Uncertainty budget for a thermocouple calibration
Thermocouples add inhomogeneity, reference-junction and extension-lead terms to the usual comparison budget — and inhomogeneity is often the largest.
- Reference thermometer: certificate, drift, readout, as for any comparison.
- Source: stability and gradients, with immersion effects larger for thermocouples' sheath conduction.
- Reference junction: ice-point realisation or compensation accuracy.
- EMF measurement: voltmeter uncertainty and resolution, divided by the Seebeck coefficient at the point.
- Inhomogeneity: evaluated by moving the thermocouple through a gradient; often ±0.5–2 °C for used base-metal thermocouples.
- Extension or compensating cable: tolerance class contribution if the cable is part of the calibrated item.
- Repeatability: Type A from repeated readings.
EURAMET cg-8 gives full examples. The practical lesson: a Type K thermocouple calibrated to 0.1 °C in a bath cannot be expected to measure to 0.1 °C in service if its inhomogeneity is 1 °C.
Frequently asked questions
- Should inhomogeneity be included if the thermocouple is calibrated at the immersion it will be used at?
- It should still be evaluated; if the immersion and gradient in use match calibration closely, the term can be smaller, but that condition must be documented.
References
- [1]EURAMET cg-8: Calibration of Thermocouples
- [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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Combine Type A and Type B components per the GUM: divisors by distribution, sensitivity coefficients, u_c, effective degrees of freedom and U at k = 2.
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