Metrology and Gauging

Instructed by: Dr. Sajid Zaidi

PhD in Advanced Mechanics, UTC, France

MS in Advanced Mechanics, UTC, France

B.Sc. in Mechanical Engineering, UET, Lahore

B.TECH Mechanical TechnologyIQRA COLLEGE OF TECHNOLOGY (ICT)INTERNATIONAL ISLAMIC UNIVERSITY, ISLAMABAD

Definition of MetrologyMetrology

◦ Comes from Greek “metron” (measure) and –logy.

◦ It refers to science of measurement, including all theoretical and practical aspects of measurement.

Measurement

◦ It is the procedure of comparing an unknown quantity to a known standard by means of consistent system of units.

◦ Provides a numerical value of quantity within certain limits of accuracy and precision.

Inspection

◦ It is the procedure in which part characteristics (e.g. dimension) are examined whether they conform to design specification.

◦ Many inspection procedures rely on measurement techniques while others use gauging methods (simply determine whether the part passes or fails inspection).

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Definition of QualityQuality

◦ It refers to a product’s fitness for use.

◦ It is the totality of features that bear on a product’s ability to satisfy a given need.

Need of Inspection

◦ To ensure that part and components are confirmed to required standards.

◦ To meet the need of Interchangeability of parts.

◦ To maintain good customer relationship by ensuring that No faulty product reaches the customer.

◦ The result of inspection are forwarded to the manufacturing department, thus helps in improving the quality.

◦ It helps to purchase good quality raw material, tool and equipment.

◦ It led to development of precision measuring instruments.

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Measure of QualityM

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Measure of Quality – What to Inspect

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Measure of Quality – When to Inspect

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Measure of Quality – How to Inspect

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Characteristics of Measurement Systems

The performance characteristics of a measuring system may be broadly divided into two groups, namely ‘static’ and ‘dynamic’ characteristics.

Static characteristics are the values given when steady-state conditions occur, i.e. the values given when the system has settled down after having received some input. Static characteristics also refer to the measurement of quantities which are constant or vary slowly with time. For example, the resistance of a resistor etc.

Dynamic characteristics refer to the behavior between the time that the input value changes and the time that the value given by the system settles down to the steady state value.

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Static CharacteristicsAccuracy

Accuracy determines the closeness of an instrument reading to the true value of the physical variable.

Accuracy is often expressed as a percentage of the full range output or full-scale deflection (f.s.d).

For example, a system might have an accuracy of ±1% of f.s.d. If the full-scale deflection is 10 A then the accuracy is ±0.1 A.

This means that when the instrument is reading 1.0 A, the possible error is 10% of this value.

The accuracy is a summation of all the possible errors that are likely to occur.

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Static CharacteristicsPrecisionThe term precision is used to describe the degree of

freedom of a measurement system from random errors.Precision is related to the repeatability of the instrument

reading.Thus, a highly precise measuring instrument will give

only a small spread of readings if repeated readings are taken of the same quantity.

A low precision system will give a large spread of readings.

For example, consider the following two sets of readings obtained for repeated measurements of the same quantity by two different instruments:

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Static CharacteristicsPrecision

◦ 20.1 mm, 20.2 mm, 20.1 mm, 20.0 mm, 20.1 mm, 20.1 mm

◦ 19.9 mm, 20.3 mm, 20.0 mm, 20.5 mm, 20.2 mm, 19.8 mm The results of the measurement give values scattered about some

value. The first set of results shows a smaller spread of readings than the

second and indicates a higher degree of precision for the instrument used for the first set.

Precision is often, though incorrectly, confused with accuracy. High precision does not imply anything about measurement

accuracy. A high precision instrument may have a low accuracy. Low accuracy measurements from a high precision instrument are

normally caused by a bias in the measurements, which is removable by recalibration.

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Static CharacteristicsPrecision Vs Accuracy

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Static CharacteristicsPrecision Vs Accuracy

Satellite Apollo 13:Low precisionlow accuracy

Satellite PolyU 18:High precisionlow accuracy

Satellite CYT 118:High precision, High accuracy

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Static CharacteristicsRepeatability and ReproducibilityRepeatability describes the closeness of output readings when

the same input is applied repetitively over a short period of time, with the same measurement conditions, same instrument and observer, same location and same conditions of use maintained throughout.

Reproducibility describes the closeness of output readings for the same input when there are changes in the method of measurement, observer, measuring instrument, location, conditions of use and time of measurement.

Both terms thus describe the spread of output readings for the same input. This spread is referred to as repeatability if the measurement conditions are constant and as reproducibility if the measurement conditions vary.

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Static CharacteristicsRangeThe range or span of an instrument defines the minimum

and maximum values of a quantity that the instrument is designed to measure. For example, a thermometer might be quoted as having a range of -200 to +800°C.

ThresholdIf the instrument input is very

gradually increased from zero there will be a minimum value required to give a detectable output change. This minimum value defines the threshold of the instrument.

input

Output

Threshold

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Static CharacteristicsLinearityIt is normally desirable that the output reading of an

instrument is linearly proportional to the quantity being measured.

The X’s marked on the Figure show a plot of the typical output readings of an instrument when a sequence of input quantities are applied to it.

Normal procedure is to draw a good fit straight line through the X’s, as shown in Figure.

The non-linearity is then defined as the maximum deviation of any of the output readings marked X from this straight line. It is usually expressed as a percentage of full-scale reading.

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Static CharacteristicsLinearity

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Static CharacteristicsSensitivityThe sensitivity indicates how much the output of an

instrument system changes when the quantity being measured changes by a given amount, i.e. the ratio of ouput to input.

For example, a thermocouple might have a sensitivity of 20 μV/ºC and so give an output of 20 μV for each 1ºC change in temperature.

If we take a series of readings of the output of an instrument for a number of different inputs and plot a graph of output against input, the sensitivity is the slope of the graph.

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Load Cell

Force, F

Output, Vo

Output, Vo (V)

Input, Fi (kN)

Slope = Output / Input

Static CharacteristicsSensitivity

By plotting a graph and finding the best straight line, you can obtain the slope m from the expression y = mx + c

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Static CharacteristicsErrorThe term error is used for the difference between the

result of the measurement and the true value of the quantity being measured, i.e.

error = measured value - true valueThus if the measured value is 10.1 when the true value is

10.0, the error is +0.10If the measured value is 9.9 when the true value is 10.0,

the error is -0.10Errors can arise in a number of ways and the following

describes some of the errors that are encountered in specifications of instrumentation systems.

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Static CharacteristicsHysteresis ErrorThe term hysteresis error is used for the difference in

outputs given from the same value of quantity being measured according to whether that value has been reached by a continuously increasing change or a continuously decreasing change.

A different value may be obtained from a thermometer used to measure the same temperature of a liquid if it is reached by the liquid warming up to the measured temperature or it is reached by the liquid cooling down to the measured temperature.

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Static CharacteristicsNon-linearity ErrorThe term non-linearity error is used for the error that

occurs as a result of assuming a linear relationship between the input and output over the working range, i.e. a graph of output plotted against input is assumed to give a straight line.

Linearity error is usually expressed as a percentage error of full range or full scale output.

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Static CharacteristicsInsertion ErrorWhen a cold thermometer is put in to a hot liquid to

measure its temperature, the presence of the cold thermometer in the hot liquid changes the temperature of the liquid. The liquid cools and so the thermometer ends up measuring a lower temperature than that which existed before the thermometer was introduced.

This effect is called loading and the consequence is the insertion error.

Loading is a problem that is often encountered when measurements are being made. For example, when an ammeter is inserted into a circuit to make a measurement of the circuit current, it changes the resistance of the circuit and so changes the current being measured.

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Static CharacteristicsApplication

◦A commercial pressure measurement system is quoted in the manufacturer's specification as having: Range 0 to 10 kPa Supply Voltage ±15 V dc Linearity error ±0.5% FS Hysteresis error ±0.15% FS Sensitivity 5 V dc for full range Thermal sensitivity ±0.02%/ºC Thermal zero drift 0.02%/ºC FS Temperature range 0 to 50ºC

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Dynamic CharacteristicsThe dynamic characteristics refer to the behavior

between the time that the input value changes and the time that the value given by the system settles down to the steady state value.

For example, the figure shows how the reading of an ammeter might change when the current is switched on. The meter pointer oscillates before settling down to give the steady-state reading.

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Dynamic CharacteristicsResponse TimeThis is the time which elapses after the input to a system

is abruptly increased from zero to a constant value up to the point at which the system gives an output corresponding to some specified percentage, e.g. 95%, of the value of the input.

Rise TimeThis is the time taken for the output to rise to some

specified percentage of the steady-state output. Often the rise time refers to the time taken for the output to rise from 10% of the steady-state value to 90 or 95% of the steady-state value.

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