The direct comparison of ³He and ⁴He vapor-pressure thermometers at LNE-Cnam within their overlapping temperature range Available to Purchase

The International Temperature Scale of 1990 defines the temperature in the range between 0.65 K and 5.0 K in terms of the vapor pressure of the two isotopes of helium³He and ⁴He. The ³He isotope is used between 0.65 K and 3.2 K, and the ⁴He isotope is employed in the range from 1.25 K to 5.0 K. LNE-Cnam has developed an apparatus for vapor pressure thermometry with two separate gas systems installed in the same cryostat. One is dedicated to ³He and the other to ⁴He. Each one is equipped with its own vapor-pressure chamber, to avoid any mixing and preserve the purity of the two gases. The two chambers are within the same copper block to ensure their temperature equality. The apparatus is cooled by a commercial dilution refrigerator [1]. The setup allows the simultaneous realization of the two temperature definitions, the one based on ³He and that based on ⁴He, and therefore a direct comparison, without any transfer standard. In this paper we present a comparison of the two temperature definitions, carried out between 2.1 K and 3.1 K.