Dean C. Ripple, Gregory F. Strouse, Michael R. Moldover
The NIST Acoustic Thermometer determines the thermodynamic temperature from measurements of ratios of the speed of sound of argon in a spherical cavity. We report recent results for T - T90 on 12 isotherms spanning the range 271 K to 552 K. (T is the thermodynamic temperature and T90 is the temperature on the International Temperature Scale of 1990.) The results are in excellent agreement with recent acoustic thermometry results reported by Benedetto et al. in the range from 273 K to 380 K and with our previously reported results at 303 K, 430 K, and 505 K. We analyzed the isotherms using several methods; the T - T90 results and related uncertainties are insensitive to the method chosen. We deduced the thermal expansion of the stainless-steel resonator from the frequencies of the microwave resonances of the cavity. To clearly identify two nearly degenerate eigenmodes in our nearly axially symmetric resonator, we used two phased coupling probes to control the azimuthal angle of the microwave excitation.