Change in inhomogeneity with temperature between 180 °C and 950 °C in base-metal thermocouples

Abstract

In order to investigate the temperature dependence of thermoelectric inhomogeneity in base-metal thermocouples, thermoelectric scan tests were performed at several temperatures using mineral-insulated metal-sheathed thermocouples of types K, N, E and J. To induce a severe inhomogeneity in the thermoelements, the thermocouples were heat-treated for 1000 h: types K and N at 950 °C, E and J at 750 °C. After the heat treatments, type K and N thermocouples were scanned at 180 °C in a stirred-liquid bath, and at 550 °C, 750 °C and 1000 °C in a sodium heat-pipe furnace. The measurement for the type E and J thermocouples was done only up to 750 °C. From the scan results, we found that the thermoelectric inhomogeneity, expressed as a ratio of maximum emf change within the working segment to the total emf generated, decreased at high temperature. We concluded that some base-metal thermocouples might have temperature-dependent inhomogeneity; therefore, applying the inhomogeneity value obtained at low temperature to assess the calibration uncertainty at higher temperatures may result in overestimation of the uncertainty.