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.