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.
- Calibration uncertainty: from the certificate, at the relevant point, divided by its coverage factor.
- Drift: estimated from calibration history; treated as rectangular with half-width equal to the expected change over the interval, or as a correction with uncertainty if predictable.
- Interpolation: if the reference was calibrated at 0, 20, 50 °C and used at 35 °C, the fit residuals or a stated interpolation uncertainty apply.
- Self-heating: for resistance thermometers, the difference between the measurement current used at calibration and in use.
- Readout: the indicator or bridge used to read the reference has its own calibration and resolution.
- Immersion: if immersion differs from the calibration condition.
A reference PRT with a 0.01 °C certificate, 0.01 °C annual drift, and a 0.005 °C readout contributes roughly u ≈ 0.009 °C — around 0.02 °C at k = 2 before any bath or UUT effects.
Frequently asked questions
- Can a reference with a 0.03 °C certificate support a 0.05 °C calibration claim?
- Only if all other contributions combine to less than about 0.04 °C at k = 2, which is unlikely once drift, bath and UUT effects are included. Budget it before promising it.
References
- [1]JCGM 100:2008 — Guide to the expression of uncertainty in measurement (GUM)
- [2]EA-4/02 M: Evaluation of the Uncertainty of Measurement in Calibration
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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