ohp-metrology

8/12/2019 ohp-metrology

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The word metrology has a long tradition and is derived from the Greek word for

measure. It is w ell-known saying that the k nowledge about anything is complete only

when it can be exp ressed in numbers and something is known about it.

Thus for every kind of quantity measured, there must be a unit to measure it and

express i t in numbers o f that unit. Further, in order t hat all and not one w ho is t akinghe m easurements follow this u nit, there m ust be a universal standard and the various

units for various parameters of importance must be standardized. Most important

parameter in metrology is the length which can be measured in several forms an d in

several ways.

Measurements p lay a v ital role i n every eld of investigation and present day scientic

and technological progress has r esulted from progress in the eld of measurements. In

general, measurements are made to increase ou r knowledge an d understanding of the

world with a v iew to lead a b etter life.

Measurement science is vital for trade and commerce and is the basis of modern

science an d technology.

In metrology, which literally is t he science o f measurements w e have to go one step

ahead and bother ourselves abo ut the cor rectness of measurement also. We h ave t o see

whether the result is given with the sufficient cor rectness and accuracy for the

particular need or no t. Thus we ar e p rimarily concerned with methods of easurement

based n

Metrology is thus concerned with the est ablishment, reproduction, conservation andtransfer o f units of measurements and their st andards. The practice of m etrology

involves precise measurements requiring the use of apparatus and equipments

(instruments an d necessary adjuncts) to permit the d egree o f accuracy required to be

obtained.


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Metrology, the science of m easurements, includes all aspects both theoretical and

practical with reference t o measurements, whatever t heir uncertainty, and in whatever

elds of science or technology they occur. Thus metrology is also the science of

measurement associated with the ev aluation of its u ncertainty.

It is i mportant to understand that only to measure i s n ot the sp ecity of metrology butthe core of metrology lies i n the validation of the result, particularly by specifying its

actual limitations. Metrology is n ot restricted only to standards of length and mass bu t

other p arameters i n sectors o f social concern, such as h ealth, safety, and environment

protection also.

Metrology is therefore concerned with the methods, execution and estimation of

accuracy of m easurements; the measuring instruments an d the inspectors. Todays

standard of precision and reliability are so high that mans b asic instincts an d senses

are inadequate to cope with them. To this end, use has to be made of precision

measuring instruments and various types o f conventional and sophisticated gauges

and comparators.

Thus it can be said that metrology is mainly concerned with

(1) Establishing the units of measurements, reproducing these units in the form of

standards, and ensuring the u niformity of measurements,

(2) Developing methods of measurement,

(3) Analysing the accuracy of m ethods of measurement, establishing uncertainty of

measurement, researching into the cau ses of measuring errors an d eliminating these.


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Measuring Instruments

Measuring Instruments ar e measuring devices t hat transform the measured quantity

or a elated quantity into an indication or i nformation.

Measuring instruments can either indicate d irectly the value o f the m easured quantity

or on ly indicate its equ alitytoaknown measure of the sam e q uantity (e. g. equal armbalance

difference bet ween t he m easured quantity and the m easure having a val ue ver y near to

it (comparator).

Measuring instruments u sually utilize a measuring sequence in which the measured

quantity is transformed into a quantity perceptible to the observer (length, angle,

sound, luminous con trast).

Measuring instruments may be used in conjunction with separate material measures

(e. g. balances using stndaas

internal p arts t o reproduce the unit ( like graduated rules, a precision thread, etc.)

Measuring range . It is t he r ange o f values of the m easured quantity for which the err or

obtained from a single measurement under normal conditions of use does no t exceed

the maximum permissible error.

The measuring range is limited by the maximum capacity and the minimum capacity.

Maximum capacity is the upper limit of the measuring range and is dictated by the

design considerations or by safety requirements or both.

Minimum capacity is t he lower l imit of the m easuring range. It is u sually dictated byaccuracy requirements. For small values o f the measured quantity in the vicinity of

zero, the rel ative e rror can be co nsiderable ev en if the a bsolute e rror i s sm all.

The m easuring range m ayor m ay n ot coincide w ith the range o f scale indication.

Sensitivity.


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The d istinction between the p recision and accuracy will become cl ear by the following

example shown in Figure, in which several measurements are made on a component by

different types of instruments an d results plotted.

Figure: Distinction between accuracy and precision From this gure, it will be o bvious

that precision is concerned with a p rocess or a of measurements, and not a singlemeasurement. In any set of measurements, the individual measurements ar e scattered

about the mean, and the precision tells us to how well the various measurements

performed by same instrument on the same component agree with each other. It will

be appreci

repeatability of the instrument is a necessary but not a sufficient condition of good

accuracy.

Accuracy can be found by taking root mean square of repeatability and systematic

error i.e.

Accuracy = [(repeatability)2 + ( Systematic err or)]1/2

Error i s the d ifference bet ween t he m ean of set of readings on same component and the

true value. Less i s t he error, more accurate is t he instrument. Since t he true value is

never known, uncertainty creeps in, and the m agnitude of error m ust be estimated by

other means. The estimate of uncertainty of a m easuring process can be made by

taking care of systematic and constant errors, an d other contributions to the

uncertainty due t o scatter of the r esults a out the m ean.

So wherever great precision is required in manufacture of mating components, they aremanufactured in a single plant, where m easurements ar e taken with same standards

and internal measuring precision can achieve the desired results. If t hey are to be

manufactured in different plants and subsequently assembled in another, the acc uracy

of the m easurement of two plants w ith true s tandard value i s important.


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Accuracy.

In mechanical inspection, the accuracy of m easurement is t he most important aspect.

The a ccuracy of an instrument is i ts ab ility to give correct r esults. It is, therefore, better

to understand the various factors w hich affect it and which are affected by it. The

accuracy of measurement to some extent is also dependent upon the sense of hearingor sense of touch or sense of sight, e.g., in certain instrument t he proportions of

subdivisions h ave t o be e stimated by the sen se o f sight; of course, in certain instances

the vern ier device m ay be em ployed in order to substitute t he es timation of proportion

by recgn

upon the rec ognition of a threshold effect, i.e. whether t he p ointer i s just moving, or

jus

One thing is very certain that there is n othing like absolute or p erfect accuracy and

there is n o instrument capable o f telling us, whether or not we h ave g ot it. The p hrases

like dead accurate or dead right become m eaningless and of only relative value.

In other w ords, no measurement c an be absolutely correct; and there is always s ome

error, the amount of which depends up on the accuracy and design of the measuring

equipment employed and the skill of the operator u sing it, and upon the method

adopted for the measurement. In some instruments, accuracy depends upon the

recognition of a threshold effect. In some instruments, proportions of s ub-divisions

have t o be es timated.

In such cases, skill of operator i s r esponsible f or accuracy.Parallaxissoerycommonand can be taken care of by installing a mirror below the pointer. How method of

measurement affect accuracy would be realized in angle measurement by sine bar, i.e.

large er rors may o ccur when sine bar is intended to be u sed for measuring large a ngles.


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Apparatus and methods should be designed so that errors in the nal results ar e small

compared with errors in actual measurements made.

The equipment chosen for a particular m easurement must bear s ome relation to the

desired accuracy in the res ult, and as a general rule, an instrument which can be re ad

to the next decimal place beyond that required in the measurement should be used,i.e.,ifameasurement is d esired to an accuracy of 0.01 mm, then instrument with the

accuracy of 0.001 mm should be used for t his pu rpose.

Classication of Methods of Measurements

In precision measurements var ious methods of measurement are followed depending

upon the acc uracy r equired and the am ount of permissible er ror.

There are nu merous ways in which a qu antity can be m easured.

Any method of measurement should be dened in such a d etail and followed by such a

standard practice t hat there i s l ittlepe for u ncertainty.The n ature o f the p rocedure

in some of the m ost common measurements is described below. Actual measurements

may employ one or more combinations of the following.

Direct method of measurement.

In this m ethod the val ue of a quantity is obt ained directlybycomparing the u nknown

with the st andard. It involves n o mathematical calculations t o arrive a t the resu lts, for

example, measurement of length by a g raduated scale. The m ethod is not very accurate

because

Indirect method of measurement.In this m ethod several parameters (to which the quantity to be measured is l inked

with) are measured directly and then the value is determined by mathematical

relationship.Forexample m easurement of density by measuring mass and geometrical

dimensions.


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This method involves measuring the difference between the given quantity and a

known master of near about the same value. For example, determination of diameter

with master cylinder on a com parator.

Coincidence method of measurement.

In this d ifferential method of measurement the ver y small difference b etween the g ivenquantity and the r eference i s d etermined by the o bservation of the co incidence o f scale

marks. For example, measurement on vernier calipers.

Null method of measurement.

In this method the quantity to be measured is compared with a known source and the

difference between t hese two is made zer o.

Deection method of measurement.

In this m ethod, the v alue o f the q uantity is d irectly indicated by deection of a p ointer

on a calibrated scale.

Interpolation method of measurement.

In this m ethod, the given quantity isompared with two or m ore kn own value of near

about same value en suring at least one sm aller and one b igger t han the q uantity to be

measured and the r eadings interpolated.

Extrapolation method of measurement.

In this method, the given quantity is compared with two or more known smaller

values an d extrapolating the r eading.

Complimentary method of measurement.This is the method of measurement by comparison in which the value of the quantity

to be m easured is combined with a known value of the same quantity so adjusted that

the sum of these two values is equal to p redetermined comparison value.

For example d etermination of the vo lume of a solid by liquid displacement.


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For every method of measurement a d etailed denition of the equ ipment to be u sed, a

sequential listof operationstobe performed, the surrounding environmental

conditions an d descriptions o f all factorsnuencing accuracy of measurement at the

required level must be p repared and followed.

When the eq uipment is no t in use, it should be p rotected from atmospheric corrosion.For this p urpose, the h ighly nished surfaces are rst wiped with a sol vent to remove

any nger m arks and then coated with mixture of heated petroleum jelly and petrol.

This mixture spreads much more easily and is applied with cloth or with ngers.

Brushing is n ot r ecommended as i t is liable to trap air w hich, with the moisture it

contains, may cau se r usting.

As t he standard temperature for m easurement is 20C, for ver y precise m easurements

the instruments an d workpieces s hould be allowed to attain this t emperature before

use an d the h andling should be as l ittle as p ossible.

Objectives of M etrology

While the basic objective of a measurement i s to provide the required accuracy at

minimum cost, metrology would have further objective in a m odern engineering plant

with different shops l ike Tool Room, Machine Sh op, Press Shop, Plastic Shop,

Pressure Die Casting Shop, Electroplating and Painting Shop, and Assembly Shop, as

also Research, Development and Engineering Department. In such an engineering

organization, the f urther o bjectives w ould be a s f ollows:

(a) Thorough evaluation of newly developed products, to ensure that componentsdesigned are w ithin the p rocess an d measuring instrument capabilities avai lable i n the

plant.

(b) To determine the process cap abilities a nd ensure that t hese are better t han the

relevant component tolerances.


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(c) To determine the measuring instrument capabilities an d ensure that t hese are

adequate for t heir r espective measurements.

(d) To minimize the cost of i nspection by effective and efficient u se of avai lable

facilities,and to reduce t he co st of rejects an d rework through application of Statistical

Quality Control Techniques.(e) Standardization of measuring methods. This is ac hieved by laying down inspection

methods for any product right at the t ime w hen production technology is prepared.

(f) Maintenance of t he accuracies of m easurement. This is achieved by periodical

calibration of the m etrological instruments u sed in the p lant.

(g) Arbitration and solution of problems ar ising on the sho p oor r egarding methods

of measurement.

(h) Preparation of designs for all gauges an d special inspection xtures.

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