Characterising a dry-block calibrator
Before using a dry block for calibration, measure its axial and radial gradients, stability, loading effect and the effect of insert wear — and repeat periodically.
A dry-block calibrator is a heated (or cooled) metal block with wells for the reference and the units under test. Its display is not the calibration reference; an external reference thermometer in a well is. The block's job is to hold a stable, uniform temperature — and its performance must be measured, not assumed.
Tests from EURAMET cg-13
- Axial homogeneity: reference at full immersion vs withdrawn by 20, 40, 60 mm.
- Radial homogeneity: two references swapped between wells.
- Stability: readings over a period at least as long as a calibration.
- Loading effect: gradient with all wells occupied vs one.
- Heat conduction (stem effect): with a probe of different diameter or a thermally conductive probe.
- Hysteresis of the block's control after approaching from above and below.
Typical results for a good block at 100 °C: axial gradient 0.05–0.2 °C over 40 mm, radial 0.02–0.1 °C. These numbers dominate the budget for loggers with fine resolution.
Frequently asked questions
- Can the block's built-in reference be used as the calibration reference?
- Only if the built-in reference is itself calibrated with stated uncertainty and the block's gradients are characterised. Most laboratories use an external reference for traceability clarity.
References
- [1]EURAMET cg-13: Calibration of Temperature Block Calibrators
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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