Drift of PR Thermocouples in Long-term Creep Testing

Abstract

Using more than 1000 creep testing machines, an extensive creep testingprogram for National Research Institute for Metals(NRIM), Creep Data Sheets has been conducted at NRIM since 1967. In conducting suchtesting program at high temperature, an essential practice is to control the test temperature within the required tolerance throughout the test duration. About 3400 PR (Pt/Pt·Rh) thermocouples have been used in the long-term creep testing in the temperature range from 400 to 1000°C up to 100000h.
The EMF (electromotive force) of PR thermocouples decreases with time at hight emperatures. Therefore, most of the thermocouples have been subjected to the calibration before and after the tests. Thus, accumulation has been made on the vast amounts of calibration data on the drift of EMF of PR thermocouples used in creep tests. This paper describes the practice on handling and the calibration of PR thermocouples at the Creep Testing Division of NRIM. Based on the numerous calibration data, anempirical equation to represent the maximum drifty Y(°C)with the temperature T(°C) and the time t(h)has been derived:
Y=-12.8+1.79(logt)+6.75×10^-2(logt)²+1.14×10^-2T.
This equation is available for l ≤Y≤5°C, 450 ≤T≤900°C, and t≤100000h.