**1.3.**

Measurement is a complex of operations carried out by means of measuring instruments to

determine the numerical value of the size which describes the object of measurement.

A physical measurement could be defined as the act of deriving quantitative information

about a physical object or action by comparison with a reference. It will be noted from this definition

that there are three important elements of a measurement, viz, (i) measurand, i.e. the physical

quantity or property like length, angle etc. being measured, (ii) comparison or comparator, i.e. the

means of comparing measurand with some reference to render a judgement and (Hi) reference, i.e.

the physical quantity or property to which quantitative comparisons are made. All these three terms

would be very clear from the following example of a direct measurement using a calibrated fixed

reference. Say, a mechanic has to measure the length of a surface table (measurand). For this, first

he lays his rule (reference) alongside the table ; he then carefully aligns the zero end of his rule

with one end of the table ; and finally he compares the length of table (measurand) with the

graduation on his rule (reference) by eye (comparator).

Such examples of direct measurement using a calibrated fixed reference are plenty in the

field of Metrology. For example, measurements of length using vernier calliper, micrometer screw

gauge, measurement of angle using bevel protractor etc. Many measurements are made using

inferential methods, e.g. measurement of pitch diameter or threads using three wire method. Such

measurements are also frequently encountered when it is not possible to measure a measurand

directly. Such methods are dependent on direct relationship (mathematical or otherwise) between

the measurand and the actual measurement being made.

In modern mersuring instruments which employ electrical principles, the measured

parameter is sensed and converted into electrical quantity such as voltage (there being a direct

relationship between the two), which is measured by a null measurement using a calibrated variable

reference. The variable reference (by potentiometric voltmeter) is adjusted until an indicator shows

that no voltage differential exists between the reference and the unknown voltage. Under such a

condition of null, the reference voltage must be equal to the unknown voltage. The purpose of making

measurement could be either to quantify the measurand (information), or, provide a historical

profile to be referred to at other times (record) or use the information to produce action such as to

produce correct size on machine tool (control). The measurement system to be employed will depend

on the objective of the measurement to be fulfilled. A modern measurement system utilising

electrical techniques responds to the measurand, effects a comparison with the reference and

provides information based on that comparison.

However, the direct measurement system using a calibrated fixed reference is more conven-

tional in the field of Metrology. Of course there are several other methods of measurements which

are also used in this field.

**Principle of measurement.** It is the physical phenomenon utilised in the measurement.

**Method of measurement.** It is the way the measuring principles and measuring means

are used.

**Nominal size (Basic Size).** It is the size on which the limits of size are based and which is

assigned to a part in accordance with its function.

**True size.** It is the theoretical size of a dimension, which is free from any errors of

measurement.

**Actual Size.** It is the value of size obtained through measurement with the permissible

measuring error.

**Exact size.** It is the value of size obtained with the highest metrological accuracy attainable

in practice.

**Approximate Size.** It is the value of size obtained with an error exceeding the permissible

error 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 to the indicated value to

obtain the actual value of the size being measured. The correction is numerically equal to the error,

but opposite in sign.

**Correctness of measurement.** It is quantitative characteristic showing how close to zero

are the systematic errors of measurement results.

**Reliability of measurement.** It is a qualitative characteristic which implies confidence in

the measured results depending on whether or not the frequency distribution characteristics of

their deviations from the true values of the corresponding quantities are known.

**Verification.** It is the process of testing an instrument for the purpose of assessing the

indication errors and determining whether setting standards or measuring instruments meet the

prescribed specifications.

**Calibration.** It is the process of determining the values of the quantity being measured

corresponding to a pre-established arbitrary scale.

# Physical Measurement (Metrology)

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