Primary Acoustic Gas Thermometer With The Fixed-Length Cylindrical Resonator And Thermodynamic Temperature Measurements

Precision measurements of thermodynamic temperature form the basis of the revision of the international temperature scale（ITS）.Up to now,the primary acoustic gas thermometry（AGT）has proved to be the method for the measurement of thermodynamic temperature with the lowest uncertainty.To measure thermodynamic temperature,a primary acoustic gas thermometer with the fixed-path cylindrical resonator（c-AGT）was developed.The important issues of the determination of the cylindrical resonator length by microwave resonant method and the measurement of speed of sound by acoustic fixed-path resonance were investigated.The thermodynamic temperature in the range between the temperature of the triple point of mercury（234.3156 K）and the melting point of gallium（302.9146 K）was obtained with this c-AGT.The experimental system of the primary c-AGT for thermodynamic temperature measurement was developed as part of this work.The corrections of the several non-ideal effects that produce perturbations to the acoustic and microwave resonance frequency measurement were investigated.A high-precision cylindrical resonant cavity that allowed simultaneous acoustic and microwave frequency measurement was built from high conductivity copper.With the application of the active temperature control system,the stability of the resonator temperature was better than±0.1 mK over 9 h.The pressure measurement system and the high purity gas handling system were improved to avoid contamination of the working gas.With active control of the gas flow rate,the stability of the pressure over 9 h was better than 10 Pa.The microwave resonant method was applied to measure the absolute length of the cylindrical cavity from the eigenfrequencies of four microwave modes.The perturbations from the resonant frequency fitting,the sweeping range of the resonance profile and the electrical resistivity of the cavity wall were investigated.An all-mode fitting procedure for determination of the cylindrical cavity length was implemented.In this way,the absolute length of the cylindrical cavity at different pressures of several isotherms in the aforementioned range was obtained from measurements of the frequencies of TM101^TM401 with a relative uncertainty better than 0.48×10^-6.The measurement of acoustic frequencies of longitudinal modes in cylindrical resonator when filled with argon were investigated.The enhancement and ring down process of the acoustic resonance field were measured.The effects of the driving voltages of the transducer on the acoustic resonance frequency measurements were investigated.A simulation of the effect of the gas flow on the temperature and pressure distributions was carried out.The effect of gas flow on the frequency measurement was also measured and resonance frequencies were corrected for perturbations due to the gas filling tube.By means of nonlinear sweeping of the resonance profile,the acoustic resonance frequencies of two longitudinal modes（200）and（300）were measured for pressures from 780 kPa to 120 kPa when the resonator was filled with argon along several isotherms in the range between 234 K and 303 K with a relative uncertainty lower than 1×10^-6.The thermodynamic temperature and the differences between the thermodynamic temperature and ITS-90,(T-T₉₀)at six isotherms in the range from the triple point of mercury and the melting point of gallium was obtained.The speed of sound of ideal argon gas was obtained by a weighted regression analysis of the speed of sound-versus-pressure curves for different modes.The second acoustic virial coefficients agreed well with the data from literatures.Referred to the triple point of water,the thermodynamic temperatures and(T-T₉₀)were obtained from the speed of sound of ideal argon gas in the aforementioned range.The estimation of(T-T₉₀)agrees well with the accurate data obtained by other methods,and should provide independent data for the revision of ITS-90.