11 Unit I Lecture Notes (Metrology)

8/10/2019 11 Unit I Lecture Notes (Metrology)

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CALIBRATION: The calibration of any measuringinstrument is necessary to measure the quantity in termsof standard unit. It is the process of framing the scale ofthe instrument by applying some standardized signals.Calibration is a pre-measurement process, generally


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carried out by manufacturers. It is carried out by makingadjustments such that the read out device produces zero

output for zero measured input. Similarly, it shoulddisplay an output equivalent to the known measured inputnear the full scale input value. The accuracy of theinstrument depends upon the calibration. Constant use ofinstruments affects their accuracy. If the accuracy is to bemaintained, the instruments must be checked andrecalibrated if necessary. The schedule of such calibrationdepends upon the severity of use, environmentalconditions, accuracy of measurement required etc. As faras possible calibration should be performed underenvironmental conditions which are very close to theconditions under which actual measurements are carriedout. If the output of a measuring system is linear and

repeatable, it can be easily calibrated.

CALIBRATION OF DIAL INDIACTOR

In most of the measurements, the dimensions to bemeasured are compared with a suitable referencestandard. Such measurements involve the use of an

instrument calledcomparator; whichmeasures the deviations ofa dimension from thestandard reference value.Dial indicators measure

the displacement of a plunger or stylus and


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indicate the magnitude of the displacement on dial bymeans of a rotating pointer.

The comparators however, maysometime indicate faultyreadings due to their inherentmanufacturing errors or due tothe wear of their parts duringservice. The comparator,therefore, needs to be calibrated

periodically.

Calibration of a dial indicatorinvolves Range of the instrument to

be calibrated, intervals ofthe readings to be taken,reference standard to be used etc.

Clean the measuring faces of the dial calibrationtester. The dial indicator is fixed on the dial testerwith the plunger just touching the anvil of themicrometer. The pointer on the dial indicator should

be adjusted such that the zero coincide the bezel. Thecircular disc of the micrometer is adjusted such thatthe zero coincides with the cross line, i.e. setting themain scale zero and the zero on the micrometer drumacross the cross line.

Rotate the thimble of the micrometer in clockwise

direction, Note down the readings of the graduated


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disc of the micrometer against the cross wire for eachinterval of movement of the dial gage.

Repeat the procedure for the range of the indicator, both in the forward and backward directions.

Since the Dial Tester is used as reference standard. Itshould be justified. Choose a proper range of the dialtester and check against the slip gauges.

Place the slip gauge between the micrometer head tipand the contact point of the electronic probe.

Take the reading in the upward direction from 0.5mm to 25 mm in a step size of 0.5 mm.

Calculate the uncertainty as per guidelines by National Accreditation Board for testing andcalibration Laboratory (NABL).

Tabulate the results as divisions on the dial gage vs.the deviations from the actual readings note from thedial tester.

Plot a graph of calibration with the dial indicatorreadings vs. the error for various turns

Tabulate the results for justification of dial tester. Study the calibration graph and comment about

sources of error in the dial indicator under test and itsmeasurement capability. Comment about the capability of the Dial tester for

its use as reference standard for calibration.

Refer file Dial Indicator Calibration Procedure.pdf for detailed calibrating procedure.


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Refer file Calibration of gauge blocks bycomparison.pdf for detailed calibrating

procedure of gauge blocks.

Refer file Micrometer Calibration.pdf fordetailed example pertaining calibrating

procedure of micrometer.

11 Unit I Lecture Notes (Metrology)

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