All measuring instruments are to prove themselves their ability to measure reliably and accurately. For this, the results of measurement are to be compared with higher standards which are traceable to national or international standards. The procedure of this is termed as calibration.
Calibration is thus a set of operation that establishes the relationship between the values that are indicated by measuring instrument and the corresponding known value of measurand.
Thus calibration of measuring instrument means introducing an accurately known sample of the variable that is to be measured and then observing the system’s response. Then the measuring instrument is checked and adjusted until its scale reads the introduced accurately known sample of the variable.
It should be further noted that an instrument is calibrated at one place and is put to use at some other place. Care should be taken to see that the instrument is used at a place where the environment has the same conditions as that of the place where the instrument was calibrated to ensure that the instrument gives correct readings.