Calibration Drift Projection
Project when an instrument's linear drift will breach tolerance and recommend the next cal interval.
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The engineering
A well-behaved instrument drifts roughly linearly between calibrations. Feed in the as-left error (where it sat leaving the last cal), the as-found error (where you caught it this cal), and the elapsed time, and this card fits a straight-line drift rate — then projects how long until that trend crosses the ±tolerance edge. Setting a guard band (e.g. 80% of tolerance) pulls the recommended interval in so the box is retired for cal before it ever produces an out-of-tolerance reading.
This is the 'reaction' half of a reliability-target interval scheme: one drift observation sets the initial interval, and successive as-found records tighten or relax it. Gotcha — a single as-found/as-left pair is a two-point line with no uncertainty. Don't stretch the interval on one favorable data point; NIST HB 145 assumes you accumulate history and treat a random adverse as-found as a signal to shorten, not average away.
Where this math comes from
Formal calibration-interval analysis grew out of Cold War missile and avionics test programs in the 1950s–60s, when the cost of pulling thousands of instruments on a fixed schedule collided with the cost of a bad measurement slipping through. The military standards (MIL-STD-45662 and its descendants) pushed labs from 'calibrate everything annually' toward interval adjustment driven by observed reliability.
The reference for U.S. commercial and legal metrology is NIST Handbook 145, the state-lab handbook, which codifies as-found/as-left recording and control-limit logic so that drift trends — not calendars — govern when an instrument comes off the bench. The linear projection here is the simplest honest version of that idea.
- 1962U.S. DoDMIL-C-45662 formalizes traceable calibration systems for defense hardware.
- 1980NCSL / metrology communityReliability-target interval-analysis methods (RP-1) mature into standard practice.
- 1986NIST (NBS)Handbook 145 pins as-found/as-left recording and control-limit interval logic for state labs.
See the full timeline of the math behind every calculator →
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