Calibration or recertification of metal detectors completed by supplier once a year.
Sorry to have to repeat myself but what (in yr case) does this mean ??
The point is that for many (most?) applications the primary calibration test is simply whether the unit maintains its sensitivity, ie rejects/kicks out the standard test piece which defines correct machine performance under the appropriate operating matrix/sample presentation conditions. I would expect the MD user to "continuously" validate this in routine usage.
The rest is frequently a "tune-up" bonus for the supplier / experts, particularly as many users are anyway unlikely to understand the technical details generated. It's a bit like the last page of yr HDTV operating manual.
Nonetheless, if it is found convenient to accumulate annual COCs for audit purposes, I suppose this may also be a secondary justification for the expenditure.
Rgds / Charles.C