National Institute of Technology Sikkim Department of Mechanical Engineering

1. ME14106
National Institute of Technology Sikkim
Department of Mechanical Engineering

2. Definitions
 Thewordmetrologyisderivedfromtwo Greekwords
Metro=measurement
Logy=science
Thusmetrologyisthescienceof measurement
Metrology is the field of knowledge concerned with
measurement and includes both theoretical and practical
problemswith referenceto measurement.
Metrologyisthe scienceofweightsandmeasures.
Metrology is the process of making precise measurements of
the relative positions and orientations of different optical and
mechanicalcomponents.
Metrology is the science concerned with the establishment,
reproduction, conversion and transfer of units of
measurementandtheir standards.

3. Need of Inspection
• Toensurethematerial,partsandcomponentsconform to the
established standards
• Tomeettheinterchangeabilityof manufacture
• T
o provide the means of finding the problem area for
meetingtheestablished standards
• T
o produce the parts having acceptable quality levels
with reducedscrapand wastage
• T
o purchase good quality of raw materials, tools and
equipmentthatgovernthequalityoffinished products
• T
o take necessary efforts to measure and reduce the
rejectionpercentage
• Tojudgethepossibilityofreworkofdefective parts

4. ElementsofMetrology
• Standard
• The most basic element of measurement is standard without which
nomeasurementis possible.
• Standard is a physical representation of unit of measurement.
• Different standards have been developed for various units including
fundamentalunitsaswell asderived units.
• Workpiece
• Workpieceisthe object to bemeasured/measuredpart
• Variationsingeometryandsurfacefinishofthe measured part
directlyaffect measuringsystem’srepeatability
• Compressiblematerialslikeplastic ornylonsposeadifferent type of
problem that any gauge pressure will distort the material. This can
beavoidedbyfixingofgaugingpressureas suggestedbytheindustry
sothat everyonewillgetuniform results

5. Instruments
• Instrument is a device with the help of which the measurement can
be done
• The instrument must be selected based on the tolerance of the
parts to be measured, the type of environment and the skill
level of operators
• It should be remembered that what type of instruments the
customerpreferto measure.
Person
• There must be some person or mechanism to carryout the
measurement
• Modern gauges are increasingly easy to use but the failure to
adequately train the operating personnel will lead a poor
performance

6. Standard
• The measurement should be performed under standard
environment
• Temperature, dirt, humidity and vibration are the four
environmental factors that influence measurement.
• Vernier scale division of vernier caliper always changes when the
measurementprocessiscarriedfor‘n’ numberoftimesfor thesame
dimension. The environment is indirectly related to temperature,
humidity,conditioningetc.,

7. ElementsofGeneralizedMeasurementSystem

8. Primarysensing element
• The primary sensing element receives signal of the physical
quantity to be measured asinput.
• It converts the signal to a suitable form (electrical,
mechanical or other form), so that it becomes easier for other
elements of the measurement system, to either convert or
manipulateit.
VariableConversionElement
• Variable conversion element converts the output of the
primary sensing element to a more suitable form. It is
used onlyif necessary.
VariableManipulation Element
• Variable manipulation element manipulates and amplifies
the output of the variable conversion element. It also
removes noise(if present)inthe signal.

9. DataProcessingElement
• It processesthe data signal received from the
variable manipulation element andproducessuitable
output.
• Data processing element may also be used to compare the
measured value with a standard value to produce required
output.
DataTransmission System
• DataTransmissionSystemissimplyusedfortransmitting data
from one element to another.
• Itacts as a communication link betweendifferentelements
ofthemeasurementsystem.
• Some of the data transmission elements used are cables,
wirelessantennae,transducers,telemetrysystemsetc.
DataPresentation Element
• It is used to presentthe measuredphysicalquantityin
a human readable form to theobserver.
• Itreceivesprocessedsignalfromdataprocessingelementand
presentsthe datainahumanreadable form.
• LEDdisplaysaremostcommonlyusedasdatapresentation
elementsinmanymeasurement systems.

10. The liquid or gas filled temperature bulb acts as primary sensing element and variable
conversion element. It senses the input quantity(temperature) and converts it into a
pressurebuilt upwithin the bulb. This pressureis transmitted throughthe capillary tube
(which acts as data transmission element) to a spiral bourdon type pressure gauge.
Bourdon tube acts as a variable conversion element which converts the pressure into
displacement. The displacement is manipulated as a variable manipulation element. The
pointerandscaleindicatethetemperature, thusservingasdatapresentation elements.

11. Accuracy
Accuracymaybedefinedastheability of aninstrumentto respondto atrue value
of measured variable under the reference conditions. It refers how closely the
measured value agrees with the true value. The difference between the
measuredvalueandthetruevalueisknowasErrorof measurement.
Precision
Precision may be defined as the degree of exactness for which an instrument is
designed or intended to perform. It refers the repeatability or consistency
of measurement when the measurements are carried out under identical
conditions

12. EffectsofelementsofmetrologyonPrecisionand
Accuracy
Factorsaffectingthestandardof measurement
• Coefficientofthermalexpansion
• Elasticpropertiesofamaterial
• Stabilitywithtime
• Calibrationinterval
• Geometriccompatibility
Factorsaffectingtheworkpieceto be measured
• Coefficientofthermalexpansionof material
• Elasticpropertiesofamaterial
• Cleanliness,surfacefinish,surfacedefectssuchasscratches,
wavinessetc.,
• Adequatedatumonthe workpiece
• Thermalequalization

13. Factorsaffectingthecharacteristicsofan instrument
• Scaleerror
• Repeatabilityandreadability
• Calibrationerrors
• Effectoffriction,zerodrift,backlashetc.,
• Inadequateamplification
• Deformationwhenheavyworkpiecesare measured
• Constantgeometryforbothworkpieceand standard.
Factorsaffectingperson
• Training/skill
• Abilityto select the measuringinstrumentsandstandard
• attitude towardsaccuracy
• Planning measurement techniques for minimum cost,
consistentwith precisionrequirementsetc.

14. Factorsaffectingenvironment
• Temperature,humidity,atmosphere,pressure etc.,
• Cleansurroundingandminimumvibrationenhance precision
• T
emperature equalization between standard, workpiece and
instrument.
• Thermal expansion effects due to heat radiation from lights,
heatingelements,sunlightandpeople.
• Manualhandlingmayalsointroducethermal expansion.

15. ErrorsinMeasurement
An error may be defined as the difference between the
measuredvalueandtheactual value
• Truevaluemaybedefinedastheaveragevalueofaninfinite
numberofmeasured values
• Measured value can be defined as the estimated value of true
value that can be found by taking several values during an
experiment.
• Errorinmeasurement=Measuredvalue-True value
TheErrorsinmeasurementcanbeexpressedeitherasan
absoluteorrelative error

16. • Absolute Error: Absolute error is the algebraic difference
between the measured value and the true value of the
quantitymeasured.Itisfurthersubdivided into
• Trueabsoluteerror
• The algebraic difference between the measured average value
and the conventional true value of the quantity measured is
calledtrue absoluteerror
• ApparentAbsoluteerror
• While taking the series of measurement, the algebraic difference
between one of the measured values of the series of
measurement and the arithmetic mean of all measured
values in the same series iscalledapparentabsolute error.
• Relative Error: It results as the results of the absolute error and
thevalueofcomparisonusedforthecalculationof absoluteerror.
• Absoluteerror=MeasuredValue-Truevalue=300-280=20units
• Relativeerror=Absoluteerror/Measured
value=20/300=0.06=6%

17. TypesofErrorsinMeasurementsystem

18. GrossErrors
• Gross errors are caused by mistakes in using instruments,
calculating measurements and recording data results.
• Eg: The operator or person reads the pressure gauge reading as
1.10N/m2insteadof1.01N/m2
• This may be the reason for gross errors in the reported data and
such error may end up in calculation of the final results, thus
producingdeviated results.
Blunders
• Blunders are caused by faulty recording or due to a wrong
value while recording a measurement, or misreading a
scaleorforgettingadigitwhilereadinga scale.

19. • Theoretical
MeasurementError
• Themeasurementerroristhe resultofthe variation ofa
measurementofthe true value.
• The best example of the measurement error is, if electronic scales
are loaded with 1kg standardweight andthe reading is 1002grams,
then
• Themeasurementerroris=(1002grams-1000grams) =2grams
• Measurement Errors are classified into two types: systematic error
andrandomerrors
Systematicerror
• Theerrorsthat occurdue to fault in the measuring device are
knownassystematicerrors.
• Theseerrorscanbedetachedbycorrectingthemeasurement
device.
• Theseerrorsmaybeclassifiedintodifferent categories.
• InstrumentalErrors
• EnvironmentalErrors
• ObservationalErrors

20. Instrumental Errors
• Instrumental errors occur due to wrong construction of the
measuring instruments.
• Theseerrorsmayoccurdueto hysteresisor friction.
• Inordertoreducetheseerrorsinmeasurement,different correction
factors must be applied and in the extreme condition instrument
mustberecalibrated carefully.
Environmental Errors
• The environmental errors occur due to some external
conditions of the instrument.
• External conditions mainly include pressure, temperature,
humidity or due to magnetic fields.
• Toreducethe environmental errors
• Tryto maintainthehumidityandtemperatureconstantinthe
laboratorybymakingsome arrangements.
• Ensure that there shall not be any external electrostatic or
magneticfield aroundthe instrument.

21. Observational Errors
• As the name suggests, these types of errors occurs due to wrong
observations or reading in the instruments particularly in case
ofenergymeter reading.
• Thewrongobservationsmaybedueto PARALLAX.
• In order to reduce the PARALLAX error highly accurate meters are
needed:metersprovidedwith mirror scales.
Theoretical Errors
• Theoretical errors are caused by simplification of the model system. For
example, atheory states that the temperature of the system surrounding
will not changethe readings takenwhenit actually does,then this factor
will beginasourceoferrorin measurement.
Random Errors
• These are errors due to unknown causes and they occur even
when all systematic errors have been accounted.
• These are caused by any factors that randomly affect the
measurement ofthe variableacrossthe sample.

22. Methods of Measurement
Directmethod
• In this method, the quantity to be measured is directly
compared with the primary or secondary standard.
• Thismethodiswidely employedinproduction field.
• In this method, a very slight difference exists between
the actual and the measured values because of the
limitation of the human being performing the
measurement.
Indirectmethod
• In this method, the value of quantity is obtained by
measuring other quantities that are functionally
related to the required value.
• Measurement of the quantity is carried out directly and then
the value is determined by using a mathematical
relationship.
• Eg: anglemeasurementusingsine bar

23. Fundamentalorabsolute method
• In this method, the measurement is based on the measurements
of base quantities used to define the quantity.
• The quantity under consideration is directly measured and is then
linkedwith the definitionof that quantity.
Comparativemethod
• The quantity to be measured is compared with the known
value of the same quantity or any other quantity practically
related to it.
• The quantity is compared with the master gauge and only the
deviationsfromthe mastergaugearerecordedafter comparison.
• Eg. Dialindicators

24. Transpositionmethod
• This method involves making the measurement by direct comparison
wherein the quantity to be measured(V) is initially balanced by a
known value (X) of the same quantity. Then, ‘X’ is replaced by the
quantity to be measured and balanced again by another known value
(Y). Ifthe quantityto bemeasuredisequalto bothX andY, then
• V= XY
• Eg. Determinationofmassbybalancing methods
Coincidence methods
• In this method, a very minute difference between the quantity to be
measured and the reference is determined by careful observation of
certain lines and signals
• Eg: Verniercaliper

25. Deflectionmethod
• This method involves the indication of the value of the
quantity to be measured by deflection of a pointer on a
calibrated scale.
• Eg.Pressuremeasurement
Nullmeasurement method
• In this method, the difference between the value of the
quantity to be measured and the known value of the same
quantity with which comparison is to be made is brought to
be zero.
Substitution method
• This method involves the replacement of the value of the
quantity to be measured with a known value of the same
quantity, so selected that the effects produced in the
indicating device by these two values are the same.

26. Contactmethod
• Inthismethod,the surface to be measured is touched by the sensor or
measuring tip of the instrument.
• Eg.Micrometer,Verniercalliperanddial indicator
Contactlessmethod
• As the name indicates, there is no direct contact with the surface to be
measured
• Eg.Toolmakersmicroscope,profile projector
Compositemethod
• The actual contour of a component to be checked is compared
with its maximum and minimum tolerance limits.
• Cumulative errors of the interconnected elements of the component which are
controlledthroughacombined tolerancecanbecheckedbythis method.
• This method is very reliable to ensure interchangeability and is effected
through the use of composite GO gauges.

27. General characteristics in metrology
Sensitivity: It is the ratio of the magnitude of output signal to the
magnitude of input signal. It denotes the smallest change in the measured
variableto whichthe instrument responds.
Sensitivity=(Infinitesimal change of output signal)/(Infinitesimal change of
inputsignal)
If the input-output relation is linear, the sensitivity will be constant for all
valuesof input.
If the instrument is having non-linear static characteristics, the sensitivity of
the instrumentdependsonthe valueofthe input quantity.

28. Hysteresis: All the energy put into the stressed component when loaded is
not recovered upon unloading. Hence, the output of a measurement system will partly
dependonitspreviousinput signalsandthisiscalledas hysteresis.
Range: It is the minimum and maximum values of a quantity for which an
instrument is designed to measure/ The region between which the instrument is to
operateiscalled range.
Range = Lower Calibration Value – Higher Calibration Value=Lcto Hc
Span: It is the algebraic difference between higher calibrationvalueandlower
calibration value.
Span=Hc- Lc
Ex:Iftherangeofaninstrumentis100̊ Cto150̊ C,its spanis150̊ C– 100̊ C=50̊ C

29. Response Time: It is the time which elapses after a sudden change in the measured
quantity, until the instrument gives an indication differing from
the true value by an amountless than a given
permissibleerror.
Speed of response of a measuring instrument is defined as the quickness with
whichaninstrumentrespondstoachangeininput signal.
Repeatability: It is the ability of the measuring instrument to give the same value
everytimethe measurementofagivenquantityis repeated.
Itistheclosenessbetweensuccessivemeasurementsofthe samequantitywith
thesameinstrumentbythesameoperatorovera shortspanoftime, withsamevalueof
inputundersameoperating conditions.

30. Stability: The ability of a measuring instrument to retain its calibration over a
long period of time is called stability. It determines an instruments consistency
over time.
Backlash: Maximum distance through which one part of an
instrumentmaybemovedwithout disturbingthe other part.
Accuracy: Thedegreeofclosenessofameasurementcomparedto the
expectedvalueisknownas accuracy.
Precision: A measure of consistency or repeatability of measurement. i.e.
successive reading does not differ. The ability of an instrument to reproduce its
readingsagainandagaininthesame mannerforaconstantinput signal.
Magnification: Human limitations or incapability to read instruments places
limitonsensitivenessofinstruments. Magnificationofthesignalfrommeasuring
instrumentcanmakeit betterreadable.

31. Resolution: Minimumvalueofinputsignalrequiredto causean
appreciable change or an increment in the output is called
resolution/ Minimum value that can be measured when the
instrumentisgraduallyincreasedfromnon-zero value.
Error: Thedeviationofthetruevaluefromthedesiredvalueis callederror.
Drift: The variation of change in output for a given input over a
periodoftimeisknownas drift.
Threshold: Minimum value of input below which no output can be
appearedisknownas threshold.
Reliability: Reliability may be explicitly defined as the probability that a
system will perform satisfactory for at least a given period of time when
used under stated conditions. The reliability function is thus same
probabilityexpressedasafunctionofthe time period.

32. Standards
Types of standards
• Linestandard
• Standardyard
• Standardmetre
• Endstandard
• Endbar
• Slipgauges
• Wavelengthstandard

33. Line standard
 Themeasurementofdistancemaybemadebetween two parallel
linesortwo surfaces.
 When a length is measured between as the distance
between centres of two engraved lines, it is called line
standard
Standard yard

34. • Yardis madeof a one inch square cross section bronzebar and
is38inches long
• The bar has a round recess (gold plug) of 0.5 inches diameter
and0.5inchesdeep.Thegoldplugis1 inchawayfromboththe
ends
• The highly polished top surfaces of these plugs contain three
transverselyandtwo longitudinally engravedlineslyingonthe
neutral axis
• The yard isthe distance between two central transverselines
on the plugs when the temperature of the bar is constant at
62OF
• To protect the gold plug from accidental damage, it is kept at
the neutral axis as the neutral axis remains unaffected even if
thebar bends

35. Standard metre
• Themetreisthedistancebetweenthecentreportions oftwo lines
engravedonthepolishedsurfaceofbar madeupofplatinum(90%)
andiridium(10%)havinga uniquecross section
• Thewebsectiongivesmaximumrigidity andeconomy intheuseof
costly material
• The upper surface of the web is
inoxidizableand needsa goodfinish
for quality measurement
• the bar is kept at 0OC and under
normal atmosphericpressure.

36. End standard
Theneedofanendstandardarisesasthe useoflinestandards and
theircopieswasdifficultat variousplacesin workshops
Theseareusuallyinthe formofendbarsandslip gauges
Endbar
• End bars made of steel having cylindrical section of 22.2 mm diameter with the
faceslappedandhardenedat the endsareavailable in variouslengths.
• Flat andparallel faced end barsare firmly usedasthe most practical endstandard
usedfor measurement.
• Theseareusedformeasurementoflarger sizes
Slipgauges
• Slipgaugesarerectangularblocksofhardenedandstabilized high gradecast steel
• Thelengthofaslipgaugeisstrictly thedimensionwhichit measures
• The blocks after being manufactured are hardened to resist wear and are allowed
to stabilizeto release internal stresses
• A combinationofslipgaugesenablesthemeasurementstobemadein therangeof
0.0025mmto 100mmbutin combinations with endbars, the measurementrange
upto1200mmis possible.

37. Wavelength standards
• Lineandendstandardsarephysicalstandardsand aremadeupofmaterialsthat
canchangetheirsize with temperatureandotherenvironmental conditions
• Insearchforsuchsuitableunitoflength,wave lengthsourceis established
• Laserisusedasprimarylevelwavelength standard
• Accordingto thisstandard,ametreisdefinedas equalto 1650763.73
wavelengthoftheredorange radiationofkrypton isotopegas
1metre
1yard
=1650763.73wavelengths
=0.9144m
=0.9144x1650763.73
=1509458.3wavelength.