1989 Volume 75 Issue 4 Pages 665-672
The drift (or decrease) of electromotive force (EMF) in the calibration of PR thermocouples used for long-time creep testing at various temperatures up to 100 000h in air is investigated. The magnitude of resistivity, ρ, which is considered to be one of indices to the inhomogeneity of thermocouples, and its distribution from the measuring junction are measured. The change in p together with the results of X-ray diffraction and spectroscopic analysis indicate that the inhomogeneity is very small in Pt but considerable in Pt-12.8%Rh limbs.
The oxides found on the surface of thermocouples with notable drifts are the mixture of Rh2O3 types. The effect of drift on the measured temperature is rather small if the ratio of immersion depth is less than unity (when it is more than unity, the scatter in the drift among thermocouples after virtually same times is pronounced). For the thermocouples used 30 000 to 50 000h at 1 000°C the drift is also responsible to the evaporation of Pt and Rh elements.
For the thermocouples with the immersion depth of 250 mm, the annealing treatment is effective to recover the drift provided that the test temperature is below 500°C ; above 500°C it is recommended to cut off a part of thermocouple limbs up to 100 mm from the measuring junction in order to get the complete recovery of EMF.
