Long-Term Stability of Germanium Resistance Thermometers

Abstract

Doped germanium resistance thermometers (GRTs) have been used as cryogenic thermometers for forty years. GRTs exhibit a negative temperature coefficient of resistance and possess a high sensitivity that allows for sub‐millikelvin control at lower temperatures. These devices also exhibit excellent short‐ and long‐term stability and were used to maintain national temperature scales below 30 K until the advent of the rhodium‐iron thermometer. Lake Shore Cryotronics uses GRTs, model GR‐200A‐1000, as the transfer thermometer for temperature calibration below 30 K. A typical GRT working standard is thermally cycled from 1.4 K to 330 K once a week on average. Every six months, to ensure stability and traceability, these working standard GRTs are compared against a set of standards‐grade germanium, platinum, and rhodium‐iron resistance thermometers calibrated by the National Institute of Standards and Technology in the US and/or the National Physical Laboratory in the UK. These comparisons yield a measure of the long‐term stability of these GRTs over a period of years. This paper reports the long‐term stability from 1.4 K to 30 K of eleven germanium resistance thermometers as a function of time and thermal cycling during their use as working standard thermometers.