Isotherms Determined by the National Bureau of Standards Acoustical Thermometer in the Liquid Helium Temperature Range.

Abstract
Isotherms of the speed of sound in helium 4 gas as a function of pressure have been measured. Five of the isotherms in the liquid helium 4 temperature range have been analyzed, and the results are discussed. The acoustically determined values of temperature are consistently higher than the values of temperature associated with the liquid helium 4 vapor pressure scale (T58). The deviations are 0.006 °K at 2.323 °K. 0.007 °K at 2.807 °K, 0.008 °K at 3.211 °K, 0.010 °K at 4.212 °K and 0.012°K at 5.024 °K.