This project is intended to compare the freezing temperatures of the copper fixed-point cells of the participants used for the calibration of contact thermometers. The intercomparison will be performed by using Pt/Pd thermocouples.
Thermocouples are no longer interpolating instruments of the International Temperature Scale of 1990 (ITS-90), but nevertheless they are used and calibrated in large numbers at National Metrology Institutes mostly at temperature fixed points, e.g. at the freezing point of copper. A copper fixed-point cell used for these calibrations is usually not suitable for measurements with radiation thermometers which assure the direct traceability to the ITS-90 by calibrations at either an Ag, Au, or Cu fixed-point cell, designed as black bodies. Otherwise, black bodies cannot be used for the calibration of contact thermometers. Therefore, a direct comparison of the different copper fixed-point cells used for contact thermometry by a preferably stable contact thermometer is necessary. Actual, the most suitable contact thermometer for such a comparison is the Pt/Pd thermocouple because of its better thermo-electrical properties compared to the common used Pt/Rh alloy thermocouples and other contact thermometers.
It is proposed to organize a intercomparison with PTB as the pilot laboratory for two rings of six or seven participants, respectively. For each ring one stable Pt/Pd thermocouple is provided by PTB. This Pt/Pd thermocouple has to be calibrated at the freezing point of copper of the participants according to a well defined calibration and annealing procedure. An additional measurement at the freezing point of silver has to be performed for the evaluation of the measurement results.
With this intercomparison valuable information on the properties of Pt/Pd thermocouples will achieved. Nevertheless, the aim of this intercomparison is not the characterization of Pt/Pd thermocouples.