COLLEGE OF ENGINEERING THIRUVANANTHAPURAM

S7 N 2010

METROLOGY AND

INSTRUMENTATION KAILAS SREE CHANDRAN

S7 INDUSTRIAL 432

kailassreechandran@yahoo.co.in

S U B M I S S I O N D A T E : 3 0 - 1 0 - 2 0 0 9

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PRINCIPLES OF MEASUREMENT

Today, the techniques of measurement are of immense importance in

most facets of human civilization. Present-day applications of measuring

instruments can be classified into three major areas. The first of these is

their use in regulating trade, and includes instruments which measure

physical quantities such as length, volume and mass in terms of standard

units.

The second area for the application of measuring instruments is in

Monitoring functions. These provide information which enables human beings

to take some prescribed action accordingly. Whilst there are thus many

uses of instrumentation in our normal domestic lives, the majority of

monitoring functions exist to provide the information necessary to allow a

human being to control some industrial operation or process. In a chemical

process for instance, the progress of chemical reactions is indicated by the

measurement of temperatures and pressures at various points, and such

measurements allow the operator to take correct decisions regarding the

electrical supply to heaters, cooling water flows, valve positions, etc. One

other important use of monitoring instruments is in calibrating the

instruments used in the automatic process control systems.

Use as part of automatic control systems forms the third area for the

application of measurement systems. The characteristics of measuring

instruments in such feedback control systems are of fundamental importance

to the quality of control achieved. The accuracy and resolution with which an

output variable of a process is controlled can never be better than the

accuracy and resolution of the measuring instruments used. This is a very

important principle, but one which is often inadequately discussed in many

texts on automatic control systems. Such texts explore the theoretical

aspects of control system design in considerable depth, but fail to give

sufficient emphasis to the fact that all gain and phase margin performance

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calculations, etc., are entirely dependent on the quality of the process

measurements obtained.

Measuring Equipments

A measuring instrument exists to provide information about the

physical value of some variable being measured. In simple cases, an

instrument consists of a single unit which gives an output reading or signal

according to the magnitude of the unknown variable applied to it. However,

in more complex measurement situations, a measuring instrument may

consist of several separate elements. These components might be contained

within one or more boxes, and the boxes holding individual measurement

elements might be either close together or physically separate. Because of

the modular nature of the elements within it, a measuring instrument is

commonly referred to as a measurement system, and this term is used

extensively to emphasize this modular nature.

Common to any measuring instrument is the primary transducer: this

gives an output which is a function of the measurand (the input applied to

it). For most but not all transducers, this function is at least approximately

linear. Some examples of primary transducers are a liquid-in-glass

thermometer, a thermocouple and a strain gauge. In the case of a mercury-

in-glass thermometer, the output reading is given in terms of the level of the

mercury, and so this particular primary transducer is also a complete

measurement system in itself. In general, however, the primary transducer

is only part of a measurement system. The types of primary transducers

available for measuring a wide range of physical quantities widely available.

The output variable of a primary transducer is often in an inconvenient

form and has to be converted to a more convenient one. For instance, the

displacement-measuring strain gauge has an output in the form of a varying

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resistance. This is converted to a change in voltage by a bridge circuit, which

is a typical example of the variable conversion element.

Signal processing elements exist to improve the quality of the output

of a measurement system in some way. A very common type of signal

processing element is the electronic amplifier, which amplifies the output of

the primary transducer or variable conversion element, thus improving the

sensitivity and resolution of measurement. This element of a measuring

system is particularly important where the primary transducer has a low

output. For example, thermocouples have a typical output of only a few mill

volts. Other types of signal processing element are those which filter out

induced noise and remove mean levels, etc.

The observation or application point of the output of a measurement

system is often some physical distance away from the site of the primary

transducer which is measuring a physical quantity, and some mechanism of

transmitting the measured signal between these points is necessary.

Sometimes, this separation is made solely for purposes of convenience, but

more often it follows from the physical inaccessibility or environmental

unsuitability of the site of the primary transducer for mounting the signal

presentation/recording unit. The signal transmission element has

traditionally consisted of single- or multi-cored cable, which is often

screened to minimize signal corruption by induced electrical noise. Now,

optical fiber cables are being used in ever increasing numbers in modem

installations, in part because of their low transmission loss and

imperviousness to the effects of electrical and magnetic fields.

The final element in a measurement system is the point where the

measured signal is utilized. In some cases, this element is omitted

altogether because the measurement is used as part of an automatic control

scheme, and the transmitted signal is fed directly into the control system. In

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other cases, this element takes the form of either a signal presentation unit

or a signal recording unit. These take many forms according to the

requirements of the particular measurement application.

PRECISION

Precision is how close the measured values are to each other. The

precision of a measurement is the size of the unit you use to make a

measurement. The smaller the unit, the more precise the measurement.

Precision depends on the unit used to obtain a measure. Consider measures

of time, such as 12 seconds and 12 days. A measurement of 12 seconds

implies a time between11.5 and 12.5 seconds. This measurement is precise

to the nearest second, with a maximum potential error of 0.5 seconds. A

time of 12 days is far less precise. Twelve days suggests a time between

11.5 and 12.5 days, yielding a potential error of 0.5 days, or 43,200

seconds! Because the potential error is greater, the measure is less precise.

Thus, as the length of the unit increases, the measure becomes less precise.

The number of decimal places in a measurement also affects precision.

A time of 12.1 seconds is more precise than a time of 12 seconds; it implies

a measure precise to the nearest tenth of a second. The potential error

in12.1 seconds is 0.05 seconds, compared with a potential error of 0.5

seconds with a measure of 12 seconds.

Although students learn that adding zeros after a decimal point is

acceptable, doing so can be misleading. The measures of 12 seconds and

12.0 seconds imply a difference in precision. The first figure is measured to

the nearest second—a potential error of 0.5 seconds. The second figure is

measured to the nearest tenth—a potential error of 0.05 seconds. Therefore,

a measure of 12.0 seconds is more precise than a measure of 12 seconds.

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Differing levels of precision can cause problems with arithmetic

operations. Suppose one wishes to add 12.1 seconds and 14.47 seconds.

The sum, 26.57 seconds, is misleading. The first time is between 12.05

seconds and12.15 seconds, whereas the second is between 14.465 and

14.475 seconds. Consequently, the sum is between 26.515 seconds and

26.625 seconds. A report of 26.57 seconds suggests more precision than the

actual result possesses.

The generally accepted practice is to report a sum or difference to the

same precision as the least precise measure. Thus, the result in the

preceding example should be reported to the nearest tenth of a second; that

is, rounding the sum to 26.6 seconds. Even so, the result may not be as

precise as is thought. If the total is actually closer to 26.515 seconds, the

sum to the nearest tenth is 26.5 seconds. Nevertheless, this practice usually

provides acceptable results.

Measurements in industrial settings such as a rubber manufacturing plant must be both accurate and

precise. Here a technician is measuring tire pressure.

Multiplying and dividing measures can create a different problem.

Suppose one wishes to calculate the area of a rectangle that measures 3.7

centimeters (cm) by 5.6 cm. Multiplication yields an area of 20.72 square

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centimeters. However, because the first measure is between 3.65 and 3.75

cm, and the second measure is between 5.55 and 5.65 cm, the area is

somewhere between 20.2575 and 21.1875 square centimeters. Reporting

the result to the nearest hundredth of a square centimeter is misleading.

The accepted practice is to report the result using the fewest number of

significant digits in the original measures. Since both 3.7 and 5.6 have two

significant digits, the result is rounded to two significant digits and an area

of 21 square centimeters is reported. Again, while the result may not even

be this precise, this practice normally produces acceptable results.

ACCURACY

Accuracy is how close a measured value is to the actual (true) value.

The accuracy of a measurement is the difference between your

measurement and the accepted correct answer. The bigger the difference,

the less accurate your measurement. Rather than the absolute error to

which precision refers, accuracy refers to the relative error in a measure. For

example, if one makes a mistake by 5 centimeters in measuring two objects

that are actually 100 and 1,000 cm, respectively, the second measure is

more accurate than the first. The first has an error of 5 percent (5 cm out of

100 cm), whereas the second has an error of only 0.5 percent (5 cm out of

1,000 cm).

Difference between Accuracy and Precision

To illustrate the difference between precision and accuracy, suppose

that a tape measure is used to measure the circumference of two circles—

one small and the other large. Suppose a result of 15 cm for the first circle

and 201 cm for the second circle are obtained. The two measures are

equally precise; both are measures to the nearest centimeter. However,

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their accuracy may be quite different. Suppose the measurements are both

about 0.3 cm too small. The relative errors for these measures are 0.3 cm

out of 15.3 cm (about 1.96 percent) and0.3 cm out of 201.3 cm (about

0.149 percent). The second measurement is more accurate because the

error is smaller when compared with the actual measurement. Consequently,

for any specific measuring tool, one can be equally precise with the

measures. But accuracy is often greater with larger objects than with smaller

ones.

Confusion can arise when using these terms. The tools one uses affect

both the precision and accuracy of one's measurements. Measuring with a

millimeter tape allows greater precision than measuring with an inch tape.

Because the error using the millimeter tape should be less than the inch

tape, accuracy also improves; the error compared with the actual length is

likely to be smaller. Despite this possible confusion and the similarity of the

ideas, it is important that the distinction between precision and accuracy be

understood.

Examples of Precision and Accuracy:

Low Accuracy

High Precision

High Accuracy Low Precision

High Accuracy

High Precision

In Figure 1, the marksman has achieved a uniformity, although it is

inaccurate. This uniformity may have been achieved by using a sighting

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scope, or some sort of stabilizing device. With the knowledge gained by

observation of the results, the marksman can apply a systematic adjustment

(aim lower and to the left of his intended target, or have his equipment

adjusted) to achieve more accurate results in addition to the precision that

his methodology and equipment have already attained. In Figure 2, the

marksman has approached the "truth", although without great precision. It

may be that the marksman will need to change the equipment or

methodology used to obtain the result if a greater degree of precision is

required, as he has reached the limitations associated with his equipment

and methodology. Figure 3 represents results indicating both accuracy and

precision. It differs from Figure 1 in that the marksman has probably made

one of the systematic adjustments that was indicated by his attainment of

precision without accuracy. The degree of precision has not changed greatly,

but its conformity with the "truth" has improved over the results obtained

inFigure 1. If the marksman from Figure 2 determines that his results are

not adequate for the task at hand, he has no choice but to change his

methodology or equipment. He has already performed to the limitations of

these.

Degree of Accuracy

Accuracy depends on the instrument you are measuring with. But as a

general rule: The degree of accuracy is half a unit each side of the unit of

measure

Examples:

If the instrument measures in "1"s then

any value between 6½ and 7½ is

measured as "7"

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If the instrument measures in "2"s then

any value between 7 and 9 is measured

as "8"

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SENSITIVITY OF MEASUREMENT

All calibrations and specifications of an instrument are only valid under

controlled conditions of temperature, pressure, etc. These standard ambient

conditions are usually defined in the instrument specification. As variations

occur in the ambient temperature, etc., certain static instrument

characteristics change, and the sensitivity to disturbance is a measure of the

magnitude of this change. Such environmental changes affect instruments in

two main ways, known as zero drift and sensitivity drift.

Zero drift describes the effect where the zero reading of an instrument

is modified by a change in ambient conditions. Typical units by which zero

drift is measured are volts/°C, in the case of a voltmeter affected by ambient

temperature changes. This is often called the zero drift coefficient related to

temperature changes. If the characteristic of an instrument is sensitive to

several environmental parameters, then it will have several zero drift

coefficients, one for each environmental parameter. The effect of zero drift is

to impose a bias in the instrument output readings; this is normally

removable by recalibration in the usual way.

Sensitivity drift (also known as scale factor drift) defines the amount

by which an instrument's sensitivity of measurement varies as ambient

conditions change. It is quantified by sensitivity drift coefficients which

define how much drift there is for a unit change in each environmental

parameter that the instrument characteristics are sensitive to.

Many components within an instrument are affected by environmental

fluctuations, such as temperature changes: for instance, the modulus of

elasticity of a spring is temperature dependent. Sensitivity drift is measured

in units of the form (angular degree/bar)/°C.

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Sensitivity may be defined as the rate of displacement of the indicating

device of an instrument, with respect to the measured quantity. In other

words, sensitivity of an instrument is the ratio of the scale spacing to the

scale division value. For example, if on a dial indicator, the scale spacing is

1.0mm and the scale division value is 0.01mm, then sensitivity is 100. It is

also called as amplification factor or gearing ratio.

CALIBRATION

Understanding instrument calibration and its proper use is an essential

element in an overall laboratory program. Proper calibration will ensure that

equipment remains within validated performance limits to accurately report

patient results. Calibration is the set of operations that establish, under

specified conditions, the relationship between the values of quantities

indicated by a measuring instrument and the corresponding values realized

by standards.

The result of a calibration permits either the assignment of values of

measurements to the indications or the determination of corrections with

respect to indications.

A calibration may also determine other metrological properties such as

the effect of influence quantities.

The result of a calibration may be recorded in a document, sometimes

called a calibration certificate or a calibration report.

A measuring instrument can be calibrated by comparison with a standard.

An adjustment of the instrument is often carried out after calibration in

order that it provides given indications corresponding to given values of the

quantity measured.

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When the instrument is made to give a null indication corresponding to

a null value of the quantity to be measured, the set of operation is called

zero adjustment .

Calibration is the process of establishing the relationship between a

measuring device and the units of measure. This is done by comparing a

device or the output of an instrument to a standard having known

measurement characteristics. For example the length of a stick can be

calibrated by comparing it to a standard that has a known length. Once the

relationship of the stick to the standard is known the stick is calibrated and

can be used to measure the length of other things.

For many operations the quality of the calibration needs to be known

and is quantified by an uncertainty estimate for the calibration. This is so

important for the scientific community and manufacturing operations that it

has been proposed that an evaluation of the measurement uncertainty was

added as part of the calibration process.

Part of calibration is to zero the measuring device, the process of

establishing that the zero point of the device corresponds to zero on the

relevant scale.

Instrument Calibration

Calibration can be called for:

with a new instrument

when a specified time period is elapsed

when a specified usage (operating hours) has elapsed

when an instrument has had a shock or vibration which potentially

may have put it out of calibration

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whenever observations appear questionable

In non-specialized use, calibration is often regarded as including the

process of adjusting the output or indication on a measurement instrument

to agree with value of the applied standard, within a specified accuracy. For

example, a thermometer could be calibrated so the error of indication or the

correction is determined, and adjusted (e.g. via calibration constants) so

that it shows the true temperature in Celsius at specific points on the scale.

Instrument calibration is one of the primary processes used to

maintain instrument accuracy. Calibration is the process of configuring an

instrument to provide a result for a sample within an acceptable range.

Eliminating or minimizing factors that cause inaccurate measurements is a

fundamental aspect of instrumentation design.

Although the exact procedure may vary from product to product, the

calibration process generally involves using the instrument to test samples

of one or more known values called ―calibrators.‖ The results are used to

establish a relationship between the measurement technique used by the

instrument and the known values. The process in essence ―teaches‖ the

instrument to produce results that are more accurate than those that would

occur otherwise. The instrument can then provide more accurate results

when samples of unknown values are tested in the normal usage of the

product.

Calibrations are performed using only a few calibrators to establish the

correlation at specific points within the instrument’s operating range. While

it might be desirable to use a large number of calibrators to establish the

calibration relationship, or ―curve‖, the time and labor associated with

preparing and testing a large number of calibrators might outweigh the

resulting level of performance. From a practical standpoint, a tradeoff must

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be made between the desired level of product performance and the effort

associated with accomplishing the calibration. The instrument will provide

the best performance when the intermediate points provided in the

manufacturer’s performance specifications are used for calibration; the

specified process essentially eliminates, or ―zeroes out‖, the inherent

instrument error at these points.

Importance of Calibration

The following figures depict how a properly performed calibration can

improve product performance.

If a product were perfect from an accuracy standpoint, then the results

for a series of tests would form for the curve in Figure 8 labeled ―Ideal

Results‖. But what if the test results form the curve labeled ―Actual

Results? Although the curve has been greatly exaggerated for the purpose

of this discussion, there is an error at any point within the operating range

with respect to the Ideal Curve. In addition, the error is not constant over

the operating range.

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Figure 8. Calibrated Graph

Calibrating the product can improve this situation significantly. In

Figure 9, the product is ―taught‖ using the known values of Calibrators 1 and

2 what result it should provide. The process eliminates the errors at these

two points, in effect moving the Actual Results curve closer to the Ideal

Results curve. The Error At Any Point has been reduced to zero at the

calibration points, and the residual error at any other point within the

operating range (exaggerated by the curve) is within the manufacturer’s

published linearity or accuracy specification.

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Figure 9. Uncalibrated Graph

Factors affecting Calibration

Once the benefits of a properly performed calibration are understood,

it becomes evident that care must be taken during the process to prevent

potential error sources from degrading the results. Several factors can occur

during and after a calibration that can affect its result. Among these are:

Using the wrong calibrator values: It is important to closely follow

the instructions for use during the calibration process. Disregarding the

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instructions and selecting the wrong calibrator values will ―teach‖ the

instrument incorrectly, and produce significant errors over the entire

operating range. While many instruments have software diagnostics that

alert the operator if the calibrators are tested in the incorrect order (i.e.

Calibrator 2 before Calibrator 1), the instrument may accept one or more

calibrators of the wrong value without detecting the operator error.

Calibrator formulation tolerance: It is important to use calibrators

that are formulated to tight tolerance specifications by a reputable

manufacturer. There is a tolerance associated with formulating a

calibrator/control due to normal variations in the instrumentation and quality

control processes. This tolerance can affect the mean value obtained when

using the calibrator.

Frequency of Calibration

The simple answer to this question, although not a very helpful one, is

―when it needs it.‖ From a more practical standpoint, daily or periodically

testing the control solutions of known values can provide a quantitative

indication of instrument performance, which can be used to establish a

history. If the controls data indicate that instrument performance is stable,

or is varying randomly well within the acceptable range of values, then there

is no need to recalibrate the instrument. However, if the historical data

indicates a trend toward, or beyond, the acceptable range limits, or if the

instrument displays a short-term pronounced shift, then recalibration is

warranted. Realize also that specific laboratory standard operating

procedures or regulatory requirements may require instrument recalibration

even when no action is warranted from a results standpoint. These

requirements should always take precedence, and the above guidance used

at times when there is uncertainty as to whether instrument recalibration

should be performed to improve accuracy.

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STANDARDS OF MEASUREMENTS

In A.D. 1120 the king of England decreed that the standard of length

in his country would be named the yard and would be precisely equal to the

distance from the tip of his nose to the end of his outstretched arm.

Similarly, the original standard for the foot adopted by the French was the

length of the royal foot of King Louis XIV. This standard prevailed until 1799,

when the legal standard of length in France became the meter, defined as

one ten-millionth the distance from the equator to the North Pole along one

particular longitudinal line that passes through Paris.

Many other systems for measuring length have been developed over

the years, but the advantages of the French system have caused it to prevail

in almost all countries and in scientific circles everywhere. As recently as

1960, the length of the meter was defined as the distance between two lines

on a specific platinum–iridium bar stored under controlled conditions in

France. This standard was abandoned for several reasons, a principal one

being that the limited accuracy with which the separation between the lines

on the bar can be determined does not meet the current requirements of

science and technology. In the 1960s and 1970s, the meter was defined as 1

650 763.73 wavelengths of orange-red light emitted from a krypton-86

lamp. However, in October 1983, the meter (m) was redefined as the

distance traveled by light in vacuum during a time of 1/299 792 458 second.

In effect, this latest definition establishes that the speed of light in vacuum

is precisely 299 792 458 m per second.

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The different types of standards of length are

1. Material Standards

(a) Line Standard – When length is measured as the distance

between centers of two engraved lines.

(b) End Standard – When length is measured as the distance between

to flat parallel faces.

2. Wavelength Standard

The wavelength of a selected orange radiation of Krtypton-86 isotope

was measured and used as the basic unit of length.

Imperial Standard YARD

After the 1 July 1959 deadline, agreed upon in 1958, the US and the

British yard were defined identically, at 0.9144 metres to match the

international yard. Metric equivalents in this article usually assume this

latest official definition. Before this date, the most precise measurement of

the Imperial Standard Yard was 0.914398416 metres.

A YARD (abbreviation: yd) is a unit of length in several different

systems, including English units, Imperial units, and United States

customary units. It is equal to 3 feet or 36 inches, although its length in SI

units varied slightly from system to system. The most commonly used yard

today is the international yard, which is equal to precisely 0.9144 meter.

The yard is used as the standard unit of field-length measurement in

American, Canadian and association football.

There are corresponding units of area and volume, the square yard

and cubic yard respectively, and these are sometimes referred to simply as

"yards" when no ambiguity is possible. For example, an American or

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Canadian concrete mixer marked with a capacity of "11 yards" or "1.5

yards", where cubic yards are obviously referred to.

Figure 10. Standard lengths on the wall of the Royal Observatory, Greenwich, London - 1 yard (3

feet), 2 feet, 1 foot, 6 inches (1/2 foot), and 3 inches. The separation of the inside faces of the

markers is exact at an ambient temperature of 60 °F (16 °C) and a rod of the correct measure,

resting on the pins, will fit snugly between them.

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Figure 11. Imperial standards of length 1876 in Trafalgar Square, London.

International Prototype METRE

The meter is the length of the path travelled by light in vacuum during

a time interval of 1/299 792 458 of a second. International Prototype meter

is defined as the straight line distance, at 0’c between the engraved lines of

a platinum iridium alloy of 1020 mm of total length and having a tresca

cross-section as shown in the figure. The graduations are on the upper

surface of the web, which coincides with the neutral axis of the section. The

sectional shape gives better rigidity for the amount of metal involved and is

therefore economic in use for an expensive metal.

The metre or meter is the basic unit of length in the International

System of Units (SI). Historically, the metre was defined by the French

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Academy of Sciences as the length between two marks on a platinum-

iridium bar, which was designed to represent one ten-millionth of the

distance from the Equator to the North Pole through Paris. In 1983, the

metre was redefined as the distance travelled by light in free space in

1⁄299,792,458 of a second.

The symbol for metre is m. Decimal multiples such as kilometre and

centimetre are indicated by adding SI prefixes to metre.

In the 1870s and in light of modern precision, a series of international

conferences were held to devise new metric standards. The Metre

Convention (Convention du Mètre) of 1875 mandated the establishment of a

permanent International Bureau of Weights and Measures (BIPM: Bureau

International des Poids et Mesures) to be located in Sèvres, France. This new

organisation would preserve the new prototype metre and kilogram

standards when constructed, distribute national metric prototypes, and

maintain comparisons between them and non-metric measurement

standards. The organisation created a new prototype bar in 1889 at the first

General Conference on Weights and Measures (CGPM: Conférence Générale

des Poids et Mesures), establishing the International Prototype Metre as the

distance between two lines on a standard bar composed of an alloy of ninety

percent platinum and ten percent iridium, measured at the melting point of

ice.

The original international prototype of the metre is still kept at the

BIPM under the conditions specified in 1889. A discussion of measurements

of a standard metre bar and the errors encountered in making the

measurements is found in a NIST document

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Figure 12. Historical International Prototype Metre bar, made of an alloy of platinum and

iridium, that was the standard from 1889 to 1960.

Standard wavelength of krypton-86 emission

In 1893, the standard metre was first measured with an

interferometer by Albert A. Michelson, the inventor of the device and an

advocate of using some particular wavelength of light as a standard of

distance. By 1925, interferometry was in regular use at the BIPM. However,

the International Prototype Metre remained the standard until 1960, when

the eleventh CGPM defined the metre in the new SI system as equal to

1,650,763.73 wavelengths of the orange-red emission line in the

electromagnetic spectrum of the krypton-86 atom in a vacuum.

Standard wavelength of helium-neon laser light

To further reduce uncertainty, the seventeenth CGPM in 1983 replaced

the definition of the metre with its current definition, thus fixing the length

of the metre in terms of time and the speed of light:

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The metre is the length of the path travelled by light in vacuum during

a time interval of 1⁄299 792 458 of a second.

This definition effectively fixed the speed of light in a vacuum at

precisely 299,792,458 metres per second. Although the metre is now

defined in terms of time-of-flight, actual laboratory realizations of the metre

are still delineated by counting the required number of wavelengths of light

along the distance. Three major factors limit the accuracy attainable with

laser interferometers:

Uncertainty in vacuum wavelength of the source,

Uncertainty in the refractive index of the medium,

Laser count resolution of the interferometer.

Use of the interferometer to define the metre is based upon the

relation:

where λ is the determined wavelength; c is the speed of light in ideal

vacuum; n is the refractive index of the medium in which the measurement

is made; and f is the frequency of the source. In this way the length is

related to one of the most accurate measurements available: frequency.

An intended byproduct of the 17th CGPM’s definition was that it

enabled scientists to measure the wavelength of their lasers with one-fifth

the uncertainty. To further facilitate reproducibility from lab to lab, the 17th

CGPM also made the iodine-stabilised helium-neon laser ―a recommended

radiation‖ for realising the metre. For purposes of delineating the metre, the

BIPM currently considers the HeNe laser wavelength to be as follows: λHeNe =

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632.99139822 nm with an estimated relative standard uncertainty (U) of

2.5×10−11.This uncertainty is currently the limiting factor in laboratory

realisations of the metre as it is several orders of magnitude poorer than

that of the second (U = 5×10−16). Consequently, a practical realisation of

the metre is usually delineated (not defined) today in labs as

1,579,800.298728(39) wavelengths of helium-neon laser light in a vacuum.

Table 1. Line and End Standards and differentiate between them.

LINE STANDARDS – When length is measured as the distance between

centers of two engraved lines, it is called Line Standards. Both material

Standards, yard and metre are line standards

E.g. Scale, Rulers, Imperial Standard Yard.

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Characteristics of Line Standards :

(i) Scale can be accurately embalmed, but the engraved lines posses

thickness and it is not possible to accurately measure

(ii) Scale is used over a wide range

(iii) Scale markings are subjected to wear. However the ends are

subjected to wear and this leads to undersize measurements

(iv) Scale does not posses built in datum. Therefore it is not possible to

align the scale with the axis of measurement

(v) Scales are subjected to parallax errors

(vi) Assistance of magnifying glass or microscope is required.

END STANDARDS – When length is expressed as the distance between

centers of two flat parallel faces, it is called End Standards. Slip Gauges, End

Bars, Ends of micrometer Anvils.

Characteristics of End Standards

(i) Highly accurate and used for measurement of closed tolerances in

precision engineering as well as standard laboratories, tool rooms,

inspection departments.

(ii) They require more time for measurement and measure only one

dimension.

(iii) They wear at their measuring faces

(iv) They are not subjected to parallax error.

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Differentiate between Line and End Standards

Sl no Characteristics Line Standard End Standard

1. Principle

Length is expressed as

distance between 2 lines

Length is expressed as

distance between 2 ends

2. Accuracy Ltd. To ± 0.2mm.

Highly accurate of closed

tolerances to ±0.001mm

3. Ease Quick and easy

Time consuming and requires

skill

4. Effect of wear Wear at only the ends wear at measuring surfaces

5. Allignment

Cannot be easily

aligned easily aligned

6. Cost low cost high cost

7. Parallax Effect

Subjected to parallax

effect not subjected to parallax effect

Wavelength standards and its advantages

A major drawback wit the material standards, that their length

changes with time. Secondly, considerable difficulty is expressed while

comparing the sizes of the gauges by using material standards.

Jacques Babinet suggested that wave length of a monochromatic light

can be used as a natural and invariable unit of length. 7th general

Conference of Weights and Measures approved in 1927, approved the

definition of standard of length relative to meter.

Accurately known wavelengths of spectral radiation emitted from

specified sources that are used to measure the wavelengths of other spectra.

In the past, the radiation from the standard source and the source under

study were superimposed on the slit of a spectrometer (prism or grating)

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and then the unknown wavelengths could be determined from the standard

wavelengths by using interpolation. This technique has evolved into the

modern computer-controlled photoelectric recording spectrometer. Accuracy

of many more orders of magnitude can be obtained by the use of

interferometric techniques, of which Fabry-Perot and Michelson

interferometers are two of the most common. See also Interferometry;

Spectroscopy.

The newest definition of the meter is in terms of the second. The

wavelength of radiation from the cesium atomic clock is not used to realize

length because diffraction problems at this wavelength are severe. Instead,

lasers at shorter wavelengths whose frequencies have been measured are

used. Frequency measurements can now be made even into the visible

spectral region with great accuracy. Hence, when the 1983 Conférence

Général des Poids et Mesures redefined the meter, it also gave a list of very

accurate wavelengths of selected stabilized lasers which may be used as

wavelength standards. Nearly ten times better accuracy can be achieved by

using these wavelengths than by using the radiation from the krypton lamp

which provided the previous standard. See also Frequency measurement;

Hyperfine structure; Laser; Laser spectroscopy; Length; Molecular structure

and spectra; Physical measurement.

The progress in laser frequency measurements since 1974 has

established wavelength standards throughout the infrared spectral region.

This has been accomplished with the accurate frequency measurement of

rotational-vibrational transitions of selected molecules. The OCS molecule is

used in the 5-micrometer spectral region. At 9–10 μm, the carbon dioxide

(CO2) laser itself with over 300 accurately known lines is used. From 10 to

100 μm, rotational transitions of various molecules are used; most are

optically pumped laser transitions. The increased accuracy of frequency

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measurements makes this technique mandatory where ultimate accuracy is

needed.

Orange radiation of isotope Krypton-86 was chosen for the new

definition of length in 1960, by the 11th General Conference of Weigths and

Measures. The committee recommended Krypton-86 and that it should be

used in hot cathode discharge lamp, maintained at a temperature of 63K.

According to this standard metre was defined as equal to 165763.73

wavelengths of the red-orange radiation of Krypton-86 isotope.

A standard can now be produced to an accuracy of about 1 part of 10^9.

Source Conditions

Krypton 86. The wavelength of the 605 nm radiation, when emitted by a

lamp conforming to the specification below, has an estimated uncertainty

(99% confidence) of ±4 parts in 109. The other radiations, under similar

conditions, have uncertainties of ± 2 parts in 108.

The source is a hot-cathode discharge lamp containing krypton-86 (purity

99%) in sufficient quantity to assure the presence of solid krypton at 64 K,

the lamp having a capillary portion with the dimensions: internal diameter

2–4 mm, wall thickness 1 mm. The conditions of operation are:

(i) The capillary is observed end-on from the anode side of the lamp;

(ii) the lower part of the lamp, including the capillary, is immersed in a

refrigerant bath maintained within 1K of the triple point of nitrogen;

(iii) the current density in the capillary is 3 ± 1 mA mm−2.

Mercury-198. The uncertainties of the wavelengths are ± 5 parts in 108

when emitted by a high-frequency electrode less discharge lamp, operated

at moderate power with the radiation observed through the side of the

capillary. The lamp should be maintained at a temperature below 10 °C and

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contain mercury-198 (purity 98%) with argon as carrier gas at a pressure

between 65 and 133 Nm−2. The internal diameter of the capillary should be

about 5 mm, with the volume of the lamp preferably >20 cm3.

Cadmium-114. The standard wavelengths have an estimated uncertainty of

±7 parts in 108 when emitted by an electrode less discharge lamp source,

maintained at a temperature such that the green line is not reversed and

containing cadmium-114 (purity 95%) with argon as carrier gas (pressure

about 150 N m−2 at ambient temperatures). The radiations should be

observed through the side of the capillary part of the tube, having an

internal diameter of about 5 mm.

Advantages :

(a) Not a material standard and hence it is not influeced by effects of

variation of environmental conditions like temperature, pressure

(b) It need not be preserved or stored under security and thus there is not

fear of being destroyed.

(c) It is subjected to destruction by wear and tear.

(d) It gives the unit of length which can be produced consistently at all

times.

(e) The standard facility can be easily available in all standard laboratories

and industries

(f) Can be used for making comparative measurements of very high

accuracy.