Why To Calibrate

Calibration may be required called for the following reasons:
a new instrument, after an instrument has been repaired or modified, when a specified time period has elapsed, when a specified usage (operating hours) has elapsed, before and/or after a critical measurement, after an event, for example after an instrument has been exposed to a shock, vibration, or physical damage, which might potentially have compromised the integrity of its calibration, sudden changes in weather, whenever observations appear questionable or instrument indications do not match the output of surrogate instruments  as specified by a requirement, e.g., customer specification, instrument manufacturer recommendation.

Ambient temperature effects
It is important to periodically calibrate an instrument at a temperature close to that at which it will be operated. Even when a calibration is performed properly, there are other factors that can affect the accuracy of results. Factors, such as the ambient temperature, can introduce errors that may not be readily evident when testing unknown values. Electronic components used in instruments may be affected by changes in operating temperature. If an instrument is calibrated at one temperature and then operated at a significantly different temperature, the temperature-induced error can also degrade the results’ accuracy.