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METROLOGYSubject Code - 141901



C HAPTER – 1

I NTRODUCTION TO M ETROLOGY

MeaninNecess ity and object ive of Metr ologySta nda rds of Measu rementE lement s of Measu r ing SystemMeth ods of Measu remen tP recision a nd AccuracySources of E r ror s

e ec on a n a re o n s r um en sSt an dar dizing Organizations



M EANING / D EFINITIONThe word metrology is der ived from the two

Greek wordsMetros means MeasurementLo means Science

F rom t he a bove,“Met rology is defin ed a s t he scien ce of measurement .Accord ing to the In t ern at iona l Bureau of Weight s

and Measu res (IBWM),“Th e science of m easurem ent em bracing bothexperim ent and theoretical determ ination at anylevel o u n certain t in a n ield o scien ce an d

technology”.



M EANING / D EFINITIONThus measu rement is th e rocess or th e result of determ ining th e ma gnitu de of a qu an tit y, such a slengt h or mass, r ela t ive t o a un it of

, .



C LASSIFICATION OF M ETROLOGYMetr olo ma be classified :

Quantity under considerationExample - Metrology of length

e ro ogy o meMetrology of temperature

Field of applicationExample - Industrial metrology

Medical metrology



N EED OF M ETROLOGYTo ensure tha t the a r t mate r ia l or com onentconform s t o th e esta blished sta nda rd.To meet t he int erchangeabilit y of man ufact ur e.

To maintain customer relat ionship by ensuringt ha t no fau lty product r eaches t he cust omer .

.To t ake decisions on t he defect ive par t s.



O BJECTIVE OF M ETROLOGYCom lete evolution of newl develo ed roducts.Determination of process capabilities and ensuret ha t t hese a re bet t er t han the relevan t

component s t o era nces.Determina t ion of t he measur ing in s t rumen tca abilit ies.Minimizing the cost of inspection by effective andefficient u se of availa ble facilit ies.To st an dar dize th e mea sur ing met hods.To ma inta in t he a ccur acies of mea sur ement .



St an da rd is u sed t o r e r esen t u niver sa llS TANDARD OF MEASURE MEN T

accepted specifications for devices.S ta n da r ds of m ea su r em en t evolved so t h a t

commun t es wou ave cer t a n commonbenchmarks.The s tandard s s tems for linear measurementar e l ine s t anda rd, end stan dard and wavelengths tandard .



S TANDARD OF MEASURE MEN TEvolution & H istory – Mat erial Sta nda rd

Need for establishing standard of length rise to determineagricultural land areas and erection of buildings andmonuments .Earliest standard of length was Cubit .One Cubit = length of forearm = from elbow to the tip of the

.In 1855 , Imperial Standard Yard was the first lengthstandard made in England .

In 1872 , International Prototype meter made in France .These two standards of length was made from material(metal alloy) and hence called as material standards .



Evolut ion & His tory –S TANDARD OF MEASURE MEN T

-Imperial Sta nda rd Yar d

Made of 1 sq . in . crossbronze bar 82 % co er13 % tin, 1% zinc) 38 in .long .It had two ½ in dia . X ½n . eep o e, eac e

with 1/ 10 in dia . goldplug .

The tem . of the bar isconstant at 62 OF.The bar is supported onrollers in a specified

flexure .



Evolu t ion & Hist ory – Ma ter ia l St anda rd -S TANDARD OF MEASURE MEN T

Int erna tiona l Sta nda rd P rototypeMade of platinum – irredium alloy (90 % Platinum and 10irredium) 598 mm long .

The upper surface was highly polished .The bar was operated at 0 OC and under normal atmosphericpressure .It was supported by rollers to avoid deflection .



Evolut ion & History – Mat eria l St anda rd - Limita t ionsS TANDARD OF MEASURE MEN T

Influenced by effects of variation of temperature, pressure,humidity etc .Stored under security to prevent damage or distraction .

Replica was not available easily .Not easily reproducible .Considerable difficult was ex erienced while com arinand verifying the sizes of gauges .



Evolut ion & History – Wavelength St anda rdS TANDARD OF MEASURE MEN T

Wavelength of monochromatic light is used to measure thelength .Cadmium 114 , krypton 86 and Mercury 198 were possible

sources of radiation of wavelength suitable as naturalstandard as length .Kr 86 is most suitable if used in a hot - cathode dischargeamp ma n a ne a .

According to this 1 meter is equal to 1650763 .73 xwavelength of radiation .

ave engt o r s . nanometers .Limited used due to costly equipment and operation cost .



Sub-divisions of StandardS TANDARD OF MEASURE MEN T

Imperial Standard Yard, Meter and Wavelength Standardscan not be used for ordinary purpose .Depending upon the impotence of standard, standards are

sub -divided into four grades :Primary StandardSecondary StandardTertiary Standard




Primary StandardStandard unit of length change its value and it is strictly followedand precisely defined .

This has no direct application to a measuring problemencountered in engineering .They are used after 10 -20 years solely for comparisons .Yard or Meter




Secondary StandardSimilar to primary standard .Any error in the instruments is defended measuring with

primary standard .Use against the loss or destruction of primary standard .Inches



Sub-divisions of StandardS

TANDARD OF MEASURE MEN T

Tertiary StandardSimilar to primary and secondary standard .Use for compare with working standard .

cm



STANDARD OF MEASURE MEN T

The two s t anda rds of t he linea r measu remen t a r eclass ified as

Line MeasurementLength is expressed as the distance between scribed lines on a

bar of metal under certain conditions of temperature and support .Example – Engineer’s steel rule

End MeasurementLength is expressed as the distance between the two flat facesparallel to each other .

Example – Vernier Calliper



Differen ce between Line and E nd Sta nda rdS

TANDARD OF MEASURE MEN T

Line Standard End Standard

Length is expressed as the distance betweenscribed lines

Length is expressed as the distance betweentwo flat parallel faces .

Measurement is quick and easy . Use of the standard requires skill and timeconsuming

Scale marking are not subjected to wear These are subjected to wear on theirmeasuring surfaces .

Limited to ± 0.2 mm for high accuracy. High accuracy for measurement close to ±0. 001 mm .

Simple to manufacture at low cost. Manufacturing process is complex and the costis high.

They are subjected to parallax error. They are not subjected to parallax error.

Exam le – Yard meter scale Exam le – Vernier calli er micrometer , ,



PROCESS OF

MEASUREMENT

The se uence of o er a t ions neces sa r for t heexecut ion of measu rement is called process of measurement .

There a re t hr ee import an t element s of process of measurement

MeasurandReferenceComparator



M EASURING S YSTEM

A measur in s s t em is made of t he five bas icelement:

Standard

or p eceInstrumentPersonEnvironment



M EASURING S YSTEM

StandardA standard is defined as something that is setup andestablished by authority as a rule for the measurement of

uantit , wei ht, value or ualit etc .

Work pieceThe job of which the dimensions or parameters are to bemeasure .

Instrument

Device used to measure a work iece arameter .Operator

Person who is measuring the work piece parameter .

EnvironmentStandard working surroundings .



M ETHODS OF M EASUREMENT

1. Direct method of measurement 2. Indirect met hod of mea su remen t3. Comparative method

4. Tra ns position met hod5. Coincidence method

6. Deflection method7. Complement ar y meth od

8. e o o m ea su r em en y su s u on9. Contact met hod

.




Direct Meth od of Measu remen tThe value of quantity to be measured is obtaineddirectly without any calculations .

– –

Indirect Met hod of Measu remen tThe value of quantity to be measured is obtained bymeasuring other quantities which are functionallyrelated to the required value .

Example – Angle measurement by using sine bar




Comparative MethodThe value of quantity to be measured is compared withknown value of the same quantity or other quantity

racticall related to it .

Example – Angle measurement by minute and second

The value of quantity to be measured to be measured isfirst balanced by initial known value of the same quantity,

en e va ue o e quan y measure s pu n p ace oknown value and is again balanced by another knownvalue .

xamp e – ass measurement y a ance an we g ts




Coincidence Met hodA very small difference between the value of the quantity tobe measured and the reference is determined by theobservation of the coincidence of certain lines or si nals .

Example – Measurement by vernier calliper and micrometer

The value of quantity to be measured is directly indicatedby the pointer on a calibrated scale .

Example – Air pressure measurement in tyres




Com lementar MethodThe value of the quantity to be measured is combinedwith the known value of the same quantity .

– – displacement

The value of quantity to be measured is replaced bythe same quantity .

Example – Different quantity for same type of measurement




Cont act Met hodThe sensor or measuring tip of the instrumentactually touches the surface to be measured .

– – , ,

thermometer etc.

There is no contact between the sensor or measuringtip of the instrument .

Example – Temperature measurement, lengthmeasurement by using laser operated devices



S YSTE M OF M EASUREMENT

A measurin s s tem is based o few fundam entalun its i.e. length , ma ss, time, t empera t ur e.F ollowing a re syst ems of measu remen t :

F.P.S. SystemMetric SystemS .I. S stem




F .P.S. SystemF.P.S. system stands for Foot Pound Second System .In this, unit of length is foot, unit of mass, weight or force is

.

MetricSystemMetric system is a decimal system of weight and measure .In this, unit of length is meter, unit of mass is kilogram,unit of weight or force is kilogram force (kgf ) and the unit of temperature is Degree Centigrade (OC) .




S.I. Syst emS.I. system stands for System International System .It covers seven basic units and provides only one base unit

.

The seven basic units are as follows :

Quantit Unit S mbol

Length Meter m

Time Second s

Mass Kilogram Kg

Temperature Kelvin K

Electric Current Ampere A

Luminous Intensity Candela cd

Quantit of Substance Mole mol



S.I. Syst em


Apart from basic units, there are fifteen derived units .

Derived units are also called as compound unit .– .



Precision

P RECISION AND ACCURACY

Precision is how close the measured values are toeach other .

AccuracyAccuracy is how close a measured value is to theactual (true) value .Accuracy describes the closeness to the target center .



P RECISION AND ACCURACY



Error

S OURCES OF ERRORS

Error is the difference between the true value of theparameter and the value found by measurement .

instrument .

Error = True Size – Actual Size

True Size : It is the theoretical size obtained throu hmeasurement . This type of size is free from any typeof error .

error .



Er ror ma occur in mea su remen t due to followin

S OURCES OF ERRORS

reasons:Defect in instrument

anua a ustment o an nstrumentManufacturing error in instrumentEnvironmental effectHuman errors



T es of er ror :

S OURCES OF ERRORS

Systematic errorsRandom errorsEnvironmenta ErrorAlignment ErrorSu ort ErrorDirt ErrorParallax Error



S stemat ic Err ors

S OURCES OF ERRORS

Systematic error is a type of error that deviates by afixed amount from the true value of measurement .

, .Systematic error results from :

faulty instrumentfaulty measuring process andpersonal bias

Exam leSuppose the first two millimeters of a ruler are wornoff, and the user is not aware of it . Everything he or

–systematic error .



Random Errors

S OURCES OF ERRORS

Random error is not unidirectional .Some of the measured values are greater than true

.



S u or t E r r or

S OURCES OF ERRORS

These errors are caused due to improper support orself weight may undergo deflection or may bend .

kept approximately half of the total distance of thecomponents .

Alignm ent E r rorThis error is due to non coincidence of the line of measurement of the measuring component and thescale or axis of the measuring instrument .

,of the actual dimension .



En vironm enta l Er ror

S OURCES OF ERRORS

This error happens due to the environmental effectson the measuring device .

, , .avoid this the instrument must be kept in remoteplace and by maintaining the atmospheric conditions

.To avoid these errors, all metrology laboratories andstandard rooms are maintained at 20 OC.

Dirt E rr or

the instrument or in the environment ..



Pa ra llax Err or

S OURCES OF ERRORS

This error occurs due to the faulty reading of theinstrument by the human eye .

.Using a proper reflecting device would avoid such anerror .



The im ort an t cha ra cter istics t o be considered in

S ELECTION AND CARE OF INSTRUMENTS

t he selection of th e measu ring inst ru men ts a re asfollows:

easur ng rangeAccuracyEase to handle / operateCost




Followin care should be taken while handinmeasuring instr ument :

Keep measuring faces clean and dry during use .g y n s e sur aces s ou not e touc e y

hands .Avoid sudden shock (dropping) as it can causeinstrument to malfunction .Coat measuring faces with rust preventing oil afteruse and return to protective case .Magnetic fields, electronic fields or excessive humidconditions will cause malfunction and unexpectederrors .




Followin care should be taken while handinmeasuring instr ument :

Batteries should be removed when instrument is not.

corrosion causing instrument to malfunction .Do not store instruments exposed to direct sunlight,

g empera ures, excess ve mo s ure or vo at egasses .Avoid taking measurements with tips / edges of jaws .

This will cause inaccurate readings .

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