(Bureau International des Poids et Mesures)
Calibration is the documented comparison of the measurement device to be calibrated against a traceable reference device.
(American Society for Quality)
The comparison of a measurement instrument or system of unverified accuracy to a measurement instrument or system of known accuracy to detect any variation from the required performance specification.
(International Society of Automation)
“A test during which known values of measurement are applied to the transducer and corresponding output readings are recorded under specified conditions.” The definition includes the capability to adjust the instrument to zero and to set the desired span.
(Fluke)
Calibration is the act of comparing a device under test (DUT) of an unknown value with a reference standard of a known value.
Note: By looking at all of the above definitions, it is clear that Calibration is basically a comparison of instruments with unknown accuracy with a reference traceable equipment. In some instances, adjustments can be done but this is not a good practice as it also affects instrument accuracy. Span and Zero adjustments can be performed which we would learn later in upcoming articles. Differences in readings can be noted down and mentioned on calibration certificates so that concerned end-user address those errors while taking measurements.