Primary acoustic thermometry between T=90 K and T=300 K

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 and T = 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 of u/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 (T(t)) from pseudo-isochoric measurements. The results were combined with measurements of u/a along isotherms to obtain highly precise estimates of the quantity A(0)/a(2) at each temperature, where A(0) is the value of u(2) in the limit of zero density. At the lowest temperature, a simplified procedure was adopted in which A(0)/a(2) was determined directly from an isotherm. Since a monatomic gas was used, the values of A(0)(T) are proportional to the thermodynamic temperature T. Finally, the values A(0)/a(2) were combined with microwave measurements of the thermal expansion of the spherical resonator to obtain the ratios A(0)(T)/A(0)(T(t)) 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. (C) 2000 Academic Press.