Design and Investigation of Pd–C Eutectic Fixed-Point Cells for Thermocouple Calibration at NMIA

Abstract

The calibration of Pt/Rh thermocouples up to 1560 \(^{\circ }\hbox {C}\) at NMIA currently uses the conventional ‘melt-wire technique’ to realize Gold (Au) and Palladium (Pd) melting points, resulting in the loss of 20 mm of wire from the junction end for each calibration. To avoid this loss, NMIA intends to replace the melt-wire technique with the use of miniature fixed-point cells. NMIA has established Copper (Cu) and Cobalt–Carbon (Co–C) eutectic cells for calibration of thermocouples to 1324 \(^{\circ }\hbox {C}\). To extend the calibration up to 1500 \(^{\circ }\hbox {C}\), miniature Palladium–Carbon (Pd–C) eutectic cells (1492 \(^{\circ }\hbox {C}\)) have been constructed and tested in collaboration with NMIJ, AIST. Although these cells are made of high-purity reference materials, careful consideration must be given to contamination introduced during the manufacture and filling of the crucibles and by their long-term use. These issues can only be assessed by measurement of cell-to-cell temperature differences within the ensemble of cells traceable to ITS-90. In the work presented here, 3 NMIA-design mini Pd–C cells were constructed: 1 at NMIA and 2 at NMIJ. These cells were compared, together with a “large” NMIJ Pd–C cell, using type-R, type-B and Pt/Pd thermocouples and radiation thermometry. Although the cells are found to be stable and repeatable, significant problems arising from migration of Pd to the thermocouples were identified.