Ice point, triple point of water and other fixed points
Fixed points realise defined temperatures without a calibrated reference. The ice point (0 °C, ~±0.002 °C) is accessible to any laboratory; the triple point of water (0.01 °C) is the anchor of ITS-90.
The ice point
- Shaved or crushed ice from pure water, packed in a Dewar and saturated with cold water so no free water sits at the bottom.
- Probe immersed to adequate depth, allowed to equilibrate; the reading should be stable to within resolution.
- Re-pack as the ice melts; a slush with floating ice is not an ice point.
- Common errors: tap water, large ice cubes, insufficient immersion, probe touching the Dewar.
Triple point of water
A sealed glass cell with pure water, ice and vapour in equilibrium defines 273.16 K exactly in ITS-90. It is used to measure R(0.01 °C) of a reference PRT before and after use, so its drift can be tracked and its coefficients corrected. Preparation requires an ice mantle and a maintenance bath; the reward is a reference check with negligible uncertainty.
Frequently asked questions
- How often should a reference PRT be ice-point checked?
- Before and after each calibration session is common practice; at minimum, on a schedule that catches drift well within the calibration interval.
References
- [1]IEC 60751:2022 — Industrial platinum resistance thermometers and platinum temperature sensors
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.
Have a question on this topic?
Ask ValiTrac AI and see the evidence and calculation behind the answer.
Ask ValiTrac AIFree calculator: Pt100 converter
Convert between resistance and temperature for Pt100 / Pt1000 sensors, with class AA, A, B and C tolerances at that temperature.
Open the calculatorCalibration on ValiTrac
Support calibration workflows, uncertainty budgets and certificate generation.
See the calibration workflowRelated articles
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.
Selecting a reference thermometer
The reference sets the floor of your uncertainty. Choose it for stability and a calibration uncertainty comfortably below what you need to deliver.