Six gold platinum thermocouples were evaluated in respect of their repeatabilities at the freezing point of silver. Short term repeatabilities determined from measurement sequences at the laboratories were generally found to be better than 0.4 uV or 0.02 C. Reproducibilities between the laboratories varied between 2 uV or 0.1 C for two thermocouples and 13 uV or 0.5 C to 21.7 uV or 0.8 C for the remaining four thermocouples. The immersion characteristics were also checked and it was found that the effect of withdrawing a thermocouple from the freezing point cell generally caused a change in the electromotive force (emf) output equivalent to about 1 uV/cm or 0.04 C over the first few centimetres. Some measurements made at the freezing points of aluminium and tin showed short term repeatabilities similar to the measurements at silver: 0.2 uV or 0.01 C at aluminium and 0.2 uV or 0.02 C at tin. The repeatability at the tin point for measurements alternated with high temperature heat treatment of the thermocouples was 0.4 uV or 0.04 C.
Temperatures between 1000 C and 1600 C are measured in industry by means of platinum-rhodium thermocouples whenever accuracy is required since Chromel Alumel thermocouples used in the lower temperature range are not sufficiently stable. Pyrometers cannot provide accurate measurements unless the emissivity of the surface is very accurately known. Noble metal thermocouples themselves need periodic recalibration because there are a number of damage mechanisms that can cause drifting.
The aim of this project was to evaluate the stability and repeatability of platinum-gold thermocouples that recent work outside Europe has indicated might be useful as new transfer standards between the primary metrology laboratories and the industry calibration laboratories. They appear to be more stable and more accurate and therefore would both facilitate the task of industry laboratories and reduce the work of primary metrology laboratories.
The reproducibility of the measurements with the platinum-gold thermocouples was not significantly better than would be expected with platinum-rhodium thermocouples with differences between the laboratories ranging from 2uV (0.1 C) to 20uV (1 C) depending upon the origin of the thermocuples. However the long term stability of platinum-gold thermocouples is still expected to be better than plutonium-rhodium ones.
Funding SchemeCSC - Cost-sharing contracts
TW11 0LW Teddington