Editor’s choice
LOOKING AT TEMPERATURE SPECIFICALLY
Temperature is an ideal example to illustrate this distinction. Consider a dry block calibrator displaying a temperature of 0.000°C, precise to three decimal places. The number looks impressively precise, but such resolution is not accuracy, nor is it an indication of uncertainty. Factors such as temperature gradients within the block, stability over time, probe immersion depth, and the quality of any reference thermometer all influence the actual measurement uncertainty. In typical calibrations with the best dry block temperature calibrators, the combined uncertainty might be around ±0.1°C or higher, even though the display might suggest precision to a thousandth of a degree. At the highest levels of metrology, such as within national metrology institutes, uncertainty budgets can be dramatically reduced to lower than a thousandth of a degree, 1 mK. Not just in national labs, for instance, Isotech’s premium UKAS-accredited calibration services achieve uncertainties as low as ±0.07 mK (0.0007°C) at the water triple point (0.01°C) and ±0.25 mK (0.00025°C) at the gallium fixed point (29.7646°C). These uncertainties can reach such low levels, but they can never be eliminated; the VIM specifically reminds us that uncertainty is always a “non-negative parameter”.
WHY IT MATTERS
Engineers, regulators, and auditors should not be asking only whether a thermometer is “accurate”. Instead, they want to know how confident you can be in the measurement. That confidence depends on two key factors:
A documented, traceable chain to recognised national or international standards.
A carefully evaluated and properly stated measurement uncertainty.
An accredited UKAS certificate provides both essentials. It confirms not only that a recognised standard was used, but also that all relevant influences such as environmental conditions, equipment drift, staff competence, and more have been considered, resulting in a clearly stated, defensible uncertainty.
CONCLUSION
For anyone involved in temperature measurement, the key lesson is straightforward: treat claims of “accuracy” with caution and always seek clarity on the associated measurement uncertainty. In temperature calibration, as in all areas of measurement, this is the difference between data you can confidently rely on and figures that might fail to withstand scrutiny.
Isotech
isotech.co.uk Instrumentation Monthly August 2025 15
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