Acoustic Thermometry Results from 271 to 552 K

The NIST acoustic thermometer determines the thermodynamic temperature from measurements of ratios of the speed of sound of argon in a nearly spherical cavity. We report recent results for T − T90 on 12 isotherms spanning the range 271–552 K. (T is the thermodynamic temperature and T90 is the temperature on the IInternational 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 to 380K and with our previously reported results at 303, 430, and 505 K. The combined data sets are sufficiently redundant and sufficiently distributed over the temperature range to support a re-determination of the reference function for standard platinum resistance thermometers for a future temperature scale. The isotherms were analyzed using several methods; the T − T90 results and related uncertainties are insensitive to the method chosen. The thermal expansion of the stainless-steel resonator was deduced from the frequencies of the microwave resonances of the cavity. To clearly identify two nearly degenerate eigenmodes in our nearly axially symmetric resonator, two phased coupling probes were used to control the azimuthal angle of the microwave excitation.