Maintaining calibration equipment between calibrations
References drift, baths contaminate, inserts wear, batteries die. A maintenance plan with intermediate checks keeps equipment within its assumed performance and catches failures early.
ISO/IEC 17025 clause 6.4 requires procedures for handling, transport, storage, use and planned maintenance of equipment, and intermediate checks where needed to maintain confidence in performance. For temperature equipment the plan is short but must be followed.
A minimal plan
- Reference PRT: ice-point check before each use or weekly; record and chart the value; investigate a shift of more than the certificate uncertainty.
- Dry block: annual re-characterisation of gradients; insert inspection for wear and contamination.
- Liquid bath: fluid level, contamination and viscosity checked; fluid change interval defined.
- Readouts: zero and reference-resistor checks.
- Loggers: battery replacement schedule; clock check against a reference time.
- All: transport cases, temperature limits for storage, and a rule that a dropped reference is checked before reuse.
A control chart of ice-point readings is the single most informative maintenance record a temperature laboratory keeps.
Frequently asked questions
- What if an intermediate check shows a shift?
- Quarantine the reference, evaluate the impact on results since the last good check under the nonconforming-work procedure, and recalibrate.
References
- [1]ISO/IEC 17025:2017 — General requirements for the competence of testing and calibration laboratories
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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The ice-point check
A properly made ice bath gives 0 °C to within a few millikelvin — a cheap, repeatable way to watch a PRT for drift between calibrations.
Equipment requirements in ISO/IEC 17025 (clause 6.4)
Equipment must be fit for purpose, calibrated where it affects results, checked, protected from adjustment, and fully recorded — including software.