In line with good metrology practices and the principles of ISO/IEC 17025, measuring instruments should be calibrated in the following situations:

1. Before first use (new equipment) to establish traceability and verify performance.
2. At scheduled intervals determined by the organization based on the instrument's stability, usage, and previous calibration history.
3. After maintenance, repair, or adjustment that may affect the instrument's measurement performance.
4. After relocation or transportation, particularly if the equipment has been subjected to shock, vibration, or environmental changes.
5. Following accidental damage, overload, or misuse that could affect measurement accuracy.
6. When measurement results are found to be inconsistent or questionable, or when there is reason to suspect the instrument is not performing correctly.
7. After prolonged periods of storage or inactivity, before returning the instrument to service.
8. Following exposure to adverse environmental conditions, such as extreme temperature, humidity, dust, or corrosive environments that may affect performance.
9. When required by applicable regulations, customer specifications, or quality management systems.
10. After a failed verification or intermediate check, where there is evidence that the instrument may no longer meet specified requirements.Before undertaking critical measurements where a high level of confidence in the measurement result is required