REALIZING THE ITS-90 BELOW 13.8 K AT NRC

摘要

The International Temperature Scale of 1990 (ITS-90) has been realized below 13.8 K at the National Research Council of Canada only recently. The establishment by the Comite consultatif de thermometrie (CCT) at its 19th meeting, held in September 1996, of a key comparison from 0.65 K to 24.5561 K using rhodium-iron resistance thermometers with the National Physical Laboratory, UK as the coordinating laboratory, provided the impetus for this work. At that meeting, NRC indicated its desire to participate in the key comparison despite having neither the required RhFe thermometers nor a realization of the ITS-90 below 13.8033 K, the lower limit of ITS-90 realized by platinum resistance thermometry. To meet the requirements for participation in the key comparison, two RhFe thermometers were purchased, an apparatus was designed to realize the vapour pressure of helium from 0.65 K to 4.2 K, and a separate interpolating gas thermometry (IGT) apparatus was designed for the 4.2 K to 24.5561 K range. The plan called for the vapour pressure thermometry to be performed first. The vapour pressure thermometry would also supply a calibration value near 4.2 K as the lowermost calibration point required by the IGT. The two additional IGT calibration points, the triple points of hydrogen (13.8033 K) and neon (24.5561 K), would be carried on calibrations of the RhFe thermometers determined in a cryostat routinely used for the calibration of capsule standard platinum resistance thermometers (CSPRTs) using sealed-cells. Thus the three calibration points of the IGT would be transferred via the RhFe thermometers. Since the NRC Thermometry group lacked a cryostat capable of reaching 0.65 K, the vapour pressure apparatus was designed to fit the demanding geometry of a top-loading ~3He cryostat insert routinely used by the Electrical Standards group to realize the resistance unit by means of the quantized Hall resistance. The vapour pressure apparatus has been operated from 0.7 K to 4 K in the ~3He cryostat and from 2.3 K to 4.2 K in a ~4He cryostat. The resistance-temperature data pairs obtained for the RhFe thermometers over the 0.7 K to 4.2 K range using the temperature derived from vapour pressure measurements for both ~3He and ~4He were fitted to 6th degree polynomials. The residuals from the fits were within +-0.3 mK for both thermometers. The IGT has recently operated over the range 4.2 K to 24.556K, achieving a standard deviation of 0.15 mK for the residuals resulting from a fit of the data to a 10th degree polynomial. The overall standard uncertainty of the measurements was approximately 0.5 mK.