Realizing the ITS-90 below 4.2 K at the National Research Council of Canada

The International Temperature Scale of 1990 (ITS-90) has only recently been realized below 4.2 K at the National Research Council of Canada (NRC). As the NRC Thermometry Group lacked a cryostat capable of reaching 0.65 K, the helium vapour-pressure thermometer 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 successfully 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 sixth-degree polynomials. The residuals from the fits were within ±0.3 mK for both thermometers.