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.
An ice point is a slurry of shaved ice made from clean water, in an insulated container, with just enough water to fill the gaps and no free water at the bottom. Made carefully it realises 0 °C closely enough for most industrial purposes.
Using it
- Immerse the probe to sufficient depth in the slurry, away from the walls, and allow it to stabilise.
- Compare with the value recorded at the last calibration (or with the certificate's 0 °C correction).
- A shift larger than the check's uncertainty indicates drift or damage and should trigger investigation.
The check does not replace calibration — it tests one point — but a PRT that holds its ice point is unlikely to have drifted grossly elsewhere, and a record of checks strengthens the case for the calibration interval.
Frequently asked questions
- How do you make an ice point?
- Shave or crush ice made from distilled or deionised water, pack it in an insulated container, add just enough cold clean water to fill the gaps, and immerse the probe deeply, away from the walls, until stable.
- How accurate is an ice point check?
- A careful ice bath is within about 0.002–0.005 °C of 0 °C. Common errors — tap water, large cubes, free water, shallow immersion — can add tens of millikelvin.
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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