How thermocouples are calibrated
Thermocouples measure with the whole wire, not just the tip — which is why immersion, homogeneity and the reference junction dominate their calibration.
A thermocouple generates an EMF from the temperature gradient along its wires. Any inhomogeneity in the wire that sits in a gradient contributes an error, so the calibration result depends on how the thermocouple is immersed, not only on its tip temperature.
Method
- Comparison against a reference thermometer in a bath, furnace or dry block at a series of temperatures across the range.
- Reference junction held at 0 °C (ice point) or measured and compensated.
- Immersion depth chosen and recorded; repeated at two depths if inhomogeneity is to be assessed.
- EMF measured with a calibrated voltmeter; deviation from the reference function in the standard reported.
Uncertainty drivers
Inhomogeneity, reference-junction error, voltmeter uncertainty and, for base-metal types, drift with thermal history. Tolerance classes in the standard are manufacturing tolerances, not calibration uncertainties.
Frequently asked questions
- Why is thermocouple calibration less accurate than PRT calibration?
- Because inhomogeneity along the wire, reference-junction error and drift with thermal history add uncertainty that a PRT does not have; 0.5–2 °C is typical for used base-metal thermocouples versus millikelvin for PRTs.
- What is a reference junction in thermocouple calibration?
- The point where the thermocouple wires join the copper measuring circuit; it must be held at a known temperature (an ice point) or measured and compensated, or its error adds directly to the result.
References
- [1]EURAMET cg-8: Calibration of Thermocouples
- [2]IEC 60584-1 — Thermocouples: EMF specifications and tolerances
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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