Calibrations using ITS-90 fixed points
Calibrations are performed using ITS-90 fixed points, high temperature fixed-point and/or the wire-bridge technique. NPL’s platinum-rhodium (Pt-Rh) thermocouple calibration service is UKAS (ISO 17025) accredited and is typically achieved by measuring a customer thermocouple at two or three reference temperatures. The results are then used to create an equation to describe the thermocouple voltage (emf) versus ITS-90 temperature.
The resulting calibration equation applies over the whole temperature range of interest, and a formal certificate is provided. Type R and S thermocouples are only suitable for calibration up to 1600 °C, whereas Type B thermocouples may be calibrated to 1800 °C.
The calibration uncertainty (k = 2) can be as low as ±0.3 °C over the range 0 °C to 1100 °C, rising to ±0.55 °C at 1330 °C, ±0.72 °C at 1500 °C, and ±1.2 °C at 1800 °C.
The types of temperature references used include:
ITS-90 fixed points
These are the reference temperature artefacts, as specified in the ITS-90, for the practical realisation of the temperature scale. At NPL, the Zn (420 °C), Ag (962 °C) and Cu (1085 °C) fixed points are used for UKAS accredited thermocouple calibrations.
High temperature fixed-points
These artefacts provide reference temperatures for contact thermometry, above those specified in the ITS-90. They are made from a eutectic alloy of metal and carbon. At NPL, the Co-C (1324 °C) and Pd-C (1492 °C) fixed points are used for UKAS accredited thermocouple calibrations.
This is an invasive technique, which involved placing a piece of pure metal directly in/around the thermocouple hot junction. The metal is then melted and the phase-transition temperature is sensed directly. This technique has wider uncertainties, but can be used to reach temperatures otherwise unavailable. At NPL, Au (1064 °C), Pd (1555 °C) and Pt (1768 °C) wire bridges are used for UKAS accredited thermocouple calibrations.
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