Design and Development of Miniature Pd–C Eutectic Fixed Point at CSIR-NPL

Abstract

The calibration of thermocouples (TCs) up to 1600 °C at CSIR-NPL, at present, uses the ITS-90 defined fixed points and the secondary fixed points such as Au and Pd using the ‘wire-bridge technique.’ In this technique, there is a loss of 2 to 3 cm of wire from the hot junction end of TCs for each calibration. To prevent this wire loss, CSIR-NPL intends to replace the wire-bridge-technique with the use of metal–carbon eutectic fixed-point cells. CSIR-NPL has established Fe–C, Co–C, and Ni–C eutectic cells for calibration of thermocouples to 1329 °C. In this paper, we present the design, material used, and construction of miniature Pd–C (1492 °C) eutectic cell with in-house development efforts at CSIR-NPL. Particular attention has been given to the repeatability and long-term stability of the phase transition temperatures using Type-S TC and the mechanical robustness of the miniature Pd–C eutectic fixed-point cell. The temperature was assigned to a miniature Pd–C cell using a Type-S TC calibrated using ITS-90 fixed points at Sn, Zn, Al, Ag, Cu, and Pd wire. The assigned temperature to the miniature Pd–C eutectic fixed-point cell is 1491.45 °C with expanded uncertainty of ± 0.64 °C (k = 2). For robustness and long-term stability, the Pd–C fixed point performance was evaluated from Nov-2022 to Sep-2024, and the observed standard deviations were 0.35 µV (0.03 °C) and 2.07 µV (0.17 °C) for melting and freezing profiles, respectively. The miniature Pd–C eutectic fixed-point cell is being used for thermocouple calibration services.