Key Comparison: Realisations of the ITS-90 scale over the range from the Ag fixed-point to 2600 °C

Project Description

In the recent European projects “Implementing the new kelvin - InK” and “Realising the new kelvin – Real-K”, a thermodynamic temperature has definitely been assigned to the freezing plateau of the Cu fixed point (1084.65 °C) and to the inflection points of the melting plateaus of the following high temperature fixed-points: Fe-C (1153.77 °C), Co-C (1324.24 °C), Pd-C (1491.90 °C), Pt-C (1738.28 °C), Ru-C (1953.84 °C), Re-C (2474.69 °C) and WC-C (2747.70 °C).

This research provides the opportunity to use high temperature fixed points (HTFPs) to perform a temperature scale directly linked to the thermodynamic temperature. The use of multi fixed-points is an alternative method to the ITS-90 allowing the calibration of radiation thermometers over a wide temperature range without the need to characterise its spectral responsivity or linearity.

This project aims to support cooperation between European NMIs. It is conducted as part of the Joint Research Project MultiFixRad 22RPT03/r02 “Improving the realisation of the kelvin by multiple fixed point radiation thermometry” and consist of a comparison of temperature scale realisations from the Ag fixed-point to 2600 °C. The reference instrument is a near-infrared radiation thermometer whose stability will be monitored throughout the comparison with a transportable copper fixed-point.

The traceability to the CCT-K10 will be established by the intermediate of LNE-LCM/Cnam.

This project is the final step of the Joint Research Project “Improving the realisation of the kelvin by multiple fixed point radiation thermometry” MultiFixRad n° 22RPT03/r02.

Progress Report 2026-03-09

The comparison covers temperature scale realisations from the Ag fixed point up to 2600 °C. The reference instrument is a near-infrared radiation thermometer whose stability is monitored throughout the comparison using a transportable Cu fixed point.
The measurements are organised in two loops involving seven laboratories, with an intermediate return to the pilot laboratory. The first loop is currently nearing completion.

Prior to the circulation, the radiation thermometer was calibrated at LNE-LCM/CNAM against Ag and Cu fixed points and a blackbody cavity set at temperatures up to 2600 °C.
Particular attention was given to the implementation of the transportable Cu fixed point in order to monitor the stability of the pyrometer throughout the comparison. For this purpose, two reference Cu cells (including one spare) were constructed and adapted to the heater element of the circulating furnace. The transportable fixed-point was subsequently compared with the LNE-LCM/CNAM’s national reference.
With these arrangements, the pyrometer stability can be controlled with an uncertainty better than 0.1 °C at 2600 °C.

The circulating pyrometer has already been calibrated at SMU (Slovakia) and RISE (Sweden) and is currently at Tubitak UME (Türkiye).
It will then be returned to LNE-LCM/CNAM to complete the first loop.

The second loop will subsequently involve UL (Slovenia), CMI (Czechia), DFM (Denmark) and JV (Norway) before the final return to LNE-LCM/CNAM.