Into to Metrology-1

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IPE 381

Introduction toMetrology

Abdullah-Al-MamunLecturer, Dept. of IPE

Outline

• What is Metrology

• Different types of Metrology

• Precision and accuracy

• Sources of errors

• Nano metrology



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What is Metrology

• “The science that deals withmeasurement”

• Measurement is a group ofoperations carried out by meansof measuring instruments todetermine the numerical value of

the size which describes the

object of measurement.

measurement

there are three important elements of a measurement,

(i) Measurand, i.e. the physical quantity or property likelength, angle etc. being measured.

(ii) Comparison or comparator, i.e. the means of

comparing measurand with some reference to render a judgement.

(iii) Reference, i.e. the physical quantity or property towhich quantitative comparisons are made.



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Measuring Means• The means of measurements could be classified as

follows :(i) Standards (Reference)-These are used to reproduceone or several definite values of a given quantity.

(ii) Fixed gauges-These are used to check thedimensions, form, and position of product features.

(iii) Measuring instruments-These are used to determinethe values of the measured quantity.

Types of Metrology

• Legal Metrology: Legal Metrology is that part of

metrology which treats units of measurement, methods ofmeasurement and the measuring instruments, in relationto the statutory, technical and legal requirements.

• Deterministic Metrology: This is a new philosophy inwhich part measurement is replaced by processmeasurement. In the deterministic metrology, fulladvantage is taken of the deterministic nature ofproduction machines (machines under automatic controlare totally deterministic in performance)



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Physical Measurement• Nominal s ize (Basic Size). It is the size on which the

limits of size are based and which isassigned to a part in accordance with its function.True size. It is the theoretical size of a dimension, whichis free from any errors ofmeasurement.Actual Size. It is the value of size obtained throughmeasurement with the permissible

measuring error.Exact size. It is the value of size obtained with thehighest metrological accuracy attainablein practice..

Physical Measurement

Approximate Size. It is the value of size obtained with an errorexceeding the permissibleerror of measurement and requiring refinement.

Error of measurement. It is the difference between the true value

of the size being measured and the value found by measurement.Error pertains to a measurement and not to an instrument.

Correction. It is the amount which should be algebraically added tothe indicated value to obtain the actual value of the size beingmeasured. The correction is numerically equal to the error,but opposite in sign.



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Precision and Accuracy

Calibration

• Calibration is a comparison betweenmeasurements – one of knownmagnitude or correctness made or setwith one device and anothermeasurement made in as similar away as possible with a second

device.

• The device with the known orassigned correctness is called thestandard. The second device is theunit under test (UUT), test instrument(TI), or any of several other names forthe device being calibrated.



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Errors in Measurement• In any measurement, there is always a degree of uncertainty

resulting from measurement error, i.e. all measurements areinaccurate to some extent Measurement error is the differencebetween the indicated and actual values of the measurand.

• During measurement several types of errors may arise such as

1. Static errors,

2. Instrument loading errors or dynamic errors,

These errors can be broadly classified into two categories viz.

a. Controllable errors

b. Random errors

Static Errors

Static errors: It result from the intrinsic imperfections or limitations in thehardware and apparatus compared to ideal instruments.

static errors stem from three basic sources :

1. Reading error: parallax, interpolation, optical resolution (readability oroutput resolution)

2. Characteristic error : It is the deviation of the output of the measuringsystem under constant environmental conditions from the theoreticallypredicted performance. If the theoretical output is a straight line, thenlinearity, hysteresis, repeatability, and resolution errors are part of thecharacteristic error.

3. Environmental error: result from effect of surrounding temperature,pressure and humidity on measuring system.



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Dynamic Errors• Dynamic errors are caused by time variations

in the measurand and results from the inability ofa measuring system to respond faithfully to a

time-varying measurand.

• Usually the dynamic response is limited by

inertia, damping, fric tion or other physical

constraints in the sensing or readout or displaysystem.

Systematic orControllable Errors

• Systematic error is just a euphemism for experimentalmistakes.

• These are controllable in both their magnitude andsense.

• These can be determined and reduced, if attempts aremade to analyse them.

• However, they can not be revealed by repeatedobservations.

• These errors either have a constant value or a valuechanging according to a definite law.



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Systematic or Controllable Errors1. Calibration Errors. The actual length of standards such as slipgauges and engraved scales will vary from nominal value by smallamount.2. Ambient Conditions.3. Stylus Pressure4. Avoidable Errors. parallax and the effect ofmisalignment of the workpiece centre.5. Experimental arrangement being different from that assumed intheory.6. Incorrect theory i.e., the presence of effects not taken intoaccount.

Random Errors

• These errors occur randomly and the specific cases of such errorscannot be determined, but likely sources of this type of errors are

• small variations in the position of setting standard and workpiece,

• slight displacement of lever joints in the measuring joints in

measuring instrument,• transient fluctuation in the friction in the measuring instrument,

• and operator errors in reading scale and pointer type displays or inreading engraved scale positions.

• Characteristics of random errors



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Error Distribution• Virtually all instrument errors are random in nature.

Exceptions to these are called systematic errors.

• Random errors have positive and negative values andtheir magnitudes are generally distributed in accordancewith the Gaussian Distribution—the familiar bell-shapedcurve.

Error Distribution

• On a frequency basis, the area under thecurve between error values xi and x2represents the percentage of all errors

lying between these two values.



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Error Accumulation• The total static error of a measurement system can be

measured in terms of root-mean-square (rms) of the

component characteristic errors, if the followingconditions are fulfilled:

– Component characteristic errors are independent and of thesame order of magnitude.

– The distribution of errors is normal (Gaussian), i.e. we consideronly the random errors

• The total static error of a measuring system, therefore,

LE = linearity errors of individual component; RE = readingerrors,CE = characteristic errors and EE = environmental errors.

Errors likely to Creep in PrecisionMeasurements—Their Care

• Effects of Environment—Temperature

• Effect of supports

• Effect of alignment

• Errors due to vibrations

• Metallurgical effects



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Errors likely to Creep in Precision

Measurements-Their Care

• Contact point penetration

• Errors due to deflection

• Errors due to looseness

• Errors due to wear in gauges

• Error due to location

• The Parallax Effect

• Errors due to poor contact

Errors likely to Creep in PrecisionMeasurements-Their Care



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• Error due to impression of measuringstylus

• Importance of gauging set up to be square – an angular error of one minute in setting can cause 1

micron error in 100 mm.

• Gauging thin blocks

Errors likely to Creep in Precision

Measurements-Their Care

That’s all for today

• To download the Soft copy of Slides visit:

http://teacher.buet.ac.bd/aamamun

Into to Metrology-1

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