Overcoming Inhomogeneity and Hysteresis Limitations of Type R Thermocouples in an IInternational Comparison

Type R thermocouples are widely used and convenient high-temperature transfer standards; however, the achievable accuracy is limited by the effects of inhomogeneity and hysteresis. In this article, we summarize the results of the recent iInternational comparison APMP-T-S1-04 and discuss the results of the thermoelectric scanning, spatially resolved over the length of the thermocouples. The thermoelectric signatures show both reversible (hysteresis) and irreversible inhomogeneities introduced by the calibration processes used by the participants. The results demonstrate that although the reversible hysteresis of Type R thermocouples limits their performance as a transfer standard in thermometry, this can be managed by appropriate design of the comparison protocol. By performing all calibrations from lower to higher temperatures from an initial 450◦C anneal state, a pilot laboratory reproducibility of typically 0.03◦C (k = 2) and a reference value uncertainty of 0.03– 0.06◦C (at k = 2) over 0–1,100◦C were achieved. This allowed statistically significant testing of the calibration capabilities of all the participants.