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

Abstract

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 °C to 1100 °C, with 11 participating countries. The use of type K material above approximately 200 °C 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 °C; the standard deviation of the thermocouple emf values of multiple cuts tested at NIST was 2.7 µV 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 °C 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.Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the SIM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

A Regional Comparison of Calibration Results for Type K Thermocouple Wire from (100 to 1,100) °C

“…where S(t) is the Seebeck coefficient at the nominal test temperature t. Because of their length, tables of the bilateral differences and associated uncertainties are omitted from the present paper and are given only in the final report for the comparison [3]. Section 5 gives a statistical summary of the results.…”

“…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 the outliers occur at temperatures of 800 • C and below, which suggests that drift of the type K wire due to high-temperature oxidation did not cause changes in the thermocouple emf comparable to or larger than the claimed uncertainties.…”