Readouts and bridges for reference thermometers
A reference PRT is only as good as the instrument reading it. Resolution, linearity, measurement current and calibration of the readout all enter the budget.
A reference thermometer's certificate is normally for the sensor alone, expressed in resistance or via ITS-90 coefficients. Converting a resistance into a temperature requires a readout whose own accuracy, resolution and stability are known. Precision thermometry readouts and resistance bridges span a range of performance from 0.01 °C to below 0.001 °C equivalent.
What to check
- Calibration of the readout's resistance measurement, or system calibration with the probe attached.
- Resolution and noise at the resistance of interest.
- Linearity across the range if calibrated at few points.
- Measurement current and whether the readout can halve it to check self-heating.
- Coefficient handling: ITS-90 or Callendar–Van Dusen coefficients entered and verified.
- Drift of the readout between calibrations.
A common error is to enter certificate coefficients into the readout and then treat the displayed temperature as if the readout contributed nothing.
Frequently asked questions
- Is a handheld thermometer adequate as a readout for a reference PRT?
- For 0.05–0.1 °C work, some are. For 0.01 °C work a laboratory readout or bridge is needed, and its uncertainty must be in the budget either way.
References
- [1]IEC 60751:2022 — Industrial platinum resistance thermometers and platinum temperature sensors
- [2]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 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.