A System for High-Temperature Homogeneity Scanning of Noble-Metal Thermocouples

Noble-metal thermocouples are amongst the most widely used thermocouples for high-temperature process measurement and as references. Although they are less susceptible to inhomogeneity effects than the more-common base-metal thermocouples, inhomogeneity is still the major source of uncertainty. Currently, most estimates of the uncertainty due to inhomogeneity are based on thermocouple specifications or historical performance of similar thermocouples. It is not common for the inhomogeneity to be measured directly, in part because there is no accepted method for measuring the inhomogeneities, and in part because there is no conclusive evidence linking the magnitude of inhomogeneities determined at the scanning temperature to the effects of the same inhomogeneities at other temperatures. This paper describes an inhomogeneity scanner able to be fitted to sodium heat-pipe furnaces to operate between and . Comparison of scans made at demonstrates the scalability of some types of inhomogeneity in Type S and R thermocouples. It is concluded that for Type R and S thermocouples, a robust uncertainty assessment can be obtained from a scan made at a single temperature.