Standards Used in Measurements & Their Classifications

For anyone performing a measurement using a particular instrument, it usually matters to know whether the instrument is calibrated or not. Calibration is required, along with precision, to enable accurate measurements to be made. Calibration means observing the instrument’s performance when measuring a standard of some kind.

A standard is a physical representation of the quantity under measurement or a unit of measurement whose true value is known with great accuracy. A unit is realized by reference to an arbitrary material standard or to natural phenomena including physical and atomic constants. The term ‘standard’ is applied to a piece of equipment having a known measure of physical quantity. For instance, the fundamental unit of mass in the SI system is the kilogram, defined as the mass of the cubic decimeter of water at its temperature of maximum of 4°C. This unit of mass is represented by a material standard; the mass of the international prototype kilogram consisting of a platinum-iridium hollow cylinder. This unit is preserved at the International Bureau of Weights and Measures at Sevres, in France and is the material representation of the Kilogram. Similar standards have been developed for other units of measurement, including the fundamental units as well as for some of the derived mechanical and electrical units.

Classifications of Standards

Standards can be classified as:

• International standards.
• Primary standards.
• Secondary standards.
• Working standards.

International Standards

International standards are defined by international agreement and are kept at the International Bureau of Weights and Measures in Sevres, France and are not available to the ordinary user utilizing measuring instruments for purposes of comparison or calibration. They represent certain units of measurement to the closest possible accuracy that production and measurement technology allow. International standards are periodically checked and evaluated by absolute measurements in terms of the fundamental units.

The table below illustrates basic SI units, Quantities and Symbols:

Table 1: Measurement Quantities, SI Units and Symbols

Quantity	Unit	Symbol
Length	Meter	m
Time	Second	s
Luminous Intensity	Candela	cd
Thermodynamic Temperature	Kelvin	K
Electric Current	Ampere	A

Primary Standards

Primary standards are maintained by national standards laboratories in different countries around the world. Primary standards, representing some of the fundamental physical and electrical units, as well as some of the derived mechanical and electrical quantities or units are independently measured and calibrated by absolute measurements at various national laboratories. The results of these measurements are compared with each other, leading to a world average figure for the primary standard. Primary standards are not available for use outside the national laboratories. One of the main uses of primary standards is the verification and calibration of secondary standards.

Secondary Standards

Secondary standards are reference standards which are initially calibrated from primary standards and then used in industry and research laboratories on a daily basis to calibrate their working standards, which are in turn used on daily basis to check and calibrate working laboratory instruments.

These standards are maintained by the particular involved industry and are checked locally against other reference standards in the area. The responsibility for maintenance and calibration rests entirely with the industrial laboratory itself. Secondary standards are generally sent to the national standards laboratory on a periodic basis for calibration and comparison against the primary standards. They are returned to the industrial user with a certification of their measured value in terms of the primary standard.

Working Standards 

Working standards are in actual fact the principle tools of a typical measurement laboratory. They are calibrated using Secondary Standards. They are used to check and calibrate general laboratory instruments for accuracy and performance or to perform comparison measurements in industrial applications. A manufacturer of precision resistances, for instance, may use a standard resistor in the quality control department of the plant to check his testing equipment. In this case, the manufacturer verifies that his measurement setup performs its function within the required limits of accuracy.

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