high surface area precious metal on to the porous hydrophobic membrane. The working electrode contacts both the electrolyte and the ambient air to be monitored usually via a porous membrane. The electrolyte most commonly used is a mineral acid, but organic electrolytes are also used for some sensors. The electrodes and housing are usually in a plastic housing which contains a gas entry hole for the gas and electrical contacts.

[edit] Theory of operation

The gas diffuses into the sensor, through the back of the porous membrane to the working electrode where it is oxidized or reduced. This electrochemical reaction results in an electric current that passes through the external circuit. In addition to measuring, amplifying and performing other signal processing functions, the external circuit maintains the voltage across the

sensor between the working and counter electrodes for a two electrode sensor or between the working and reference electrodes for a three electrode cell. At the counter electrode an equal and opposite reaction occurs, such that if the working electrode is an oxidation, then the counter electrode is a reduction.HHGGHY

[edit] Diffusion controlled response

The magnitude of the current is controlled by how much of the target gas is oxidized at the working electrode. Sensors are usually designed so that the gas supply is limited by diffusion and thus the output from the sensor is linearly proportional to the gas concentration. This linear output is one of the advantages of electrochemical sensors over other sensor technologies, (e.g. infrared), whose output must be linearized before they can be used. A linear output allows for more precise measurement of low concentrations and much simpler calibration (only baseline and one point are needed).

Diffusion control offers another advantage. Changing the diffusion barrier allows the sensor manufacturer to tailor the sensor to a particular target gas concentration range. In addition, since the diffusion barrier is primarily mechanical, the calibration of electrochemical sensors tends to be more stable over time and so electrochemical sensor based instruments require much less maintenance than some other detection technologies. In principle, the sensitivity can be calculated based on the diffusion properties of the gas path into the sensor, though experimental errors in the measurement of the diffusion properties make the calculation less accurate than calibrating with test gas.[1]

[edit] Cross sensitivity

For some gases such as ethylene oxide, cross sensitivity can be a problem because ethylene oxide requires a very active working electrode catalyst and high operating potential for its oxidation. Therefore gases which are more easily oxidized such as alcohols and carbon monoxide will also give a response. Cross sensitivity problems can be eliminated though through the use of a chemical filter, for example filters that allows the target gas to pass through unimpeded, but which reacts with and removes common interferences.

While electrochemical sensors offer many advantages, they are not suitable for every gas. Since the detection mechanism involves the oxidation or reduction of the gas, electrochemical sensors are usually only suitable for gases which are electrochemically active, though it is possible to detect electrochemically inert gases indirectly if the gas interacts with another species in the sensor that then produces a response.[2] Sensors for carbon dioxide are an example of this approach and they have been commercially available for several years.

Electronic noseFrom Wikipedia, the free encyclopedia

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An electronic nose is a device intended to detect odors or flavors.

Over the last decade, “electronic sensing” or “e-sensing” technologies have undergone important developments from a technical and commercial point of view. The expression “electronic sensing” refers to the capability of reproducing human senses using sensor arrays and pattern recognition systems. Since 1982[1], research has been conducted to develop technologies, commonly referred to as electronic noses, that could detect and recognize odors and flavors. The stages of the recognition process are similar to human olfaction and are performed for identification, comparison, quantification and other applications. However, hedonic evaluation is a specificity of the human nose given that it is related to subjective opinions. These devices have undergone much development and are now used to fulfill industrial needs.

Contents

[hide]

1 Other techniques to analyze odors 2 Electronic Nose working principle

3 How to perform an analysis

4 Range of applications

5 See also

6 References

7 External links

[edit] Other techniques to analyze odors

In all industries, odor assessment is usually performed by human sensory analysis, Chemosensors or by gas chromatography (GC, GC/MS). The latter technique gives information about volatile organic compounds but the correlation between analytical results and actual odor perception is not direct due to potential interactions between several odorous components.

[edit] Electronic Nose working principle

The electronic nose was developed in order to mimic human olfaction that functions as a non-separative mechanism: i.e. an odor / flavor is perceived as a global fingerprint.

Electronic Noses include three major parts: a sample delivery system, a detection system, a computing system.

The sample delivery system enables the generation of the headspace (volatile compounds) of a sample, which is the fraction analyzed. The system then injects this headspace into the detection

system of the electronic nose. The sample delivery system is essential to guarantee constant operating conditions.

The detection system, which consists of a sensor set, is the “reactive” part of the instrument. When in contact with volatile compounds, the sensors react, which means they experience a change of electrical properties. Each sensor is sensitive to all volatile molecules but each in their specific way. Most electronic noses use sensor arrays that react to volatile compounds on contact: the adsorption of volatile compounds on the sensor surface causes a physical change of the sensor. A specific response is recorded by the electronic interface transforming the signal into a digital value. Recorded data are then computed based on statistical models.[2]

The more commonly used sensors include metal oxide semiconductors (MOS), conducting polymers (CP), quartz crystal microbalance, surface acoustic wave (SAW), and field effect transistors (MOSFET).

In recent years, other types of electronic noses have been developed that utilize mass spectrometry or ultra fast gas chromatography as a detection system.[2]

The computing system works to combine the responses of all of the sensors, which represents the input for the data treatment. This part of the instrument performs global fingerprint analysis and provides results and representations that can be easily interpreted. Moreover, the electronic nose results can be correlated to those obtained from other techniques (sensory panel, GC, GC/MS).

[edit] How to perform an analysis

As a first step, an electronic nose need to be trained with qualified samples so as to build a database of reference. Then the instrument can recognize new samples by comparing volatile compounds fingerprint to those contained in its database. Thus they can perform qualitative or quantitative analysis.

[edit] Range of applications

Electronic nose instruments are used by Research & Development laboratories, Quality Control laboratories and process & production departments for various purposes:

in R&D laboratories for:

Formulation or reformulation of products Benchmarking with competitive products

Shelf life and stability studies

Selection of raw materials

Packaging interaction effects

Simplification of consumer preference test

in Quality Control laboratories for at line quality control such as:

Conformity of raw materials, intermediate and final products Batch to batch consistency

Detection of contamination, spoilage, adulteration

Origin or vendor selection

Monitoring of storage conditions.

In process and production departments for:

Managing raw material variability Comparison with a reference product

Measurement and comparison of the effects of manufacturing process on products

Following-up cleaning in place process efficiency

Scale-up monitoring

Cleaning in place monitoring.

Various application notes describe analysis in areas such as Flavor & Fragrance, Food & Beverage, Packaging, Pharmaceutical, Cosmetic & Perfumes, Chemical companies. More recently they can also address public concerns in terms of olfactive nuisance monitoring with networks of on-field devices.[3]

Electrolyte–insulator–semiconductor sensorFrom Wikipedia, the free encyclopedia

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The introduction to this article provides insufficient context for those unfamiliar with the subject. Please help improve the article with a good introductory style. (October 2009)

An Electrolyte–insulator–semiconductor (EIS) sensor is a sensor that is made of these three components:

an electrolyte with the chemical that should be measured an insulator that allows field-effect interaction, without leak currents between the two other

components

a semiconductor to register the chemical changes

The EIS sensor can be used in combination with other structures, for example to construct a light-addressable potentiometric sensor (LAPS).

Hydrogen sensorFrom Wikipedia, the free encyclopedia

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A hydrogen sensor is a gas detector that detects the presence of hydrogen. They contain micro-fabricated point-contact hydrogen sensors and are used to locate leaks. They are considered low-cost, compact, durable, and easy to maintain as compared to conventional gas detecting instruments.[1]

Contents

[hide]

1 Key Issues o 1.1 Additional requirements

2 Types of microsensors

o 2.1 Optical fibre hydrogen sensors

o 2.2 Other type hydrogen sensors

o 2.3 Enhancement

3 See also

4 References

5 External links

[edit] Key Issues

There are five key issues with hydrogen detectors:[2]

Reliability : Functionality should be easily verifiable. Performance : Detection 0.5% hydrogen in air or better

Response time < 1 second.

Lifetime : At least the time between scheduled maintenance.

Cost : Goal is $5 per sensor and $30 per controller.

[edit] Additional requirements

Measurement range coverage of 0.1%–10.0% concentration[3]

Operation in temperatures of -30°C to 80°C

Accuracy within 5% of full scale

Function in an ambient air gas environment within a 10%–98% relative humidity range

Resistance to hydrocarbon and other interference.

Lifetime greater than 10 years

[edit] Types of microsensors

There are various types of hydrogen microsensors, which use different mechanisms to detect the gas. Palladium is used in many of these, because it selectively absorbs hydrogen gas and forms the compound palladium hydride.[4] Palladium-based sensors have a strong temperature dependence which makes their response time too large at very low temperatures.[5] Palladium sensors have to be protected against carbon monoxide, sulfur dioxide and hydrogen sulfide.

[edit] Optical fibre hydrogen sensors

Several types of optical fibre surface plasmon resonance (SPR) sensor are used for the point-contact detection of hydrogen:

Fiber Bragg grating coated with a palladium layer - Detects the hydrogen by metal hindrance. Micromirror - With a palladium thin layer at the cleaved end, detecting changes in the

backreflected light.

Tapered fibre coated with palladium - Hydrogen changes the refractive index of the palladium, and consequently the amount of losses in the evanescent wave.

[edit] Other type hydrogen sensors

MEMS hydrogen sensor - The combination of nanotechnology and microelectromechanical systems (MEMS) technology allows the production of a hydrogen microsensor that functions properly at room temperature. The hydrogen sensor is coated with a film consisting of nanostructured indium oxide (In2O3) and tin oxide (SnO2).[6]

Thin film sensor - A palladium thin film sensor is based on an opposing property that depends on the nanoscale structures within the thin film. In the thin film, nanosized palladium particles swell when the hydride is formed, and in the process of expanding, some of them form new electrical connections with their neighbors. The resistance decreases because of the increased number of conducting pathways.[2][7]

Thick film sensor - Thick film hydrogen sensors rely on the fact that palladium hydride's electrical resistance is greater than the metal's resistance. The absorption of hydrogen causes a measurable increase in electrical resistance.

Chemochromic hydrogen sensor - Reversible and irreversible chemochromic hydrogen sensors, a smart pigment paint that visually identifies hydrogen leaks by a change in color. The sensor is also available as tape. [8] [9]

Diode based Schottky sensor - A Schottky diode-based hydrogen gas sensor employs a palladium-alloy gate. Hydrogen can be selectively absorbed in the gate, lowering the Schottky energy barrier.[10] A Pd/InGaP metal-semiconductor (MS) Schottky diode can detect a concentration of 15 parts per million (ppm) H2 in air.[11] Silicon carbide semiconductor or silicon substrates are used.

Metallic La-Mg2-Ni which is electrical conductive, absorbs hydrogen near ambient conditions, forming the nonmetallic hydride LaMg2NiH7 an insulator [12] .

Sensors are typically calibrated at the manufacturing factory and are valid for the service life of the unit.

[edit] Enhancement

Siloxane enhances the sensitivity and reaction time of hydrogen sensors.[4] Detection of hydrogen levels as low as 25 ppm can be achieved; far below hydrogen's lower explosive limit of around 40,000 ppm.

Hydrogen sulfide sensorFrom Wikipedia, the free encyclopedia

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A hydrogen sulfide sensor or H2S sensor is a gas sensor for the measurement of hydrogen sulfide [1] .

Contents

[hide]

1 Principle 2 Applications

3 Research

4 See also

5 References

[edit] Principle

The H2S sensor is a metal oxide semiconductor (MOS) sensor which operates by a reversible change in resistance caused by adsorption and desorption of hydrogen sulfide in a film with hydrogen sulfide sensitive material like tin oxide thick films and gold thin films. Current response time is 25 ppb to 10 ppm < one minute.

[edit] Applications

This type of sensor has been under constant development because of the toxic and corrosive nature of hydrogen sulfide:

The H2S sensor is used to detect hydrogen sulfide in the hydrogen feed stream of fuel cells to prevent catalyst poisoning and to measure the quality of guard beds used to remove sulfur from hydrocarbon fuels[2].

[edit] Research

2004 - a nanocrystalline SnO2–Ag on ceramic wafer sensor is reported[3].

Infrared point sensorFrom Wikipedia, the free encyclopedia

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A infrared point sensor is a point gas detector based on the nondispersive infrared sensor technology.

Contents

[hide]

1 Principle 2 Micro heaters

3 Range

4 See also

5 External links

[edit] Principle

Dual source and dual receivers are used for self compensation of changes in alignment, light source intensity and component efficiency. The transmitted beams from two infrared sources are superimposed onto an internal beam splitter. 50% of the overlapping sample and reference signal is passed through the gas measuring path and reflected back onto the measuring detector. The presence of combustible gas will reduce the intensity of the sample beam and not the reference beam, with the difference between these two signals being proportional to the concentration of gas present in the measuring path. The other 50% of the overlapped signal passes through the beam splitter and onto the compensation detector. The compensation detector monitors the intensity of the two infrared sources and automatically compensates for any long term drift.

Mean time between failures may go up to 15 years.

[edit] Micro heaters

Micro heaters can be used to raise the temperature from optical surfaces above ambient to enhance performance and to prevent condensation on the optical surfaces.

[edit] Range

Toxic gases are measured in the low parts per million (ppm) range. Flammable gases are measured in the 0 - 100 % lower flammable limit (LFL) or lower explosive limit (LEL) range.

Ion selective electrodeFrom Wikipedia, the free encyclopedia

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An ion-selective electrode (ISE), also known as a specific ion electrode (SIE), is a transducer (or sensor) that converts the activity of a specific ion dissolved in a solution into an electrical potential, which can be measured by a voltmeter or pH meter. The voltage is theoretically dependent on the logarithm of the ionic activity, according to the Nernst equation. The sensing part of the electrode is usually made as an ion-specific membrane, along with a reference electrode. Ion-selective electrodes are used in biochemical and biophysical research, where measurements of ionic concentration in an aqueous solution are required, usually on a real time basis.

Contents

[hide]

1 Types of ion-selective membrane

o 1.1 Glass membranes

o 1.2 Crystalline membranes

o 1.3 Ion-exchange resin membranes

1.3.1 Construction

1.3.2 Applications

o 1.4 Enzyme electrodes

2 Interferences

3 See also

4 References

5 External links

[edit] Types of ion-selective membrane

There are four main types of ion-selective membrane used in ion-selective electrodes: glass, solid state, liquid based, and compound electrode.

[edit] Glass membranes

Glass membranes are made from an ion-exchange type of glass (silicate or chalcogenide). This type of ISE has good selectivity, but only for several single-charged cations; mainly H+, Na+, and Ag+. Chalcogenide glass also has selectivity for double-charged metal ions, such as Pb2+, and Cd2+. The glass membrane has excellent chemical durability and can work in very aggressive media. A very common example of this type of electrode is the pH glass electrode.

[edit] Crystalline membranes

Crystalline membranes are made from mono- or polycrystallites of a single substance. They have good selectivity, because only ions which can introduce themselves into the crystal structure can interfere with the electrode response. Selectivity of crystalline membranes can be for both cation and anion of the membrane-forming substance. An example is the fluoride selective electrode based on LaF3 crystals.

[edit] Ion-exchange resin membranes

Ion-exchange resins are based on special organic polymer membranes which contain a specific ion-exchange substance (resin). This is the most widespread type of ion-specific electrode. Usage of specific resins allows preparation of selective electrodes for tens of different ions, both single-atom or multi-atom. They are also the most widespread electrodes with anionic selectivity. However, such electrodes have low chemical and physical durability as well as "survival time".

An example is the potassium selective electrode, based on valinomycin as an ion-exchange agent.

[edit] Construction

These electrodes are prepared from glass capillary tubing approximately 2 millimeters in diameter, a large batch at a time. Polyvinyl chloride is dissolved in a solvent and plasticizers (typically phthalates) added, in the standard fashion used when making something out of vinyl. In order to provide the ionic specificity, a specific ion channel or carrier is added to the solution; this allows the ion to pass through the vinyl, which prevents the passage of other ions and water.

One end of a piece of capillary tubing about an inch or two long is dipped into this solution and removed to let the vinyl solidify into a plug at that end of the tube. Using a syringe and needle, the tube is filled with salt solution from the other end, and may be stored in a bath of the salt solution for an indeterminate period. For convenience in use, the open end of the tubing is fitted through a tight o-ring into a somewhat larger diameter tubing containing the same salt solution, with a silver or platinum electrode wire inserted. New electrode tips can thus be changed very quickly by simply removing the older electrode and replacing it with a new one.

[edit] Applications

In use, the electrode wire is connected to one terminal of a galvanometer or pH meter, the other terminal of which is connected to a reference electrode, and both electrodes are immersed in the solution to be tested. The passage of the ion through the vinyl via the carrier or channel creates an electrical current, which registers on the galvanometer; by calibrating against standard solutions of varying concentration, the ionic concentration in the tested solution can be estimated from the galvanometer reading.

In practice there are several issues which affect this measurement, and different electrodes from the same batch will differ in their properties. Leakage between the vinyl and the wall of the capillary, thereby allowing passage of any ions, will cause the meter reading to show little or no change between the various calibration solutions, and requires that that electrode be discarded. Similarly, with use the ion-sensitive channels in the vinyl appear to gradually become blocked or otherwise inactivated, causing the electrode to lose sensitivity. The response of the electrode and galvanometer is temperature sensitive, and also 'drifts' over time, requiring recalibration frequently during a series of measurements, ideally at least one calibration sample before and after each test sample. On the other hand, after immersion in the solution there is a transient 'settling time' which can be five minutes or even longer, before the electrode and galvanometer equilibrate to a new reading; so that timing of the reading is critical in order to find the most accurate 'window' after the response has settled, but before it has drifted appreciably.

[edit] Enzyme electrodes

Enzyme electrodes definitely are not true ion-selective electrodes but usually are considered within the ion-specific electrode topic. Such an electrode has a "double reaction" mechanism - an enzyme reacts with a specific substance, and the product of this reaction (usually H+ or OH-) is

detected by a true ion-selective electrode, such as a pH-selective electrodes. All these reactions occur inside a special membrane which covers the true ion-selective electrode, which is why enzyme electrodes sometimes are considered as ion-selective. An example is glucose selective electrodes.

[edit] Interferences

The most serious problem limiting use of ion-selective electrodes is interference from other, undesired, ions. No ion-selective electrodes are completely ion-specific; all are sensitive to other ions having similar physical properties, to an extent which depends on the degree of similarity. Most of these interferences are weak enough to be ignored, but in some cases the electrode may actually be much more sensitive to the interfering ion than to the desired ion, requiring that the interfering ion be present only in relatively very low concentrations, or entirely absent. In practice, the relative sensitivities of each type of ion-specific electrode to various interfering ions is generally known and should be checked for each case; however the precise degree of interference depends on many factors, preventing precise correction of readings. Instead, the calculation of relative degree of interference from the concentration of interfering ions can only be used as a guide to determine whether the approximate extent of the interference will allow reliable measurements, or whether the experiment will need to be redesigned so as to reduce the effect of interfering ions. The nitrate electrode has various ionic interferences, i.e. perchlorate, iodide, chloride, and sulfate. These interferences vary markedly in the extent to which they interfere. Thus, perchlorate gives a response which is about 50,000x as great as an equal amount of nitrate, while 1000x as much sulfate produces about a 10% error in the reading.[1] Chloride causes a 10% error when present at about 30x the nitrate level, but can be removed by the addition of silver sulfate. Alternately, nitrate can be determined by using an ammonia gas sensing electrode. This technique allows the user to determine both ammonium and nitrate ions sequentially. The procedure makes use of the reducing ability of titanium chloride. Trivalent titanium reduces any nitrate ion, up to 20 ppm, to ammonium ion (i.e., reverse nitrification). At pH 12-13, any ammonium ion in the sample is converted to ammonia gas and is ultimately detected by the electrode[2].

Nondispersive infrared sensorFrom Wikipedia, the free encyclopedia

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A nondispersive infrared sensor (or NDIR) sensor is a simple spectroscopic device often used as gas detector.

NDIR-Analyzer with one double tube for CO and another double tube for hydrocarbons

[edit] Principle

The main components are an infrared source (lamp), a sample chamber or light tube, a wavelength filter, and the infrared detector. The gas is pumped (or diffuses) into the sample chamber, and gas concentration is measured electro-optically by its absorption of a specific wavelength in the infrared (IR). The IR light is directed through the sample chamber towards the detector. In parallel there is an other chamber with an enclosed reference gas, typically nitrogen. The detector has an optical filter in front of it that eliminates all light except the wavelength that the selected gas molecules can absorb. Ideally other gas molecules do not absorb light at this wavelength, and do not affect the amount of light reaching the detector.

As many gases absorb well in the IR area, it is often necessary to compensate for interfering components. For instance, CO2 and H2O often initiate cross sensitivity in the infrared spectrum. As many measurements in the IR area are cross sensitive to H2O it is difficult to analyse for instance SO2 and NO2 in low concentrations using the infrared light principle.

The IR signal from the source is usually chopped or modulated so that thermal background signals can be offset from the desired signal[1].

Microwave chemistry sensorFrom Wikipedia, the free encyclopedia

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This article does not cite any references or sources.Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (August 2008)

Microwave chemistry sensor or Surface acoustic wave (SAW) sensors consist of an input transducer, a chemically adsorbent polymer film, and an output transducer on a piezoelectric substrate, which is typically quartz. The input transducer launches an acoustic wave that travels through the chemical film and is detected by the output transducer. The Sandia-made device runs at a very high frequency (approximately 525 MHz), and the velocity and attenuation of the signal are sensitive to the viscoelasticity and mass of the thin film . SAWS have been able to distinguish organophosphates, chlorinated hydrocarbons, ketones, alcohols, aromatic hydrocarbons, saturated hydrocarbons, and water . The SAW used in these tests have four channels—each channel consists of a transmitter and a receiver, separated by a small distance. Three of the four channels have a polymer deposited on the substrate between the transmitter and receiver. The purpose of the polymers is to adsorb chemicals of interest, with different polymers having different affinities to various chemicals. When a chemical is adsorbed, the mass of the polymer increases, causing a slight change in phase of the acoustic signal relative to the reference (fourth) channel, which does not contain a polymer. The SAW device also contains three Application Specific Integrated Circuit chips (ASICs), which contain the electronics to analyze the signals and provide a DC voltage signal proportional to the phase shift. The SAW device, containing the transducers and ASICs, is bonded to a piece of quartz glass, which is placed in a leadless chip carrier (LCC). Wire bonds connect the terminals of the leadless chip carrier to the SAW circuits.

[edit] Apllication

The Microwave chemistry sensor can detect several chemical materials including:

Lead Mercury

Oxygen

Nitrogen oxide sensorFrom Wikipedia, the free encyclopedia

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A nitrogen oxide sensor or NOx sensor is typically a high temperature device built to detect nitrogen oxides in combustion environments such as an automobile or truck tailpipe or a smokestack.

Contents

[hide]

1 Availability

2 Motivating factors

3 Difficulties

o 3.1 Harsh environment

o 3.2 High sensitivity and durability required

4 Conclusion

5 See also

6 External links

[edit] Availability

Continental Automotive Systems/NGK are in production of a NOx sensor for automotive and truck applications. Several automobile and related companies such as Delphi, Ford, Chrysler, and Toyota have also put extensive research into development of NOx sensors. Many academic and government labs are pushing to develop the sensors as well. The term NOx actually represents several forms of nitrogen oxides such as NO (nitric oxide), NO2 (nitrogen dioxide) and N2O (nitrous oxide aka laughing gas). In a gasoline engine NO is the most common form of NOx being around 93% while NO2 is around 5% and the rest is N2O. There are other forms of NOx such as N2O4 (the dimer of NO2), which only exists at lower temperatures, and N2O5, for example. However, owing to lower combustion temperatures, diesel engines produce lower engine-out NOx emissions than do spark-ignition gasoline engines, but the nonexistent NOx after-treatment causes diesel engines to emit significantly more NOx at the tailpipe compared to a typical gasoline engine with a 3-way catalyst. In addition, the diesel oxidation catalyst significantly increases the fraction of NO2 in "NOx" by oxidizing over 50% of NO using the excess oxygen in the diesel exhaust gases.

[edit] Motivating factors

The drive to develop a NOx sensor comes from environmental factors. NOx gases can cause various problems such as smog and acid rain. Many governments around the world have passed laws to limit their emissions (along with other combustion gases such as SOx (oxides of sulfur), CO (carbon monoxide) and CO2 (carbon dioxide) and hydrocarbons). Companies have realized that one way of minimizing NOx emissions is to first detect them and then employ some sort of feedback loop in the combustion process, minimizing NOx production by, for example, combustion optimization or regeneration of NOx traps.

[edit] Difficulties

[edit] Harsh environment

Due to the high temperature of the combustion environment only certain types of material can operate in situ. The majority of NOx sensors developed have been made out of ceramic type metal oxides with the most common being yttria stabilized zirconia (YSZ), which is currently used in the decades old oxygen sensor. The YSZ is compacted into a dense ceramic and actually conducts oxygen ions (O2-) at the high temperatures of a tailpipe such at 400 °C and above. To get a signal from the sensor a pair of high temperature electrodes such as noble metals (platinum, gold, or palladium) or other metal oxides are placed onto the surface and an electrical signal such as the change in voltage or current is measured as a function of NOx concentration.

[edit] High sensitivity and durability required

The levels of NO are around 100-2000 ppm (parts per million) and NO2 20-200 ppm in a range of 1-10% O2. The sensor has to be very sensitive to pick up these levels.

The main problems that have limited the development of a successful NOx sensor (which are typical of many sensors) are selectivity, sensitivity, stability, reproducibility, response time, limit of detection and cost. In addition due to the harsh environment of combustion the high gas flow rate can cool the sensor which alters the signal or it can delaminate the electrodes over time and soot particles can degrade the materials.

[edit] Conclusion

The NOx sensing element mentioned above is only a part of the whole device, which also includes the packaging and electronics. The development of a very good NOx sensor is highly desirable and has the potential to serve a large market. Thus there is a large push to produce a robust sensor that works reliably and accurately over a wide temperature range. It is currently one of the most sought after combustion gas sensors. Degradation by CO2 and SO2 gases, however, remains a major problem of NOx sensors.

OlfactometerFrom Wikipedia, the free encyclopedia

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An olfactometer is an instrument typically used to detect and measure ambient odor dilution. Olfactometers are utilized in conjunction with human subjects in laboratory settings, most often in market research, to quantify and qualify human olfaction.[1] Olfactometers are used to gauge the odor detection threshold of substances. To measure intensity, olfactometers introduce an odorous gas as a baseline against which other odors are compared.

Alternatively, an olfactometer is a device used for producing aromas in a precise and controlled manner.

OptodeFrom Wikipedia, the free encyclopedia

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This article does not cite any references or sources.Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (December 2009)

An optode or optrode is an optical sensor device that optically measures a specific substance usually with the aid of a chemical transducer.

Contents

[hide]

1 Construction 2 Operation

3 Popularity

4 See also

[edit] Construction

An optode requires three components to function: a chemical that responds to an analyte, a polymer to immobilise the chemical transducer and instrumentation (optical fibre, light source, detector and other electronics). Optodes usually have the polymer matrix coated onto the tip of an optical fibre, but in the case of evanescent wave optodes the polymer is coated on a section of fibre that has been unsheathed.

[edit] Operation

Optodes can apply various optical measurement schemes such as reflection, absorption, evanescent wave, luminescence (fluorescence and phosphorescences), chemiluminescence, surface plasmon resonance. By far the most popular methodology is luminescence.

Luminescence in solution obeys the linear Stern-Volmer relationship. Fluorescence of a molecule is quenched by specific analytes, e.g. ruthenium complexes are quenched by oxygen. When a fluorophore is immobilised within a polymer matrix a myriad of micro-environments are created. The micro-environments reflect varying diffusion co-efficients for the analyte. This leads to a non-linear relationship between the fluorescence and the quencher (analyte). This

relationship is modelled in various ways, the most popular model is the two site model created by James Demas (University of Virginia).

The signal (fluorescence) to oxygen ratio is not linear, and an optode is most sensitive at low oxygen concentration, i.e. the sensitivity decreases as oxygen concentration increases. The optode sensors can however work in the whole region 0–100% oxygen saturation in water, and the calibration is done the same way as with the Clark type sensor. No oxygen is consumed and hence the sensor is stirring insensitive, but the signal will stabilize more quickly if the sensor is stirred after being put into the sample.

[edit] Popularity

Optical sensors are growing in popularity due to the low-cost, low power requirements and long term stability. They provide viable alternatives to electrode-based sensors or more complicated analytical instrumentation.

Major international conferences are devoted to their development e.g. Europtrode VIII Tübingen 2006, OFS 18, Cancun 2006.

Oxygen sensorFrom Wikipedia, the free encyclopedia

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It has been suggested that AFR sensor be merged into this article or section. (Discuss)

Contents

[hide]

1 Automotive applications o 1.1 Function of a lambda probe

o 1.2 The probe

o 1.3 Operation of the probe

1.3.1 Zirconia sensor

1.3.2 Wideband zirconia sensor

1.3.3 Titania sensor

o 1.4 Location of the probe in a system

o 1.5 Sensor surveillance

o 1.6 Sensor failures

2 Diving applications

3 Scientific applications

o 3.1 Electrodes

o 3.2 Optodes

4 See also

5 References

An oxygen sensor, or lambda sensor, is an electronic device that measures the proportion of oxygen (O2) in the gas or liquid being analyzed. It was developed by Robert Bosch GmbH during the late 1960s under supervision by Dr. Günter Bauman. The original sensing element is made with a thimble-shaped zirconia ceramic coated on both the exhaust and reference sides with a thin layer of platinum and comes in both heated and unheated forms. The planar-style sensor entered the market in 1998 (also pioneered by Robert Bosch GmbH) and significantly reduced the mass of the ceramic sensing element as well as incorporating the heater within the ceramic structure. This resulted in a sensor that both started operating sooner and responded faster. The most common application is to measure the exhaust gas concentration of oxygen for internal combustion engines in automobiles and other vehicles. Divers also use a similar device to measure the partial pressure of oxygen in their breathing gas.

Scientists use oxygen sensors to measure respiration or production of oxygen and use a different approach. Oxygen sensors are used in oxygen analyzers which find a lot of use in medical applications such as anesthesia monitors, respirators and oxygen concentrators.

There are many different ways of measuring oxygen and these include technologies such as zirconia, electrochemical (also known as Galvanic), infrared, ultrasonic and very recently laser. Each method has its own advantages and disadvantages.

[edit] Automotive applications

A three-wire oxygen sensor suitable for use in a Volvo 240 or similar.

Automotive oxygen sensors, colloquially known as O2 sensors, make modern electronic fuel injection and emission control possible. They help determine, in real time, if the air fuel ratio of a combustion engine is rich or lean. Since oxygen sensors are located in the exhaust stream, they do not directly measure the air or the fuel entering the engine. But when information from oxygen sensors is coupled with information from other sources, it can be used to indirectly determine the air-to-fuel ratio. Closed-loop feedback-controlled fuel injection varies the fuel injector output according to real-time sensor data rather than operating with a predetermined (open-loop) fuel map. In addition to enabling electronic fuel injection to work efficiently, this emissions control technique can reduce the amounts of both unburnt fuel and oxides of nitrogen entering the atmosphere. Unburnt fuel is pollution in the form of air-borne hydrocarbons, while oxides of nitrogen (NOx gases) are a result of combustion chamber temperatures exceeding 1,300 kelvins due to excess air in the fuel mixture and contribute to smog and acid rain. Volvo was the first automobile manufacturer to employ this technology in the late 1970s, along with the 3-way catalyst used in the catalytic converter.

The sensor does not actually measure oxygen concentration, but rather the amount of oxygen needed to completely oxidize any remaining combustibles in the exhaust gas. Rich mixture causes an oxygen demand. This demand causes a voltage to build up, due to transportation of oxygen ions through the sensor layer. Lean mixture causes low voltage, since there is an oxygen excess.

Modern spark-ignited combustion engines use oxygen sensors and catalytic converters in order to reduce exhaust emissions. Information on oxygen concentration is sent to the engine management computer or ECU, which adjusts the amount of fuel injected into the engine to compensate for excess air or excess fuel. The ECU attempts to maintain, on average, a certain air-fuel ratio by interpreting the information it gains from the oxygen sensor. The primary goal is a compromise between power, fuel economy, and emissions, and in most cases is achieved by an air-fuel-ratio close to stoichiometric. For spark-ignition engines (such as those that burn gasoline, as opposed to diesel), the three types of emissions modern systems are concerned with are: hydrocarbons (which are released when the fuel is not burnt completely, such as when misfiring or running rich), carbon monoxide (which is the result of running slightly rich) and NOx (which dominate when the mixture is lean). Failure of these sensors, either through normal aging, the use of leaded fuels, or fuel contaminated with silicones or silicates, for example, can lead to damage of an automobile's catalytic converter and expensive repairs.

Tampering with or modifying the signal that the oxygen sensor sends to the engine computer can be detrimental to emissions control and can even damage the vehicle. When the engine is under low-load conditions (such as when accelerating very gently, or maintaining a constant speed), it is operating in "closed-loop mode." This refers to a feedback loop between the ECU and the oxygen sensor(s) in which the ECU adjusts the quantity of fuel and expects to see a resulting change in the response of the oxygen sensor. This loop forces the engine to operate both slightly lean and slightly rich on successive loops, as it attempts to maintain a mostly stoichiometric ratio on average. If modifications cause the engine to run moderately lean, there will be a slight increase in fuel economy, sometimes at the expense of increased NOx emissions, much higher exhaust gas temperatures, and sometimes a slight increase in power that can quickly turn into misfires and a drastic loss of power, as well as potential engine damage, at ultra-lean air-to-fuel ratios. If modifications cause the engine to run rich, then there will be a slight increase in power to a point (after which the engine starts flooding from too much unburned fuel), but at the cost of decreased fuel economy, and an increase in unburned hydrocarbons in the exhaust which causes overheating of the catalytic converter. Prolonged operation at rich mixtures can cause catastrophic failure of the catalytic converter (see backfire). The ECU also controls the spark engine timing along with the fuel injector pulse width, so modifications which alter the engine to operate either too lean or too rich may result in inefficient fuel consumption whenever fuel is ignited too soon or too late in the combustion cycle.

When an internal combustion engine is under high load (e.g. wide open throttle), the output of the oxygen sensor is ignored, and the ECU automatically enriches the mixture to protect the engine, as misfires under load are much more likely to cause damage. This is referred to an engine running in 'open-loop mode'. Any changes in the sensor output will be ignored in this state. In many cars (with the exception of some turbocharged models), inputs from the air flow meter are also ignored, as they might otherwise lower engine performance due to the mixture being too rich or too lean, and increase the risk of engine damage due to detonation if the mixture is too lean.

[edit] Function of a lambda probe

Lambda probes are used to reduce vehicle emissions by ensuring that engines burn their fuel efficiently and cleanly. Robert Bosch GmbH introduced the first automotive lambda probe in 1976[1], and it was first used by Volvo and Saab in that year. The sensors were introduced in the US from about 1980, and were required on all models of cars in many countries in Europe in 1993.

By measuring the proportion of oxygen in the remaining exhaust gas, and by knowing the volume and temperature of the air entering the cylinders amongst other things, an ECU can use look-up tables to determine the amount of fuel required to burn at the stoichiometric ratio (14.7:1 air:fuel by mass for gasoline) to ensure complete combustion.

[edit] The probe

The sensor element is a ceramic cylinder plated inside and out with porous platinum electrodes; the whole assembly is protected by a metal gauze. It operates by measuring the difference in

oxygen between the exhaust gas and the external air, and generates a voltage or changes its resistance depending on the difference between the two.

The sensors only work effectively when heated to approximately 316 °C (600 °F), so most newer lambda probes have heating elements encased in the ceramic that bring the ceramic tip up to temperature quickly. Older probes, without heating elements, would eventually be heated by the exhaust, but there is a time lag between when the engine is started and when the components in the exhaust system come to a thermal equilibrium. The length of time required for the exhaust gases to bring the probe to temperature depend on the temperature of the ambient air and the geometry of the exhaust system. Without a heater, the process may take several minutes. There are pollution problems that are attributed to this slow start-up process, including a similar problem with the working temperature of a catalytic converter.

The probe typically has four wires attached to it: two for the lambda output, and two for the heater power, although some automakers use a common ground for the sensor element and heaters, resulting in three wires. Earlier non-electrically-heated sensors had one or two wires.

[edit] Operation of the probe

[edit] Zirconia sensor

A planar zirconia sensor (schematic picture)

The zirconium dioxide, or zirconia, lambda sensor is based on a solid-state electrochemical fuel cell called the Nernst cell. Its two electrodes provide an output voltage corresponding to the quantity of oxygen in the exhaust relative to that in the atmosphere. An output voltage of 0.2 V (200 mV) DC represents a "lean mixture" of fuel and oxygen, where the amount of oxygen entering the cylinder is sufficient to fully oxidize the carbon monoxide (CO), produced in burning the air and fuel, into carbon dioxide (CO2). An output voltage of 0.8 V (800 mV) DC represents a "rich mixture", one which is high in unburned fuel and low in remaining oxygen. The ideal setpoint is approximately 0.45 V (450 mV) DC. This is where the quantities of air and fuel are in the optimum ratio, which is ~0.5% lean of the stoichiometric point, such that the exhaust output contains minimal carbon monoxide.

The voltage produced by the sensor is nonlinear with respect to oxygen concentration. The sensor is most sensitive near the stoichiometric point and less sensitive when either very lean or very rich.

The engine control unit (ECU) is a control system that uses feedback from the sensor to adjust the fuel/air mixture. As in all control systems, the time constant of the sensor is important; the ability of the ECU to control the fuel-air-ratio depends upon the response time of the sensor. An aging or fouled sensor tends to have a slower response time, which can degrade system performance. The shorter the time period, the higher the so-called "cross count" [2] and the more responsive the system.

The zirconia sensor is of the "narrow band" type, referring to the narrow range of fuel/air ratios to which it responds.

[edit] Wideband zirconia sensor

A planar wideband zirconia sensor (schematic picture)

A variation on the zirconia sensor, called the "wideband" sensor, was introduced by Robert Bosch in 1994 but is (as of 2006) used in only a few vehicles (such as the Subaru Impreza WRX when equipped with a manual transmission). It is based on a planar zirconia element, but also incorporates an electrochemical gas pump. An electronic circuit containing a feedback loop controls the gas pump current to keep the output of the electrochemical cell constant, so that the pump current directly indicates the oxygen content of the exhaust gas. This sensor eliminates the lean-rich cycling inherent in narrow-band sensors, allowing the control unit to adjust the fuel delivery and ignition timing of the engine much more rapidly. In the automotive industry this sensor is also called a UEGO (for Universal Exhaust Gas Oxygen) sensor. UEGO sensors are also commonly used in aftermarket dyno tuning and high-performance driver air-fuel display equipment. The wideband zirconia sensor is used in stratified fuel injection systems, and can now also be used in diesel engines to satisfy the forthcoming EURO and ULEV emission limits.

Wideband sensors have three elements:

Ion Oxygen pump Narrowband zirconia sensor

Heating element

The wiring diagram for the wideband sensor typically has six wires:

resistive heating element (two wires) sensor

pump

calibration resistor

common

[edit] Titania sensor

A less common type of narrow-band lambda sensor has a ceramic element made of titanium dioxide (titania). This type does not generate its own voltage, but changes its electrical resistance in response to the oxygen concentration. The resistance of the titania is a function of the oxygen partial pressure and the temperature. Therefore, some sensors are used with a gas temperature sensor to compensate for the resistance change due to temperature. The resistance value at any temperature is about 1/1000th the change in oxygen concentration. Luckily, at lambda = 1, there is a large change of oxygen, so the resistance change is typically 1000 times between rich and lean, depending on the temperature.

As titania is an N-type semiconductor with a structure TiO2-x, the x defects in the crystal lattice conduct the charge. So, for fuel-rich exhaust the resistance is low, and for fuel-lean exhaust the resistance is high. The control unit feeds the sensor with a small electrical current and measures the resulting voltage across the sensor, which varies from near 0 volts to about 5 volts. Like the zirconia sensor, this type is nonlinear, such that it is sometimes simplistically described as a

binary indicator, reading either "rich" or "lean". Titania sensors are more expensive than zirconia sensors, but they also respond faster.

In automotive applications the titania sensor, unlike the zirconia sensor, does not require a reference sample of atmospheric air to operate properly. This makes the sensor assembly easier to design against water contamination. While most automotive sensors are submersible, zirconia-based sensors require a very small supply of reference air from the atmosphere. In theory, the sensor wire harness and connector are sealed. Air that leaches through the wire harness to the sensor is assumed to come from an open point in the harness - usually the ECU which is housed in an enclosed space like the trunk or vehicle interior.

[edit] Location of the probe in a system

The probe is typically screwed into a threaded hole in the exhaust system, located after the branch manifold of the exhaust system combines, and before the catalytic converter. New vehicles are required to have a sensor before and after the exhaust catalyst to meet U.S. regulations requiring that all emissions components be monitored for failure. Pre and post-catalyst signals are monitored to determine catalyst efficiency. Additionally, some catalyst systems require brief cycles of lean (oxygen-containing) gas to load the catalyst and promote additional oxidation reduction of undesirable exhaust components.

[edit] Sensor surveillance

The air-fuel ratio and naturally, the status of the sensor, can be monitored by means of using an air-fuel ratio meter that displays the read output voltage of the sensor.

[edit] Sensor failures

Normally, the lifetime of an unheated sensor is about 30,000 to 50,000 miles (50,000 to 80,000 km). Heated sensor lifetime is typically 100,000 miles (160,000 km). Failure of an unheated sensor is usually caused by the buildup of soot on the ceramic element, which lengthens its response time and may cause total loss of ability to sense oxygen. For heated sensors, normal deposits are burned off during operation and failure occurs due to catalyst depletion, similar to the reason a battery stops producing current. The probe then tends to report lean mixture, the ECU enriches the mixture, the exhaust gets rich with carbon monoxide and hydrocarbons, and the mileage worsens.

Leaded gasoline contaminates the oxygen sensors and catalytic converters. Most oxygen sensors are rated for some service life in the presence of leaded gasoline but sensor life will be shortened to as little as 15,000 miles depending on the lead concentration. Lead-damaged sensors typically have their tips discolored light rusty.

Another common cause of premature failure of lambda probes is contamination of fuel with silicones (used in some sealings and greases) or silicates (used as corrosion inhibitors in some antifreezes). In this case, the deposits on the sensor are colored between shiny white and grainy light gray.

Leaks of oil into the engine may cover the probe tip with an oily black deposit, with associated loss of response.

An overly rich mixture causes buildup of black powdery deposit on the probe. This may be caused by failure of the probe itself, or by a problem elsewhere in the fuel rationing system.

Applying an external voltage to the zirconia sensors, e.g. by checking them with some types of ohmmeter, may damage them.

Symptoms of a failing oxygen sensor includes:

Sensor Light on dash indicates problem Increased tailpipe emissions

Increased fuel consumption

Hesitation on acceleration

Stalling

Rough idling

[edit] Diving applications

Main article: electro-galvanic fuel cell

A diving breathing gas oxygen analyser

The diving type of oxygen sensor, which is sometimes called an oxygen analyser or ppO2 meter, is used in scuba diving. They are used to measure the oxygen concentration of breathing gas mixes such as nitrox and trimix.[3] They are also used within the oxygen control mechanisms of closed-circuit rebreathers to keep the partial pressure of oxygen within safe limits.[4] This type of sensor operates by measuring the electricity generated by a small electro-galvanic fuel cell.

[edit] Scientific applications

In marine biology or limnology oxygen measurements are usually done in order to measure respiration of a community or an organism, but have also been used to measure primary production of algae. The traditional way of measuring oxygen concentration in a water sample has been to use wet chemistry techniques e.g. the Winkler titration method. There are however commercially available oxygen sensors that measure the oxygen concentration in liquids with great accuracy. There are two types of oxygen sensors available: electrodes (electrochemical sensors) and optodes (optical sensors).

[edit] Electrodes

A dissolved oxygen meter for laboratory use.

The Clark-type electrode is the most used oxygen sensor for measuring oxygen dissolved in a liquid. The basic principle is that there is a cathode and an anode submersed in an electrolyte. Oxygen enters the sensor through a permeable membrane by diffusion, and is reduced at the cathode, creating a measurable electrical current.

There is a linear relationship between the oxygen concentration and the electrical current. With a two-point calibration (0% and 100% air saturation), it is possible to measure oxygen in the sample.

One drawback to this approach is that oxygen is consumed during the measurement with a rate equal to the diffusion in the sensor. This means that the sensor must be stirred in order to get the correct measurement and avoid stagnant water. With an increasing sensor size, the oxygen consumption increases and so does the stirring sensitivity. In large sensors there tend to also be a

drift in the signal over time due to consumption of the electrolyte. However, Clark-type sensors can be made very small with a tip size of 10 µm. The oxygen consumption of such a microsensor is so small that it is practically insensitive to stirring and can be used in stagnant media such as sediments or inside plant tissue.

[edit] Optodes

An oxygen optode is a sensor based on optical measurement of the oxygen concentration. A chemical film is glued to the tip of an optical cable and the fluorescence properties of this film depend on the oxygen concentration. Fluorescence is at a maximum when there is no oxygen present. When an O2 molecule comes along it collides with the film and this quenches the photoluminescence. In a given oxygen concentration there will be a specific number of O2 molecules colliding with the film at any given time, and the fluorescence properties will be stable.

The signal (fluorescence) to oxygen ratio is not linear, and an optode is most sensitive at low oxygen concentration. That is, the sensitivity decreases as oxygen concentration increases following the Stern-Volmer relationship. The optode sensors can, however, work in the whole region 0% to 100% oxygen saturation in water, and the calibration is done the same way as with the Clark type sensor. No oxygen is consumed and hence the sensor is insensitive to stirring, but the signal will stabilize more quickly if the sensor is stirred after being put in the sample. These type of electrode sensors can be used for insitu and realtime monitoring of Oxygen production in water splitting reactions. The platinized electrodes can accomplish the real time monitoring of Hydrogen production in water splitting device. Calzaferri and his co workers employed this type of electrodes very extensively for photoelectrochemical water splitting research.

PellistorFrom Wikipedia, the free encyclopedia

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A pellistor is a solid-state device[1] used to detect gases which are either combustible or which have a significant difference in thermal conductivity to that of air. The word "pellistor" is a combination of pellet and resistor.

Contents

[hide]

1 Principle 2 History

3 Types

o 3.1 Catalytic

o 3.2 Thermal Conductivity

4 References

[edit] Principle

The detecting element consist of small "pellets" of catalyst loaded ceramic whose resistance changes in the presence of gas.

[edit] History

The pellistor was developed in the early 1960s for use in mining operations as the successor of the flame safety lamp and the canary. It was invented by English scientist Alan Baker.

[edit] Types

[edit] Catalytic

The catalytic pellistor as used in the catalytic bead sensor works by burning the target gas; the heat generated producing a change in the resistance of the detecting element of the sensor proportional to the gas concentration.

[edit] Thermal Conductivity

The thermal conductivity (TC) pellistor works by measuring the change in heat loss (and hence temperature/resistance) of the detecting element in the presence of the target gas.

Glass electrodeFrom Wikipedia, the free encyclopedia

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A glass electrode is a type of ion-selective electrode made of a doped glass membrane that is sensitive to a specific ion. It is an important part of the instrumentation for chemical analysis and physico-chemical studies. In modern practice, widely used membranous ion-selective electrodes (ISE, including glasses) that are part of a galvanic cell. The electric potential of the electrode system in solution is sensitive to changes in the content of a certain type of ions, which is reflected in the dependence of the electromotive force (EMF) of galvanic element concentrations of these ions.

Contents

[hide]

1 History 2 Applications

3 Types

4 Interfering ions

o 4.1 Metallic function of GE

5 Range of a pH glass electrode

6 Construction

7 Storage

8 See also

9 References

10 External links

[edit] History

Pioneering publication about GE (1909)

Already the first studies of glass electrodes (GE) found different sensitivities of different glasses to change of the medium's acidity (pH), due to the effect of alkali metal ions.

1906 — M. Cramer determined that the electric potential that arises between parts of the fluid, located on opposite sides of the glass membrane is proportional to the concentration of acid (hydrogen ion concentration) [1].

1909 — S. P. L. Sørensen introduced the concept of pH.

1909 — F. Haber and Z. Klemensiewicz publicized on January 28, 1909 results of their research on the glass electrode in The Society of Chemistry in Karlsruhe (first publication — The Journal of Physical Chemistry by W. Ostwald and J. H. van 't Hoff) — 1909) [2]

1922 — W. S. Hughes proved that the alkali-silicate GE are similar to hydrogen electrode, reversible with respect to H+ [3].

[edit] Applications

Glass electrodes are commonly used for pH measurements. As well, there are specialized ion sensitive glass electrodes used for determination of concentration of lithium, sodium, ammonium, and other ions. Glass electrodes have been utilized in a wide range of applications — from pure research, control of industrial processes, to analyze foods, cosmetics and comparison of indicators of the environment and environmental regulations: a microelectrode measurements of membrane electrical potential of a biological cell, analysis of soil acidity, etc.

[edit] Types

Almost all commercial electrodes respond to single charged ions, like H+, Na+, Ag+. The most common glass electrode is the pH-electrode. Only a few chalcogenide glass electrodes are sensitive to double-charged ions, like Pb2+, Cd2+ and some others.

There are two main glass-forming systems:

silicate matrix based on molecular network of silicon dioxide (SiO2) with additions of other metal oxides, such as Na, K, Li, Al, B, Ca, etc.

chalcogenide matrix based on molecular network of AsS, AsSe, AsTe.

[edit] Interfering ions

A silver chloride reference electrode (left) and glass pH electrode (right)

Because of the ion-exchange nature of the glass membrane, it is possible for some other ions to concurrently interact with ion-exchange centers of the glass and to distort the linear dependence of the measured electrode potential on pH or other electrode function. In some cases it is possible to change the electrode function from one ion to another. For example, some silicate pNa electrodes can be changed to pAg function by soaking in a silver salt solution.

Interference effects are commonly described the semiempirical Nicolsky-Eisenman equation (also known as Nikolsky-Eisenman equation)[4], an extension to the Nernst equation. It is given by

where E is the emf, E0 the standard electrode potential, z the ionic valency including the sign, a the activity, i the ion of interest, j the interfering ions and kij is the selectivity coefficient. The smaller the selectivity coefficient, the less is the interference by j.

To see the interfering effect of Na+ to a pH-electrode:

[edit] Metallic function of GE

Before the 1950's, there was no explanation for some important aspects of the behavior of glass electrodes (GE) and the factual reversibility of this behavior. Some authors have denied the existence of a particular function at GE in such solutions where they do not behave fully as hydrogen electrode, denying the phenomena of these functions, which were attributed by researchers as an incorrect interpretation of the structural changes in the surface layers of glass [5]; it is mistakenly was attributed the changes of EMF element to change the capacity of GE, and therefore received too large values of pH. [6].

George Eisenman wrote in his retrospective review [7]:

The pioneering studies of Lengiel and Blum were extended by others, who were primarily interested in the existence of Na+ sensitivity per se (i.e., the Na+ selectivity relative to H+ only) and in establishing whether or not the electrodes were reversible in the thermodynamic sense. A review of this work is given by Shultz, whose studies and those of Nicolskii and Tolmacheva are noteworthy. In fact, Shultz was the first to demonstrate, by direct comparison with a sodium amalgam electrode, that glasses behave as reversible electrodes for Na+ at neutral and alkaline pH.

In 1951 Mikhail Schultz, first proved rigorously the thermodynamical reversibility of the Na-function of different glasses in different pH ranges (later the functions for other metal ions) that confirmed the validity of one of the key hypotheses of ion exchange theory, now the Nikolsky-Shultz-Eisenman [6][8] thermodynamic ion-exchange theory of GE.

This fact is important because ion-exchange theory «began to work» after a thermodynamically rigorous experimental confirmation of metallic function only. Before, it could be called only as a hypothesis (an epistemological). This opened the way for industrial technology GE, forming ionometry with them, later with membrane electrodes. In the context of «generalized» theory of glass electrodes, Shultz has created a framework for interpreting a mechanism of the influence of diffusion processes in glasses and resin in their electrode properties, giving new quantitative relationship, which take into account the dynamic and energetic characteristics of ion exchangers. Schulz introduced a thermodynamic consideration of processes in the membranes. Considering the different abilities of the dissociation of ionogenic groups of the glasses, his theory allows a rigorous analytical way to connect the electrode properties of glasses and ion-exchange resins with their chemical characteristics [9].

[edit] Range of a pH glass electrode

The pH range at constant concentration can be divided into 3 parts:

Scheme of the typical dependence E-pH for ion-selective electrode

Complete realization of general electrode function, where dependence of potential on pH has linear behavior and within which such electrode really works as ion-selective electrode for pH.

Alkali error range - at low concentration of hydrogen ions (high values of pH) contributions of interfering alkali metals (like Li, Na, K) are comparable with the one of hydrogen ions. In this situation dependence of the potential on pH become non-linear.

The effect is usually noticeable at pH > 12, and concentrations of lithium or sodium ions of 0.1 moles per litre or more. Potassium ions usually cause less error than sodium ions.

Acidic error range - at very high concentration of hydrogen ions (low values of pH) the dependence of the electrode on pH becomes non-linear and the influence of the anions in the solution also becomes noticeable. These effects usually become noticeable at pH < 1.

There are different types of pH glass electrode, some of them have improved characteristics for working in alkaline or acidic media. But almost all electrodes have sufficient properties for working in the most popular pH range from pH=2 to pH=12. Special electrodes should be used only for working in aggressive conditions.

Most of text written above is also correct for any ion-exchange electrodes.

[edit] Construction

Scheme of typical pH glass electrode

A typical modern pH probe is a combination electrode, which combines both the glass and reference electrodes into one body. The combination electrode consists of the following parts (see the drawing):

1. a sensing part of electrode, a bulb made from a specific glass2. sometimes the electrode contains a small amount of AgCl precipitate inside the glass electrode

3. internal solution, usually 0.1 mol/L HCl for pH electrodes or 0.1 mol/L MeCl for pMe electrodes

4. internal electrode, usually silver chloride electrode or calomel electrode

5. body of electrode, made from non-conductive glass or plastics.

6. reference electrode, usually the same type as 4

7. junction with studied solution, usually made from ceramics or capillary with asbestos or quartz fiber.

The bottom of a pH electrode balloons out into a round thin glass bulb. The pH electrode is best thought of as a tube within a tube. The inside most tube (the inner tube) contains an unchanging saturated KCl and a 0.1 mol/L HCl solution. Also inside the inner tube is the cathode terminus of the reference probe. The anodic terminus wraps itself around the outside of the inner tube and ends with the same sort of reference probe as was on the inside of the inner tube. Both the inner tube and the outer tube contain a reference solution but only the outer tube has contact with the solution on the outside of the pH probe by way of a porous plug that serves as a salt bridge.

o The details of this section describe the functioning of two separate types of glass electrodes as one unit. It needs clarification.

This device is essentially a galvanic cell that can be represented as:

Ag | AgCl | KCl solution || glass membrane || test solution || ceramic junction || KCl solution | AgCl | Ag

The measuring part of the electrode, the glass bulb on the bottom, is coated both inside and out with a ~10 nm layer of a hydrated gel. These two layers are separated by a layer of dry glass. The silica glass structure (that is, the conformation of its atomic structure) is shaped in such a way that it allows Na + ions some mobility. The metal cations (Na+) in the hydrated gel diffuse out of the glass and into solution while H+ from solution can diffuse into the hydrated gel. It is the hydrated gel, which makes the pH electrode an ion selective electrode.

H+ does not cross through the glass membrane of the pH electrode, it is the Na+ which crosses and allows for a change in free energy. When an ion diffuses from a region of activity to another region of activity, there is a free energy change and this is what the pH meter actually measures. The hydrated gel membrane is connected by Na+ transport and thus the concentration of H+ on the outside of the membrane is 'relayed' to the inside of the membrane by Na+.

All glass pH electrodes have extremely high electric resistance from 50 to 500 MΩ. Therefore, the glass electrode can be used only with a high input-impedance measuring device like a pH meter, or, more generically, a high input-impedance voltmeter which is called an electrometer.

[edit] Storage

Between measurements any glass and membrane electrodes should be kept in the solution of its own ion (Ex. pH glass electrode should be kept in 0.1 mol/L HCl or 0.1 mol/L H2SO4). It is necessary to prevent the glass membrane from drying out.

Potentiometric sensorFrom Wikipedia, the free encyclopedia

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A potentiometric sensor is a type of chemical sensor that may be used to determine the analytical concentration of some components of the analyte gas or solution. These sensors measure the electrical potential of an electrode when no current is flowing.

Contents

[hide]

1 Principle 2 Classification of sensors

3 See also

4 References

[edit] Principle

The signal is measured as the potential difference (voltage) between the working electrode and the reference electrode. The working electrode's potential must depend on the concentration of the analyte in the gas or solution phase. The reference electrode is needed to provide a defined reference potential.

[edit] Classification of sensors

Potentiometric solid state gas sensors have been generally classified into three broad groups.

Type I sensors have an electrolyte containing mobile ions of the chemical species in the gas phase that it is monitoring. The commercial product, YSZ oxygen sensor,[1] is an example of type I.

Type II sensors do not have mobile ions of the chemical species to be sensed, but an ion related to the target gas can diffuse in the solid electrolyte to allow equilibration with the atmosphere. Therefore, type I and type II sensors have the same design with gas electrodes combined with metal and an electrolyte where oxidized or reduced ions can be electrochemically equilibrated through the electrochemical cell. In the third type of electrochemical sensor, auxiliary phases are added to the electrodes to enhance the selectivity and stability.

Type III sensors make the electrode concept even more confusing. With respect to the design of a solid state sensor, the auxiliary phase looks as part of the electrode. But it cannot be an electrode because auxiliary phase materials are not generally good electrical conductor. In spite of this confusion, type III design offers more feasibility in terms of designing various sensors with different auxiliary materials and electrolytes.

Working redox electrode

From Wikipedia, the free encyclopedia

(Redirected from Redox electrode)

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A gold disk working electrode with a Teflon shroud insulating the disk.

The working electrode, is the electrode in an electrochemical system on which the reaction of interest is occurring.[1][2][3] The working electrode is often used in conjunction with an auxiliary electrode, and a reference electrode in a three electrode system. Depending on whether the reaction on the electrode is a reduction or an oxidation, the working electrode can be referred to as either cathodic or anodic. Common working electrodes can consist of inert metals such as gold, silver or platinum, to inert carbon such as glassy carbon or pyrolytic carbon, and mercury drop and film electrodes.

Contents

[hide]

1 Special types of working electrodes 2 See also

3 References

4 External links

[edit] Special types of working electrodes

Ultramicroelectrode (UME) Rotating disk electrode (RDE)

Rotating ring-disk electrode (RRDE)

Hanging mercury drop electrode (HMDE)

Dropping mercury electrode (DME)

Smoke detectorFrom Wikipedia, the free encyclopedia

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A smoke detector is a device that detects smoke, typically as an indicator of fire. Commercial, industrial, and mass residential devices issue a signal to a fire alarm system, while household detectors, known as smoke alarms, generally issue a local audible and/or visual alarm from the detector itself.

Smoke detectors are typically housed in a disk-shaped plastic enclosure about 150 millimetres (6 in) in diameter and 25 millimetres (1 in) thick, but the shape can vary by manufacturer or product line. Most smoke detectors work either by optical detection (photoelectric) or by physical process (ionization), while others use both detection methods to increase sensitivity to smoke. Smoke detectors in large commercial, industrial, and residential buildings are usually powered by a central fire alarm system, which is powered by the building power with a battery backup. However, in many single family detached and smaller multiple family housings, a smoke alarm is often powered only by a single disposable battery.

Contents

[hide]

1 History 2 Design

o 2.1 Optical

o 2.2 Ionization

o 2.3 Air-sampling

o 2.4 Carbon monoxide and carbon dioxide detection

o 2.5 Performance differences

3 Commercial smoke detectors

o 3.1 Conventional

o 3.2 Addressable

4 Standalone smoke alarms

o 4.1 Batteries

o 4.2 Reliability

o 4.3 Installation and placement

5 References

6 External links

[edit] History

The first automatic electric fire alarm was invented in 1890 by Francis Robbins Upton (US patent no. 436,961). Upton was an associate of Thomas Edison, but there is no evidence that Edison contributed to this project.

In the late 1930s the Swiss physicist Walter Jaeger tried to invent a sensor for poison gas. He expected that gas entering the sensor would bind to ionized air molecules and thereby alter an electric current in a circuit in the instrument. His device failed: small concentrations of gas had no effect on the sensor's conductivity. Frustrated, Jaeger lit a cigarette—and was soon surprised to notice that a meter on the instrument had registered a drop in current. Smoke particles had apparently done what poison gas could not. Jaeger's experiment was one of the advances that paved the way for the modern smoke detector.

It was 30 years, however, before progress in nuclear chemistry and solid-state electronics made a cheap sensor possible. While home smoke detectors were available during most of the 1960s, the price of these devices was rather high. Before that, alarms were so expensive that only major businesses and theaters could afford them.

The first truly affordable home smoke detector was invented by Duane D. Pearsall in 1965, featuring an individual battery powered unit that could be easily installed and replaced. The first units for mass production came from Duane Pearsall’s company, Statitrol Corporation, in Lakewood, Colorado.[1][2]

These first units were made from strong fire resistant steel and shaped much like a bee's hive. The battery was a rechargeable specialized unit created by Gates Energy. The need for a quick replace battery didn't take long to show itself and the rechargeable was replaced with a pair of AA batteries along with a plastic shell encasing the detector. The small assembly line sent close to 500 units per day before Statitrol sold its invention to Emerson Electric in 1980 and Sears’s retailers picked up full distribution of the 'now required in every home' smoke detector.

The first commercial smoke detectors came to market in 1969. Today they are installed in 93% of US homes and 85% of UK homes. However it is estimated that any given time over 30% of these alarms don't work, as users remove the batteries, or forget to replace them.

Although commonly attributed to NASA, smoke detectors were not invented as a result of the space program, though a variant with adjustable sensitivity was developed for Skylab.[3]

[edit] Design

[edit] Optical

Optical Smoke Detector with the cover removed.

Optical Smoke Detector1: optical chamber2: cover3: case moulding4: photodiode (detector)5: infrared LED

Inside a basic ionization smoke detector. The black, round structure at the right is the ionization chamber. The white, round structure at the upper left is the piezoelectric buzzer that produces the alarm sound.

An optical detector is a light sensor. When used as a smoke detector, it includes a light source (incandescent bulb or infrared LED), a lens to collimate the light into a beam, and a photodiode or other photoelectric sensor at an angle to the beam as a light detector. In the absence of smoke, the light passes in front of the detector in a straight line. When smoke enters the optical chamber across the path of the light beam, some light is scattered by the smoke particles, directing it at the sensor and thus triggering the alarm.

Also seen in large rooms, such as a gymnasium or an auditorium, are devices to detect a projected beam. A unit on the wall sends out a beam, which is either received by a receiver or reflected back via a mirror. When the beam is less visible to the "eye" of the sensor, it sends an alarm signal to the fire alarm control panel.

Optical smoke detectors are quick in detecting particulate (smoke) generated by smoldering (cool, smoky) fires. Many independent tests indicate that optical smoke detectors typically detect particulates (smoke) from hot, flaming fires approximately 30 seconds later than ionization smoke alarms.[dubious – discuss][citation needed]

They are less sensitive to false alarms from steam or cooking fumes generated in kitchen or steam from the bathroom than are ionization smoke alarms. For the aforementioned reason, they are often referred to as 'toast proof' smoke alarms.[citation needed]

[edit] Ionization

This type of detector is cheaper than the optical detector; however, it is sometimes rejected because it is more prone to false (nuisance) alarms than photoelectric smoke detectors [4][5]. It can detect particles of smoke that are too small to be visible. It includes about 37 kBq or 1 µCi of radioactive americium-241 (241Am), corresponding to about 0.3 µg of the isotope.[6][7] The radiation passes through an ionization chamber, an air-filled space between two electrodes, and permits a small, constant current between the electrodes. Any smoke that enters the chamber

absorbs the alpha particles, which reduces the ionization and interrupts this current, setting off the alarm.

241 Am , an alpha emitter, has a half-life of 432 years. This means that it does not have to be replaced during the useful life of the detector, and also makes it safe for people at home, since it is only slightly radioactive. Alpha radiation, as opposed to beta and gamma, is used for two additional reasons: Alpha particles have high ionization, so sufficient air particles will be ionized for the current to exist, and they have low penetrative power, meaning they will be stopped by the plastic of the smoke detector and/or the air. About one percent of the emitted radioactive energy of 241Am is gamma radiation.

[edit] Air-sampling

An air-sampling smoke detector is capable of detecting microscopic particles of smoke. Most air-sampling detectors are aspirating smoke detectors, which work by actively drawing air through a network of small-bore pipes laid out above or below a ceiling in parallel runs covering a protected area. Small holes drilled into each pipe form a matrix of holes (sampling points), providing an even distribution across the pipe network. Air samples are drawn past a sensitive optical device, often a solid-state laser, tuned to detect the extremely small particles of combustion. Air-sampling detectors may be used to trigger an automatic fire response, such as a gaseous fire suppression system, in high-value or mission-critical areas, such as archives or computer server rooms.

Most air-sampling smoke detection systems are capable of a higher sensitivity than spot type smoke detectors and provide multiple levels of alarm threshold, such as Alert, Action, Fire 1 and Fire 2. Thresholds may be set at levels across a wide range of smoke levels. This provides earlier notification of a developing fire than spot type smoke detection, allowing manual intervention or activation of automatic suppression systems before a fire has developed beyond the smoldering stage, thereby increasing the time available for evacuation and minimizing fire damage.

[edit] Carbon monoxide and carbon dioxide detection

Some smoke alarms use a carbon dioxide sensor or carbon monoxide sensor in order to detect extremely dangerous products of combustion.[8][9] However, not all smoke detectors that are advertised with such gas sensors are actually able to warn of poisonous levels of those gases in the absence of a fire.

[edit] Performance differences

Optical or "toast-proof" smoke detectors are generally quicker in detecting particulate (smoke) generated by smoldering (cool, smokey) fires. Ionization smoke detectors are generally quicker in detecting particulate (smoke) generated by flaming (hot) fires.[10]

According to fire tests conformant to EN 54, normally the CO2 cloud from smoke can be detected before particulate.[9]

Obscuration is a unit of measurement that has become the standard definition of smoke detector sensitivity. Obscuration is the effect that smoke has on reducing visibility. Higher concentrations of smoke result in higher obscuration levels, lowering visibility.

Typical smoke detector obscuration ratings [11]

Type of Detector Obscuration Level

Ionization 2.6-5.0% obs/m (0.8-1.5% obs/ft)

Photoelectric 6.5-13.0% obs/m (2-4% obs/ft)

Beam 3% obs/m (0.9% obs/ft)[citation needed]

Aspirating 0.005-20.5% obs/m (0.0015-6.25% obs/ft)

Laser 0.06-6.41% obs/m (0.02-2.0% obs/ft) [12]

[edit] Commercial smoke detectors

This section requires expansion.

An integrated locking mechanism for commercial building doors. Inside an enclosure are a locking device, smoke detector and power supply.

Commercial smoke detectors are either conventional or analog addressable, and are wired up to security monitoring systems or fire alarm control panels (FACP). These are the most common type of detector, and usually cost a lot more than a household smoke alarms. They exist in most commercial and industrial facilities, such as high rises, ships and trains. These detectors don't need to have built in alarms, as alarm systems can be controlled by the connected FACP, which will set off relevant alarms, and can also implement complex functions such as a staged evacuation.

[edit] Conventional

The word Conventional is slang used in to distinguish the method used to communicate with the control unit from that used by addressable detectors whose methods were unconventional at the time of their introduction. So called “Conventional Detectors” cannot be individually identified by the control unit and resemble an electrical switch in their information capacity. These detectors are connected in parallel to the signaling path or (initiating device circuit) so that the current flow is monitored to indicate a closure of the circuit path by any connected detector when smoke or other similar environmental stimulus sufficiently influences any detector. The resulting increase in current flow is interpreted and processed by the control unit as a confirmation of the presence of smoke and a fire alarm signal is generated.

[edit] Addressable

This type of installation gives each detector on a system an individual number, or address. Thus, addressable detectors allow an FACP, and therefore fire fighters, to know the exact location of an alarm where the address is indicated on a diagram.

Analog addressable detectors provide information about the amount of smoke in their detection area, so that the FACP can decide itself, if there is an alarm condition in that area (possibly considering day/night time and the readings of surrounding areas). These are usually more expensive than autonomous deciding detectors.[13]

[edit] Standalone smoke alarms

The main function of a standalone smoke alarm is to alert persons at risk. Several methods are used and documented in industry specifications published by Underwriters Laboratories [14] Alerting methods include:

Audible tones o usually around 3200 Hz due to component constraints (Audio advancements for persons

with hearing impairments have been made; see External links)

o 85 dB A at 10 feet

Spoken voice alert

Visual strobe lights

o 110 candela output

Tactile stimulation, e.g., bed or pillow shaker (No standards exist as of 2008 for tactile stimulation alarm devices.)

Some models have a hush or temporary silence feature that allows silencing without removing the battery. This is especially useful in locations where false alarms can be relatively common (i.e. due to "toast burning") or users could remove the battery permanently to avoid the annoyance of false alarms, but removing the battery permanently is strongly discouraged.

While current technology is very effective at detecting smoke and fire conditions, the deaf and hard of hearing community has raised concerns about the effectiveness of the alerting function in awakening sleeping individuals in certain high risk groups such as the elderly, those with hearing loss and those who are intoxicated.[15] Between 2005 and 2007, research sponsored by the United States' National Fire Protection Association (NFPA) has focused on understanding the cause of a higher number of deaths seen in such high risk groups. Initial research into the effectiveness of the various alerting methods is sparse. Research findings suggest that a low frequency (520 Hz) square wave output is significantly more effective at awakening high risk individuals. Wireless Wi-Safe smoke and carbon monoxide detectors linked to alert mechanisms such as vibrating pillow pads, strobes and remote warning handsets have been found to support the groups above.[16]

[edit] Batteries

Photoelectric smoke detector equipped with strobe light for the hearing impaired

Most residential smoke detectors run on 9-volt alkaline or carbon-zinc batteries. When these batteries run down, the smoke detector becomes inactive. Most smoke detectors will signal a low-battery condition. The alarm may chirp at intervals if the battery is low, though if there is more than one unit within earshot, it can be hard to locate. It is common, however, for houses to have smoke detectors with dead batteries. It is estimated, in the UK, that over 30% of smoke alarms may have dead or removed batteries. As a result, public information campaigns have been

created to remind people to change smoke detector batteries regularly. In Australia, for example, it is advertised that all smoke alarm batteries should be replaced on the first day of April every year. In regions using daylight saving time, these campaigns may suggest that people change their batteries when they change their clocks or on a birthday.

Some detectors are also being sold with a lithium battery that can run for about 7 to 10 years, though this might actually make it less likely for people to change batteries, since their replacement is needed so infrequently. By that time, the whole detector may need to be replaced. Though relatively expensive, user-replaceable 9-volt lithium batteries are also available.

Common NiMH and NiCd rechargeable batteries have a high self-discharge rate, making them unsuitable for use in smoke detectors. This is true even though they may provide much more power than alkaline batteries if used soon after charging, such as in a portable stereo. Also, a problem with rechargeable batteries is a rapid voltage drop at the end of their useful charge. This is of concern in devices such as smoke detectors, since the battery may transition from "charged" to "dead" so quickly that the low-battery warning period from the detector is either so brief as to go unnoticed, or may not occur at all.

The NFPA, recommends that home-owners replace smoke detector batteries with a new battery at least once per year, when it starts chirping (a signal that its charge is low), or when it fails a test, which the NFPA recommends to be carried out at least once per month by pressing the "test" button on the alarm.[17]

[edit] Reliability

In 2004, NIST issued a comprehensive report [5]. The report concludes, among other things, that "smoke alarms of either the ionization type or the photoelectric type consistently provided time for occupants to escape from most residential fires", and "consistent with prior findings, ionization type alarms provided somewhat better response to flaming fires than photoelectric alarms, and photoelectric alarms provided (often) considerably faster response to smoldering fires than ionization type alarms".

The NFPA strongly recommends the replacement of home smoke alarms every 10 years. Smoke alarms become less reliable with time, primarily due to aging of their electronic components, making them susceptible to nuisance false alarms. In ionization type alarms, decay of the 241Am radioactive source is a negligible factor, as its half-life is far greater than the expected useful life of the alarm unit.

Regular cleaning can prevent false alarms caused by the build up of dust or other objects such as flies, particularly on optical type alarms as they are more susceptible to these factors. A vacuum cleaner can be used to clean ionization and optical detectors externally and internally. However, on commercial ionisation detectors it is not recommended for a lay person to clean internally. To reduce false alarms caused by cooking fumes, use an optical or 'toast proof' alarm near the kitchen. [18]

A jury in the United States District Court for the Northern District of New York decided in 2006 that First Alert and its parent company, BRK Brands, was liable for millions of dollars in damages because the ionization technology in the smoke alarm in the Hackert's house was defective, failing to detect the slow-burning fire and choking smoke that filled the home as the family slept.[19]

[edit] Installation and placement

A 2007 U.S. guide to placing smoke detectors, suggesting that one be placed on every floor of a building, and in each bedroom.

In the United States, most state and local laws regarding the required number and placement of smoke detectors are based upon standards established in Article 72 of the NFPA fire code.

Laws governing the installation of smoke detectors vary depending on the locality. Homeowners with questions or concerns regarding smoke detector placement may contact their local fire marshal or building inspector for assistance. However, some rules and guidelines for existing homes are relatively consistent throughout the developed world. For example, Canada and Australia require a building to have a working smoke detector on every level. The United States[citation needed] requires smoke detectors on every habitable level and within the vicinity of all bedrooms. Habitable levels include attics that are tall enough to allow access.

In new construction, minimum requirements are typically more stringent. All smoke detectors must be hooked directly to the electrical wiring, be interconnected and have a battery backup. In addition, smoke detectors are required either inside or outside every bedroom, depending on local codes. Smoke detectors on the outside will detect fires more quickly, assuming the fire does not begin in the bedroom, but the sound of the alarm will be reduced and may not wake some people. Some areas also require smoke detectors in stairways, main hallways and garages.

Wired units with a third "interconnect" wire allow a dozen or more detectors to be connected, so that if one detects smoke, the alarms will sound on all the detectors in the network, improving the chances that occupants will be alerted, even if they are behind closed doors or if the alarm is triggered one or two floors from their location. Wired interconnection may only be practical for use in new construction, especially if the wire needs to be routed in areas that are inaccessible without cutting open walls and ceilings. As of the mid-2000s, development has begun on wirelessly networking smoke alarms, using technologies such as ZigBee, which will allow interconnected alarms to be easily retrofitted in a building without costly wire installations. Some wireless systems using Wi-Safe technology will also detect smoke or carbon monoxide through the detectors, which simultaneously alarm themselves with vibrating pads, strobes and remote warning handsets. As these systems are wireless they can easily be transferred from one property to another.

In the UK the placement of detectors are similar however the installation of smoke alarms in new builds need to comply to the British Standards BS5839 pt6. BS 5839: Pt.6: 2004 recommends that a new-build property consisting of no more than 3 floors (less than 200sqm per floor)) should be fitted with a Grade D, LD2 system. Building Regulations in England,Wales & Scotland recommend that BS 5839: Pt.6 should be followed, but as a minimum a Grade D, LD3 system should be installed. Building Regulations in Northern Ireland require a Grade D, LD2 system to be installed, with smoke alarms fitted in the escape routes and the main living room and a heat alarm in the kitchen, this standard also requires all detectors to have a main supply and a battery back up.

Zinc oxide nanorod sensorFrom Wikipedia, the free encyclopedia

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A zinc oxide nanorod sensor or ZnO nanorod sensor is an electronic device detecting presence of certain gas or liquid molecules (e.g. NO, hydrogen[1][2], etc.) in the ambient atmosphere. The sensor exploits enhanced surface area (and thus surface activity) intrinsic to all nano-sized materials, including ZnO nanorods. Adsorption of molecules on the nanorods can be detected through variation of the nanorods properties, such as electrical conductivity, vibration frequency, mass, etc. The simplest and thus most popular way is to pass electrical current through the nanorods and observe its changes upon gas exposure.

LIST OF ANALYTIC INSTRUMENTS

This page introduces our major analytical instruments.

Analytical Instrument Abbreviation Major Uses

Elemental Analysis

Organic Elemental Analyzer   These instruments analyze products and materials developed in all industrial fields and analyze the purity, components and impurities of materials used in manufacturing processes and production activities. They analyze the compositions of exhaust gases and wastewater generated in manufacturing processes.They are helpful to analyze gases generated by heating.

<Related service fields>Environmental Research ServicesElectronicsMaterial

Flame and Flameless Atomic Absorption Spectrometer

AAS/FL-AAS

Emission Spectrophotometer  

Inductively Coupled Plasma Emission Spectrometer ICP-AES

X-ray Fluorescence Analyzer XRF

Inductively Coupled Plasma Mass Spectrometer ICP-MS

Ion Chromatograph IC

Molecular Absorption Spectrophotometer  

Particle Analyzer  

Total Nitrogen Analyzer TN

Nitrogen and Carbon Analyzer NC

Evaluation ServicesInstrument Sales

Surface Analysis

X-ray Photoelectron Spectrometer XPS/ESCA These instruments analyze surface and interface conditions of electronic materials, metals, ceramics, catalysts and other substances. They are helpful to analyze the foreign particles on substance surfaces and interfaces.

<Related service fields>ElectronicsMaterial Evaluation Services

Auger Electron Spectrometer AES/SAM

Secondary Ion Mass Spectrometer SIMS

Time-of-flight secondary ion mass spectrometer TOF-SIMS

Electron Probe X-ray Microanalyzer EPMA/XMA

Field Emission Scanning Electron Microscope FE-SEM-EDX

Low Level Alpha particle measuring instrument  

Thermal Desorption Mass Spectrometer TDS

Form Observation

Transmission Electron Microscope TEM The surface and interface conditions of substances produced in all industrial fields can be observed directly through these electron microscopes.The inside of substances can be directly observed.

<Related service fields>ElectronicsMaterial Evaluation Services

Scanning Electron Microscope SEM

Atomic Force Microscope AFM

Optical Microscope OM

Field Emission Scanning Microscope FE-SEM

Compound Structure Analysis

Organic Mass Spectrometer MS

Time-of-Flight Mass Spectrometer TOF-MS

Nuclear Magnetic Resonance Analyzer NMR

Visible/Ultraviolet Spectrochemical Analyzer UV•VIS

Fourier Transform Infrared Spectrometer FT-IR

Raman Spectrometer RSS

X-ray Diffraction Analyzer XRD

Electron Spin Resonance Analyzer ESR

Fourier Transform Infrared Microspectrometer μ-FT-IR

Scanning Infrared Microprobe Analyzer IRμS/SIRM

Thermal Analysis/Thermal Properties

Thermogravimetric Analyzer TG

Differential Thermal Analyzer DTA

Differential Scanning Calorimeter DSC

Specific Heat Measuring Instrument  

Reaction Heat Measuring Instrument  

Vaporization Heat Measuring Instrument  

Thermal Expansion Coefficient Measuring Instrument  

Thermal Conductivity Measuring Instrument  

Chromatography and Separation Analysis

Gas Chromatography GC

Gas Chromatography-Mass Spectrometer GC-MS

Gas Chromatography-Fourier Transform Infrared Spectrometer

GC-IR

Gas Chromatography-Atomic Emission Detector GC-AED

Liquid Chromatograph LC

Liquid Chromatography-Mass Spectrometer LC-MS

Liquid Chromatography-Tandem Mass Spectrometer LC-MS/MS

Gel Permeation Chromatograph GPC

Thin Layer Chromatograph TLC

Pyrolysis Gas Chromatograph PyGC

Gas Chromatography-Tandem Mass Spectrometer GS-MS/MS

Instruments for Liquid Chromatography-Mass Spectrometer

LC-MS

Gel Permeation Chromatograph - Scattering Method GPC-LALLS

Capillary Electrophoresis CE