Procedure for an Investigation of Drift Behavior of Noble Metal Thermocouples at High Temperature

Abstract

This paper presents the effects of short-term and long-term temperature exposure on noble metal thermocouples in the range from 1000 °C to 1720 °C in oxidizing atmosphere (air). As thermocouples voltage output depends on the wire material of which the sensors are constructed, high-temperature and long continuous exposure to limit temperatures can introduce changes to the materials composition and structure. This can result in drift of generated voltage independent of the thermal environment and into reduced lifetime of the sensors. The intensions of conducted measurements were to determine the drift and lifetime of commonly available alumina-sheathed noble metal thermocouples and furthermore to establish traceable techniques to enable lifetime testing and thermoelectric stability evaluation of noble metal thermocouples at high temperatures. Result presented within this paper is not intended to test the capabilities of thermocouple Types B, R and S in general, but to show a possible behavior in the above-specified temperature conditions. This work puts more emphasis on the procedures that can be used for regular checks of intensely used thermocouple. Results obtained by this study show that the long-term temperature drift of Type B thermocouples at 1600 °C, and R-, S-type thermocouples at 1000 °C are much smaller than thermoelectric stability declared by IEC 60584-1:2013 standard (International standard IEC 60584-1:2013, Thermocouples—Part 1: EMF specifications and tolerances, 2013) tolerance classes after exposure to thermal stress up to 4 months. More specifically tolerance for class 1 thermocouple Types R, S from 0 °C up to 1100 °C is ± 1 °C and for class 2 tolerance for Type B at 1600 °C is ± 1.5 °C. The short-term thermoelectric stability of R- and S-type thermocouples exposed to 1600 °C for repeated 8 h periods has been within 2 °C when measured by comparison with a reference Pt–Pd thermocouple at 960 °C. The short-term thermoelectric stability of B-type thermocouple when exposed to 1720 °C for repetition of 8 h has been within 1 °C.