Uncertainty from the calibration bath or dry block
The source's stability, axial and radial gradients, and (for dry blocks) loading and stem-conduction effects determine how well the reference and the unit under test see the same temperature.
In a comparison calibration the reference and unit under test are assumed to be at the same temperature. They are not. The source fluctuates in time and varies in space; the budget must include both.
Components
- Temporal stability: standard deviation of the source temperature over the acquisition window, evaluated from the reference readings.
- Radial (well-to-well) gradient: the difference between the reference well and the UUT well, from a characterisation with two references.
- Axial gradient: temperature variation along the immersion depth, relevant when sensors have different sensing lengths.
- Loading: in dry blocks, several probes or a large probe alter the gradient.
- Stem conduction: heat flow along the probe stem in dry blocks affects the sensing element.
EURAMET guidance on dry-block calibrators describes how to characterise these effects; the results should be recorded per source and per temperature range.
Frequently asked questions
- Is a stirred liquid bath always better?
- Metrologically, usually yes for uniformity and stability. Dry blocks are chosen for portability, speed and cleanliness; their larger gradients must then be characterised and budgeted.
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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