THERMOELECTRIC PROPERTIES OF A SELECTED LOT OF GOLD VERSUS PLATINUM THERMOCOUPLES

摘要

Gold versus platinum thermocouples have uncertainties superior to all other thermocouples over the range 0 ℃ to 1000 ℃. In this paper, we describe the properties of 21 specially-constructed gold versus platinum (Au/Pt) thermocouple thermometers, each calibrated on the International Temperature Scale of 1990. The best 18 thermometers were selected as a Standard Reference Material, with an expanded uncertainty (k=2) less than 9 mK from 0 ℃ to 962 ℃. Each thermocouple was calibrated at fixed points over the range 0 ℃ to 962 ℃. At both the aluminum (660.323 ℃) and silver (961.78 ℃) freezing points, the thermocouple inhomogeneity was characterized by measuring the immersion profiles in the fixed-point cells during a freeze. The resulting body of data for the best 18 thermocouples revealed that: a. Thermocouple inhomogeneity over a 10 cm range of immersion contributed only 2 mK to the standard uncertainty; b. The emf values for all 18 thermocouples differed by less than the equivalent of 8.5 mK at every fixed point; c. The average emf at each fixed point deviated from the NIST reference function by less than 14 mK; and d. For individual thermocouples, emf values measured at fixed points adjacent in temperature (e.g., aluminum and silver, or indium and tin) were correlated, indicating that the variance in emf values was partly due to metallurgical differences of the thermoelements. These data demonstrate the level of performance that can be achieved with careful wire selection and preparation.