Standard

            All the instruments are calibrated at the time manufacture against a measurement standard. A standard of measurement is a physical representation of a unit of measurement. A standard means known accurate measure of a physical quantity. Other physical quantities are compared with the standards to obtain their values.

          A unit is realized by reference to an arbitrary material standard or to natural phenomenon including physical and atomic constants. For example, the fundamental unit of mass i.e.kilogramme, defined as the mass of a cubic decimeter of water as its temperature of maximum density of 4° C. This unit is represented by material standard i.e. by the mass of international prototype kilogram, consisting of a platinum-iridium alloy cylinder which is preserved at the International Bureau of Weights and measures at Severus, near Paris and is the material representation of the unit kilogram. The unit of length i.e. meters is represented by the distance between two fine lines engraved on gold plugs near the ends of a platinum–iridium   alloy at 0°c and mechanically supported in a prescribed manner. Similarly, for all the units including fundamental and derived units, the different standards have been developed. All these standards have been developed. All these standards are preserved at the International Bureau of Weights and Measures at Severes, near Paris.

      The different types of standards are classified as

1.     International standards

2.    Primary standards

3.    Secondary standards

4.    Working standards

Let us discuss in brief, each of these categories of standards. 

  International standards

     International standards are defined as the international agreement. These standards, as motioned above are mentioned at the International Bureau of Weights and Measures and are periodically evaluated and checked by absolute measurements in terms of fundamental units of physics. These international standards are not available to the ordinary users for the calibration purpose. For the improvements in the accuracy of absolute measurements, the international units replaced by the absolute units in 1948. Absolute units are more accurate than the international units.

Primary standards

                    These are highly accurate standards, which can be used as ultimate reference standards. These primary standards are maintained at National Standard Laboratories in different countries. These standards representing fundamental units as well as some electrical and mechanical derived units are calibrated independently by absolute measurements at each of the national laboratories. These are not available for use, outside the national laboratories.

                    The main function of the primary standards is the calibration and verification of secondary standards.

Secondary standards

                    As mentioned above, the primary standards are not available for the use outside the national laboratories. These various industries need some reference standards. So, to protect highly accurate primary standards the secondary standards are maintained, which are designed and constructed from the absolute standards. These are used by the measurement and calibration laboratories in industries and are maintained by the particular industry to which they belong. Each industry has its own standards.

Working standards

                    These are the basic tools of a measurement laboratory and are used to check and calibrate the instruments used in laboratory for accuracy and the performance. For example, the resistor manufacturing industry maintains a standard resistor in the laboratory for checking the values of the manufactured resistors. The manufacture verifies that the values of the manufactured resistors as well within the specified accuracy limits. Thus, the working standards are somewhat less accurate than the primary standards. Thus the working standards are used to check and calibrate general laboratory instruments for accuracy and performance.