How often does an instrument really need calibrating?
The honest answer is that it depends, and here is what it depends on.
Annual calibration is a convention, not a requirement. The right interval is set by how much the instrument drifts, how much drift your process can absorb, and what happens if the instrument goes out of tolerance between checks.
Setting the interval from data
Start with annual calibration, record the as-found deviation at every calibration, and adjust. An instrument that comes back well within tolerance three years running can move to eighteen months. One that arrives out of tolerance needs a shorter interval and a look at its installation.
What needs a shorter interval regardless of the data
- An instrument that was dropped, knocked or repaired — calibrate before it returns to service, not on the next due date.
- A measurement used to release a batch to an external customer.
- An installation that has been moved. Calibration is valid for the installation, not just the instrument.
What justifies a longer interval
Three consecutive checks in which the as-found deviation is within a quarter of the tolerance are a reasonable basis for extending. Record the decision and the data behind it — an external auditor will ask, and an answer backed by data is the only one that is accepted.
