Calibrations against NPL reference standard thermometers
NPL’s comparison calibration service is UKAS (ISO 17025) accredited. A liquid bath or “dry-block”, with a well-known temperature profile, are used to realise temperatures required. The results of the customer thermometer are compared against a reference thermometer. These results are then used to create an equation to describe the reading versus ITS-90 temperature. The resulting calibration equation applies over the whole temperature range of interest, and a formal certificate is provided.
The uncertainty is assessed for each instrument at the time of calibration, but it can be as low as ±0.003 °C for liquid bath comparisons, and ±0.037 °C for dry-block comparisons.
Liquid bath comparisons
UKAS (ISO17025) accredited calibrations against NPL standard thermometers can be carried out at -196 °C (boiling point of liquid nitrogen) and over the range -90 °C to +100 °C in liquid baths. The reference thermometers are calibrated in-house and are directly traceable to the NPL national standards. Although this is a comparatively fast technique compared to using temperature fixed points, the achievable uncertainty is still very low.
The dry-block comparison facility is available for calibration ranges which extend beyond +100 °C. This UKAS accredited technique can be used between +50 °C and +420 °C for resistance thermometers and between +50 °C and +700 °C for thermocouples. The customer instrument is compared against an NPL reference thermometer, but they are immersed in a metal block (with a known temperature profile) rather than a liquid bath. The use of the dry-block and the uncertainty assessment meet the requirements of Euramet cg-13 02/2015 'Calibration of Temperature Block Calibrators'.
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