Calibration may be called for:
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 had a shock, vibration, or has been exposed to an adverse condition which potentially may have put it out of calibration or damage
A measuring device should be calibrated:
According to recommendation of the manufacturer.
After any mechanical or electrical shock.
Periodically (annually, quarterly, monthly)