Abstract
Thermocouples are widely used temperature sensors and convenient high temperature transfer standards. However, the achievable accuracy is limited by the effects of inhomogeneity, drift and contamination. The work reported in this thesis focuses on approaches to reduce the uncertainty of temperature measurement with thermocouples up to 2000C. These approaches are qualification of new thermocouple types and the investigation of two approaches to self-calibration at high temperatures utilising recently developed high temperature fixed points whose transition temperatures are in excess of the Cu fixed point (1084C). Robust high performance elemental thermocouples (Pt/Pd) have been developed and a comparative study performed relative to more conventional types to quantitatively demonstrate the enhanced performance of the new thermocouple. It was found that the Pt/Pd thermocouple showed substantially better stability and lower homogeneity than the two type R thermocouples during a 500 h ageing programme at 1350C. The special type R thermocouple, which was given the same preparatory treatment as the Pt/Pd thermocouple, showed better stability and homogeneity than the normal type R thermocouple. The development of in-situ calibration methods is described. The initial development of the “integrated cell” method where the mini-fixed point cells are integrated and formed into a measurement junction was investigated. Four fixed point materials were investigated; Ag (962C) and the metal-carbon eutectic fixed point cells of Co-C (1324C), Fe-C (1153C) and Pd-C (1492C). It was found that this approach is significantly influenced by the thermal gradient within the furnace. Nonetheless, the experimental results show good repeatability of melting plateaux for monitoring calibration drift of the sensor in-situ. Because this approach is restricted to bare wire thermocouples it cannot be easily used above 1500C hence a different “immersion cell” approach was followed. In order to extend the possibility of self-calibration above 1500C, “immersion cells”, for the in-situ calibration of mineral insulated metal sheathed W/Re thermocouples (type C), were developed. The refractory metal W/Re thermocouple, is widely used to at least 2300C in industry and hence an appropriate subject for study. The development and construction of immersion cells made from metal-carbon eutectic fixed points; Co-C (1324C), Pd-C (1492C), and Ru-C (1954C) is discussed. A multi-mini cell for the W/Re thermocouple was also trialled. Given the significant drift observed with W/Re thermocouples it is clear that the immersion cell is required to reduce thermocouple temperature measurement uncertainty at high temperatures. These developments were supported by appropriate thermal modelling. This is described at the appropriate part of the relevant chapters.