The 1962 He³ Scale of Temperatures. I. New Vapor Pressure Comparisons

A comparison of He³ and He⁴ vapor pressures, (P ₃, P ₄), has been made in an apparatus designed to reduce the number and magnitude of corrections associated with the refluxing film in the He⁴ pressure sensing tube and the attached bulb. The critical pressure of He³ has been redetermined to be at 873.0 ±1.5 mm Hg at 0° C and standard gravity; the corresponding temperature as measured by a He⁴ thermometer is 3.3240±0.0018° K on the 1958 He⁴ scale. These (P ₃, P ₄) comparisons and the 1958 He⁴ scale are the basis of the 1962 He³ scale of temperatures derived and evaluated in the papers following this one. Empirical interpolation equations containing only P ₃ and P ₄ are described by means of which existing P ₄ measurements may be converted to an equivalent P ₃. A comparison has been made between this interpolation and a conversion in which the 1958 He⁴ and 1962 He³ scales are used as parameters. Deviations between the two procedures are within the estimated errors of the (P ₃, P ₄) measurements. In subsidiary experiments on techniques for He⁴ thermometry a typical vapor pressure bulb arrangement was tested. It is shown that the refluxing film introduces a heat flux, Q̇, and a resulting Kapitza temperature drop, ΔT , between the He ii and its container, which may amount to many millidegrees. The feasibility of calculating ΔT for a particular He⁴ vapor pressure bulb was studied. The necessity of measuring ΔT /Q at least once in situ is pointed out. In addition, it is necessary to redetermine the film flow rate periodically at the same time that He⁴ vapor pressure measurements are being made.