Evaluation of Cobalt–Carbon and Palladium–Carbon Eutectic Point Cells for Thermocouple Calibration

Abstract

Two cobalt-carbon (Co-C) eutectic point (1,324 • C) cells and one palladium-carbon (Pd-C) eutectic point (1,492 • C) cell were constructed for thermocouple calibration. The lengths of the Co-C and Pd-C cells were 297 mm, 140 mm, and 140 mm, respectively. The melting and freezing plateaux at the Co-C and Pd-C eutectic points were observed using Pt/Pd thermocouples. The repeatability of the plateau, the effect of the surrounding temperature, and the temperature profile in the cell were measured, and the heat flux effect along the thermometer well was evaluated. When the plateaux of Co-C (297 mm height), Co-C (140 mm height), and Pd-C cells, were measured three times, seven times, and six times, respectively, the standard deviations of the melting points were 0.1 µV, 0.1 µV, and 0.4 µV, respectively. According to the temperature profiles along the thermometer well during the melting plateaux, it was found that the Pt/Pd thermocouple should be inserted at least 9.5 cm, 5 cm, and 6 cm below the surface of the eutectic alloys in the Co-C (297 mm height), Co-C (140 mm height), and Pd-C cells with the furnace set-point 16 • C above the melting point.

Comparison of Co–C and Pd–C Eutectic-Point Cells for Thermocouple Calibration Between LNE-Cnam and NMIJ

“…CoC-L-2 and CoC-NMIJ were designed and constructed at LNE-Cnam and NMIJ, respectively. A schematic diagram and construction procedure of CoC-L-2 are described in [9], while those of CoC-NMIJ are described in [5] as the "Co-C (Cell-B)" cell. Crucible pieces of CoC-L-2 were made of high-purity graphite (grade R6710) supplied by SGL Carbon and had an ash content of less than 5 ppm.…”

“…Schematic diagrams of PdC-LNE and PdC-NMIJ are described in [6] and [5], respectively. The purity and supplier of crucibles Palladium powder of 99.998+ % nominal purity (metals basis) was supplied by Alfa Aesar.…”

“…In this temperature range, the melting temperatures of cobaltcarbon (Co-C, 1324 • C) and palladium-carbon (Pd-C, 1492 • C) eutectic points are considered as useful practical fixed points [2][3][4][5][6][7], since they are ideally distributed between the freezing point of copper and the upper application limit of the Pt/Pd thermocouple (1500 • C), the stable contact sensor developed for accurate measurements [8].…”

Melting Temperature of High-Temperature Fixed Points for Thermocouple Calibrations

“…The melting points of the metal-carbon eutectic alloys Co-C (1324 • C) and Pd-C (1492 • C) are at particularly convenient temperatures for the calibration of thermocouples [1][2][3][4][5][6][7], being located between the freezing points of the conventionally used Cu (1085 • C) and Pd (1553 • C) fixed points. However, one pressing problem is still the assignment of an agreed temperature to these eutectic fixed points.…”

Palladium–Carbon Eutectic Fixed Point for Thermocouple Calibration

“…A number of studies have been carried out to investigate the use of eutectics for thermocouple calibration [3,4,[5][6][7][8][9][10][11][12]. Edler et al [6] used four Pt/Pd thermocouples calibrated at two national measurement institutes (NMIs) (PTB and Inmetro) to obtain 1323.81°C ± 0.27 C (k = 2) for the melting temperature of Co-C. Sadli et al [13] compiled temperatures from many radiation thermometry studies and suggested a temperature of 1324.0°C ± 0.6 C (k = 2) for Co-C.…”