Final report on SIM supplementary comparison 3.9: Type K thermocouple wire over the range 100 DGC to 1100 DGC

Type K thermocouples are one of the most commonly used temperature sensors in industry. The skills, personnel, and facilities necessary for calibrating type K thermocouples are also applicable to the calibration of other base metal thermocouples, and, to a lesser extent, calibration of platinum-rhodium alloy thermocouples. Under the auspices of the Inter-american Metrology System (SIM), the National Institute of Standards and Technology (NIST) initiated a regional comparison for type K thermocouples from 100 DGC to 1100 DGC, with 11 participating countries. The use of type K material above approximately 200 DGC is considered destructive. Therefore, each participating laboratory was sent new, unused wire from a lot of material characterized by NIST. The uniformity of the lot was remarkable, especially at temperatures above 500 DGC; the standard deviation of the thermocouple emf values of multiple cuts tested at NIST was 2.7 mV or less over the full temperature range. The high uniformity eliminated any need to correct for variations of the transfer standard among the laboratories, greatly simplifying the analysis. The level of agreement among the laboratories'results was quite good. Of the 380 total bilateral combinations of the data at the eight test temperatures, only 13 (i.e., 3.4% of all combinations) are outside the k = 2 limits, and of these 13, only 3 are outside k = 3 limits. All of the outliers occur at temperatures of 800 DGC and below, which suggests that drift of the type K wire due to high-temperature oxidation did not cause changes in thermocouple emf comparable to or larger than the claimed uncertainties.