The combined use of a gas-controlled heat pipe and a copper point to improve the calibration of thermocouples up to 1100 °C

The calibration of platinum-based thermocouples from 420 °C to 1100 °C is currently carried out at INRIM making use of two different apparatus: for temperatures below 930 °C, a potassium gas-controlled heat pipe (GCHP) is used, whereas a metal-block furnace is adopted for higher temperatures. The standard uncertainty of the reference temperature obtained in the lower temperature range is almost one order of magnitude better than in the higher temperature range. A sealed copper cell was investigated to see if it could be used to calibrate thermocouples above 930 °C with a lower uncertainty than our current procedures allowed. The cell was characterized with Type S and Pt/Pd thermocouples and with an HTPRT. The freezing plateaux were flat within 0.01 °C and lasted up to 1 h with a repeatability of 0.02 °C. The temperature of the cell was determined with a standard uncertainty of 0.04 °C. Hence, the copper cell was found to be superior to the comparator furnace for the calibration of platinum- based thermocouples because of the significant decrease in the uncertainty that it provides. An analysis was also carried out on the calibration of Pt/Pd thermocouples, and it was found that the combined use of the potassium GCHP and the Cu fixed-point cell is adequate to exploit the potential of these sensors in the range from 420 °C to 1084 °C. A comparison with a fixed-point calibration was also made which gave rise to agreement within 0.07 °C between the two approaches.