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.
A reference thermometer for comparison calibration should have an uncertainty several times smaller than the target uncertainty of the calibrations it supports, and a stability that keeps it there between its own calibrations.
Options
- Standard platinum resistance thermometers (SPRTs) for the lowest uncertainties in baths; fragile and expensive.
- High-quality industrial PRTs (often described as secondary or reference PRTs) for most laboratory work; robust, low drift.
- Thermistor references for narrow ranges near ambient, with excellent resolution.
- Reference thermocouples for high temperatures.
Beyond the sensor
The readout (bridge or digital thermometer) is part of the reference; its calibration and resolution enter the budget. Keep a history of the reference's calibration values — a drifting reference quietly moves every calibration it supports.
Frequently asked questions
- What is the difference between an SPRT and a reference PRT?
- An SPRT is a strain-free, high-purity platinum thermometer meeting ITS-90 criteria, capable of millikelvin uncertainty but fragile; a reference PRT is a rugged industrial-style sensor with low drift, typically calibrated to 0.01–0.03 °C.
- How often should a reference thermometer be recalibrated?
- Commonly every one to two years, supported by ice-point or triple-point checks between calibrations; the interval is set from drift history.
References
- [1]ISO/IEC 17025:2017 — General requirements for the competence of testing and calibration laboratories
- [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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The reference thermometer's contribution to uncertainty
The reference contributes its certificate uncertainty, its drift since calibration, and any interpolation or self-heating effects. Together they set the floor of what can be claimed.
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.