New Procedures and Capabilities for the Calibration of Cryogenic Resistance Thermometers at Nist

Abstract

Historically, cryogenic thermometer calibrations at national standards laboratories have been made with respect to several different, and somewhat incongruous, practical scales which lacked agreement with the thermodynamic temperature scale. The recent adoption of the International Temperature Scale of 1990 (ITS-90)[1] has significantly improved the scale reproducibility and thermodynamic agreement. With the advent of the ITS-90, the National Institute of Standards and Technology (NIST) has embarked on a program to directly realize the new scale from its definitions and to disseminate this scale to its user community. Two new facilities for the scale realization below 83.8 K and for customer calibrations between 0.65 K and 83.8 K are being constructed. We describe here the changes in the scale relevant to cryogenic users and the methods employed in comparison calibrations. These include comparisons of standard platinum resistance thermometers (SPRTs) of the capsule type between 13.8 K and 83.8 K, and of rhodium-iron resistance thermometers (RIRTs) and germanium resistance thermometers (GRTs) between 0.65 K and 30 K. The use of cryogenic fixed points for interpolating resistance thermometry in the range 13.8 K to 273.16 K is also described. The resulting improvements from this effort will be a single temperature scale for dissemination above 0.65 K which is a very good representaion of thermodynamic temperature.