Calibration is an essential process to be undertaken for each instrument and measuring system frequently. A reference standard atleast ten times more accurate than the instrument under test is normally used. Calibration is the process where. the test instru:dLent (the instrument to he calibrated) is compased with the standard instrument. It consists of. reading the standard and testinstruments simultaneously when the input quantity is held constant at several values over the range of the test instrument. The calibration is better carried out under the stipulated environme~tal conditions. All industrial grade instruments can be checked for accuracy in the laboratory by using the working standards.

Generally, certification of an instrument’ manufactured by ,an industry is  ‘undertaken by the National Physical Laboratory and. other authorized laboratories where the secondary standards and the working standards are kept. Generalized ‘performance characteristics of Instruments

Static calibration

The static performance characteristics are obtained by one form or another of the process ofstatic calibration.

·         In general, static calibration refers to a situation in which all inputs except one are kept at some constant values.

·         Then the one input under study is varied over some range of constant values, which causes the outputs to vary over some range of constant values.

·         The input-output relations developed ‘in this way comprise a static calibration valid under ,the stated’ constant conditions of all the other inputs,

·         This procedure may be repeated, by varying in turn each input considered to be’ of interest and thus developing a family of static input-output relations.

Procedure for calibration

1. Exarninc th.e construction of the instrument, and identify and list all the’ possible inputs,

2. Decide, which of the inputs will be significant in the application for which the instrument is to be calibrated.

a. Select the apparatus that will allow you to vary all the significant inputs over ‘the ranges considered necessary. Select standards to measure each inpu.   holding ‘some inputs constant, varying others and recording the outputs develop the desired static input-output relations.