On the Use of the Co-C Fixed Point for Calibration of Pt/Pd Thermocouples

At INRIM, different Co–C fixed-point cells have been constructed and investigated. Two cells of different design and volume and filled with highly pure cobalt (99.998%) were used to extend the fixed-point calibration of five Pt/Pd thermocouples that had been previously calibrated at the triple point of water and at the fixed points of In, Sn, Zn, Al, and Ag. The calibration at the Cu point was also added during this exercise. Because a previous calibration from 962 °C up to 1500°C against the local standard radiation thermometer was available, a comparison was possible with the Co–C fixed-point calibration. Agreement within 0.10 °C was found when the value of 1324.0 °C, the same value proposed for the Co–C point to be included as a secondary reference point of the ITS-90, was assumed.