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.