LONG-TERM DRIFT IN MINERAL-INSULATED NICROSIL-SHEATHED TYPE-K THERMOCOUPLES

The drift in signal for Nicrosil-sheathed type K thermocouple probes has been measured over 1000 h at temperatures up to 1300 °C. It is shown that thermoelectric hysteresis is the dominant process for temperatures below 800 °C, giving drifts of up to 1.5 °C. At higher temperatures the drift is influenced by irreversible changes in Seeback coefficient and is below 3 °C when A1 and Mn are present in the negative thermoelement and Nicrosil is used as the sheathing alloy. This represents a ten-fold improvement in stability over conventional type K probes. Since hysteresis in type N alloys extends over twice the temperature range of that in type K, some comparisons have been made between the two types at 500 and 1000 °C. The drift is larger in the type N probes, being + 5 °C at 1000 °C. © 1990.