Drift: how instruments change between calibrations
Drift is a slow change in indication over time. It is estimated from calibration history and enters the budget as an uncertainty for the interval — or is corrected if it is predictable.
Every reference standard and working instrument drifts. A platinum resistance thermometer's ice-point resistance creeps; a thermocouple's Seebeck coefficient changes with thermal history; a humidity sensor's response shifts with contamination. Drift is why calibration intervals exist.
Putting it in the budget
- Compare the last two or more certificates for the same point; the change over the interval is the observed drift.
- For an instrument used across its interval, a rectangular component of half the expected change is a common treatment.
- If drift is steady and well characterised, a time-dependent correction may be applied with its own uncertainty.
A reference whose drift is not tracked silently moves every calibration it supports; a drift record is part of the traceability evidence.
Frequently asked questions
- Is drift the same as instability?
- Drift is a slow, largely monotonic change; short-term instability is fluctuation over minutes or hours. Both are uncertainty components but they are evaluated differently.
References
- [1]JCGM 200:2012 — International vocabulary of metrology (VIM), 3rd edition
- [2]JCGM 100:2008 — Guide to the expression of uncertainty in measurement (GUM)
General technical guidance written against the cited sources. It is not regulatory or legal advice and does not replace the applicable standard, guideline or a qualified reviewer's judgement.
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How to set calibration intervals
There is no universal interval. Set it from risk, manufacturer data and — above all — the instrument's own as-found history.
Drift as an uncertainty component
Between calibrations an instrument's error moves. Its history tells you how much; that amount enters the budget for any result obtained during the interval.