Physical Measurement (Metrology)

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
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
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.

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