Metrology and Measurements

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Anna university syllabus Mechanical Engineering VII sem For Unit 1 Concepts of Measurements PPT

Transcript of Metrology and Measurements

Unit 1 Concept of Measurement

GENERAL CONCEPTMeasurement is a complex operation carried out by means of measuring instrument to determine the numerical value of the size which describes the object of measurement. Act of deriving quantitative information about a object or action by comparison with reference.

Three important element of measurements are

1. Measurand: Physical quantity or property like length, angle being measured. 2. Comparison or comparator:The mean of comparing measurand with some reference 3. Reference: Physical quantity or property to which quantitative comparisons are made.

STANDARD

MEASURAND

COMPARISON PROCESS

RESULT

Example:

Direct measurement using calibrated fixed reference.Mechanic has to measure the length of a surface table (measurand). First he lays his rule(reference) alongside the table. He then carefully align the zero end of his rule with one end of the table; and finally he compares the length of table (measurand) with graduation on his rule (reference) by eye (comparator)

Two important requirement of measurement: 1. 2. The standard used for comparison must be accurate and internationally accepted. The instrument and the process used for the comparison must be provable.

The basic aim of measurement in industries is to check whether a component has been manufactured to the requirement of a specification or not.

Measurement is defined as the process of numerical evaluation of a dimension or the process or comparison, with standard measuring instrument.

GENERALIZED MEASURING SYSTEM

Physical variable to be measured

Input signal

Primary sensing element

Variable conversion element

Variablemanipulation elementData processing element

Data presentation element

Observer

Functional elements of an instrument system

Primary sensing element: First element which receives energy from measured medium and produces an output corresponding to the measurand.This output is then converted into an analog electical signal by transducer.

Variable conversion element:It converts the output electrical signal of the primary sensing element (voltage,frequency or someother electrical parameters) into a more suitable form without changing the Information content of the input signal.Some instruments it may not be required.

Variable manipulation element:It is used to manipulate the signal presented to it and preserve original nature of the signal. It amplifies the input signal to the required magnification. An electronic voltage amplifier receives a small voltage as input and produces a greater magnitude voltage as out put.

Data transmission element:It transmits the data from one element to the other, as simple as shaft and gear assy. system or as complicated as a telemetry system which transmit signal from one place to other.

Data processing element:which is used to modify the data before display or recording. To convert the data into useful form. It may provide correction to the measured physical variable to compensate for zero offset, temperature error etc.

The data presentation element:They finally communicate to human observer for monitoring, control or analysis purpose. The value of the measured variable my be indicated by an analog indicator (pointer and scale), Digital indicator (ammeter, voltmeter etc.) or by recorder (magnetic taps, cameras etc.).

GENERALIZED MEASURING SYSTEM Example

Temperature measurement of a thermal system

Temperature measurement of a thermal systemLiquid or gas filled temperature bulb

Temperature

Capillary tube

Physical variable to be measured

Primary sensing element

Variable conversion element Pressure

Data transmission element

Displacement

Magnification

Variable conversion element Spiral bourdon tube

Variable manipulation element

Data presentation element

Observer

Pointer and scale Linkage and gear

UNITSTo specify and perform calculation with physical quantities, the physical quantity must be defined both in kind and magnitude. The standard measuring of each kind of physical quantity is the unit.

Earlier days number of systems of units has been used in various countries. M.K.S and C.G.S systems were used. However for the sake of uniformity of units all over the world S.I has been developed ( System international units ).FUNDAMENTAL UNITS: independantly chosen and not depend on anyother units. Base units. Quantity Length Mass Time Unit Meter Kilogram Second symbol m kg s

TemperatureElectric current

KelvinAmpere

KA

DERIVED UNITS: expressed in terms fundamental units by defining equation

Mechanical units Force Pressure Weight Torque Acceleration velocity density Area Volume

Electric and magnetic units Power Energy Electrical resistance Electrical field strength

Thermal

unitsSpecific heat Latent heat Sensible heat

STANDARDSLINE MEASUREMENT

LINE STANDARD

END STANDARD

WAVE LENGTH STANDARD

When the length is measured between two lines it is called line standard. Eg. Protype meter and imperial yard When the distance(length) is measured between two surfaces or faces it is called end standard Eg. Slip gauge,gap gauge micrometer

When the distance(length) is measured between two surfaces or faces it is called end standard Eg. Slip gauge,gap gauge micrometer

Material standards are liable to destruction and their dimensions change due to variations of environmental temperature, pressure,humidity and ageing with time. But with the monochromatic light we have the advantage of constant wavelength and since the wavelength is not physical one, it need not be preserved. This is reproducible standard of length. The error of reproduction can be in the order of 1 part in 100 millions. Because of this reason that international standard measures the meter in terms of wave length of krypton 86 (Kr 86). 1meter = 1650763.73 wave length of Kr86 in vaccume 1 yard = 0.9144m =1509458.25wave length of Kr86 in vaccume

Sub division of standards:

Primary or international Standards: The material is preserved carefully and is and used only at rare intervals for comparison with secondary standard.eg. Protype meter, Imperial yard Secondary Standards: These are kept at number of places for occasional comparison with tertiary standards when ever required. Tertiary Standards: Reference purpose in laboratories and work shops. True copy of secondary std, and used for comparison with working std.

Workshop Standard: More frequently in laboratories and work shops. Made of low grade material

Measuring InstrumentsDepending upon the accuracy required in measurement various measuring instruments are selected. Some instrument measure the exact dimension of the component . eg. Vernier caliper and micrometer etc. Some devices are used to check whether the size of the component is within the specified limits.They are called Limit gauges. eg. Plug gauge, Ring gauge and Snap gauges. Some instrument compare the size of the work piece with the standard and show deviations.These instruments are known as Comparators. eg. All mechanical,electrical,pneumatic and optical comparators

Linear Measuring instrument

Direct measuring instrument

Indirect measuring instrument

Graduated Vernier calipers Vernier height gauge Vernier depth gauge Micrometers and Dial indicators

Non graduated telescopic gauge, Surface gauge, Screw pitch gauge radius gauges thickness gauge, slip gauge

Precision instrument Vernier instruments Micrometers Dial gauges

Non precision instrument Outside Caliper Inside caliper Steel rule

The term accuracy denotes the closeness of the measured value with the true value. Accuracy is the degree to which the measured value agrees with the true value. The difference between the true value and measured value is known as error of measurement. The lesser the error, the more is the accuracy

Precision is the repeatability of the measuring instrument.

It refers to the group of measurements for the same characteistics taken under identical conditions.It indicates to what extent the identically performed measurements agrees with each other. If the instrument is not precise will give different (widely varying)results for the same dimension when measured again and again. The set of observations will scatter about the mean true value.

The less the scattering more precise the instrument.

Range

Fine and widely spaced graduation lines ordinarily improve the readability.If the graduation lines are very finely spaced,the scale will be more readable by using microscope, however,with the naked eye the readability will be poor.

It is the susceptibility of a measuring device to have its indications converted into meaningful number

PERFORMANCE OF INSTRUMENTThe knowledge system characteristics or system response of an instrument is essential for selecting the most suitable instrument for specific measuring jobs.

System Characteristic (or) System response

Static Characteristic or static response ConsideredIf the instrument is required to measure condition not varying with time. Static charecteristics are obtained by the process called Calibration

Dynamic characteristic or Dynamic response Considered If the instrument is required to measure time varying process-variable measurements

STATIC RESPONSEIf the instrument is required to measure condition not varying with time static characterestics are considered. Static performance charecteristics are obtained by one form or other of a process called Calibratio