Effect of bushings in thermometric fixed-point cells

Abstract

Bushings are tubular inserts that can be used with temperature fixed-point cells for the calibration of standard platinum resistance thermometers in accordance with the International Temperature Scale of 1990. They are made out of different materials such as glass and various metals with high thermal conductivities. Their function is to increase the thermal contact between the thermometer’s sensor and the phase boundary in the thermometer well of the cell. In the paper, the effect of bushings on the self-heating and on the immersion profiles of thermometers in fixed-point cells was researched. Three different thermometers were used: one fused silica sheathed and two Inconel sheathed. The self-heating was measured in the temperature range from the mercury triple-point to the zinc freezing-point with and without metal bushings: brass, aluminum and copper. The immersion profiles were measured in the zinc cell with and without an aluminum and a glass bushing. Besides experimental measurements, some qualitative results of numerical modeling are also presented. The bushings considerably lower the self-heating, in some cases even by more than 50%. But, on the other hand, bushings seem to increase the measured temperature, worsen the immersion profile and act as a radiation shield in the gap between the thermometer and the fixed-point cell at higher temperatures, where radiation is the dominant mode of heat transfer.