Calibration, verification, adjustment
The three are routinely confused. Calibration measures and documents the deviation of the instrument against a reference; it does not correct the instrument. Adjustment intervenes to remove that deviation. Verification — a gas detector's daily bump test, for example — checks quickly that the instrument still responds. A daily check does not replace calibration.
What sets the interval
The interval comes from the manufacturer's advice, how heavily the instrument is used, how harsh its environment is, and the history of its deviations. If the deviation is consistently far inside the limit the interval can be extended; if it creeps towards the limit it is shortened. That decision must be recorded, because what an auditor asks about is not the interval itself but how it was chosen.
In Optifora the instrument record, its calibration date and the measurements taken with it are linked; a measurement taken with an out-of-date instrument is flagged.