Primary Acoustic Thermometry

D. Ripple (836), M.R. Moldover, and K.A. Gillis

Objectives: (1) To reduce the uncertainty in the determination of the thermodynamic temperature by a factor of 3 to 8 in the range from 500 K to 900 K using speed-of-sound measurements in low density argon as a primary standard and (2) to improve the accuracy of the high-temperature fixed points (e.g. tin point, zinc point) and radiometry tied to these fixed points.

Problem: The most accurate determinations of thermodynamic temperature above 700 K use relative radiance measurements referenced to a black body near 700 K. The thermodynamic temperature of the black body is known from NIST constant volume gas thermometry (CVGT) experiments. Unfortunately, two NIST CVGT experiments differ from each other for reasons that are not well understood. The difference leads to an estimated uncertainty of 13 mK in temperatures near 700 K and 50 mK in temperatures near the gold point (1337.33 K).

Approach: We measure the frequencies of both acoustic and microwave resonances in a spherical, argon-filled cavity bounded by a thick, metal shell, enclosed by a high-performance thermostat. The data determine the speed of sound in the argon from which the thermodynamic temperature is deduced. The temperature is transferred to platinum resistance thermometers and then to fixed-point devices.

Results and Future Plans: Microwave and acoustic data in the temperature interval 217 K to 303 K were acquired with a prototype resonator. These data determined (T – T₉₀), the difference between the Kelvin thermodynamic temperature T and the International Temperature Scale of 1990 (ITS-90) with a standard uncertainty of 0.6 mK, depending mostly upon the model fitted to the acoustic data. The graph compares these data with results from other laboratories.

These results were recognized with the "Best Oral Presentation" Award at the 7^th International Symposium on Temperature and Thermal Measurements in Science and Industry. The work with the prototype resonator led to many improvements in the high-temperature apparatus. During the past year, acoustic resonances were successfully measured at 250 ° C, in static argon and also in flowing argon. The latter indicates that the purity of the argon can be maintained at high temperatures. It appears that the performance targets for this apparatus will be met, if not exceeded.

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