Calibration Interval

Two questions every gauge has to answer. Is it fine enough? — the old 10:1 rule says the instrument resolution should be a tenth of the tolerance (4:1 is the usual floor). How long until it drifts out? — straight-line drift against the tolerance you have left turns "it drifts a bit" into a date.

Linear drift is a crude model — real instruments drift in steps after knocks and temperature excursions. Set the first interval short, then extend it once you have a drift history. A guard band reserves part of the tolerance so the instrument stays usable right up to its due date.