Primary acoustic thermometry betweenT = 90 K andT = 300 K

Abstract

Differences between thermodynamic temperature and temperatures on the International Temperature Scale of 1990 (ITS-90) have been measured at seven temperatures between T= 90 K andT= 300 K using a primary acoustic thermometer. The thermometer, which is based on a spherical acoustic resonator, yields values of the quantity u/a, where u is the speed of sound in the thermometric fluid (argon) and a is the radius of the resonator. Values ofu/a in a single sample of argon were obtained at each of the temperatures, except the lowest, and at the temperature of the triple point of water (Tt) from pseudo-isochoric measurements. The results were combined with measurements of u/a along isotherms to obtain highly precise estimates of the quantity A0/a2at each temperature, where A0is the value of u2in the limit of zero density. At the lowest temperature, a simplified procedure was adopted in which A0/a2was determined directly from an isotherm. Since a monatomic gas was used, the values of A0(T) are proportional to the thermodynamic temperature T. Finally, the values A0/a2were combined with microwave measurements of the thermal expansion of the spherical resonator to obtain the ratiosA0 (T)/A0(Tt) from which thermodynamic temperatures were evaluated. The results, which have an estimated standard uncertainty of between 0.9 mK and 1.3 mK, deviate significantly from ITS-90.