Calibration Interval from Drift Calculator
Estimate a calibration interval from past calibration results: how long the instrument can drift from its as left error before it uses up the tolerance, with a safety margin.
- Drift rate = |as found − previous as left| ÷ months = 0.008333 %/month
- Room = tolerance × fraction − as left = 0.35 %
- Interval = room ÷ drift rate = 42 months
Drift Based Interval
Interval = (tolerance × fraction − |as left|) ÷ drift rate
This is a simple linear drift estimate. Formal interval methods (for example in ILAC G24) use the history of many calibrations and in or out of tolerance statistics. Use the fraction to keep a safety margin, such as 80 % of the tolerance.
Worked example
A transmitter found at +0.12 % after being left at +0.02 % 12 months earlier drifts 0.0083 % per month. With a 0.5 % tolerance, 80 % usable and a 0.05 % as left error, the interval estimate is 42 months, so a 12 month interval is well supported.
Frequently Asked Questions
How are calibration intervals set?
Initially from the manufacturer recommendation and criticality, then adjusted using calibration history, as described in ILAC G24.
What is drift?
The slow change of an instrument reading over time at the same input, found by comparing as found results with the previous as left results.
Should critical instruments use shorter intervals?
Yes. Safety instruments follow the proof test interval from the SIL calculation, which may be shorter than the drift estimate.
Learn more on the blog
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