The difference between the thermodynamic temperature T above 335 K and the International Temperature Scale ITS-90 temperature(T90)T-T90,is currently the focus and difficulty of frontier research in international temperature metrology.Acoustic gas thermometry,based on the determination of theremperature from the speed of sound in gases,is one of the promising methods for this temperature range.The speed of sound in gases could be measured from acoustic resonant frequency and the inner dimension of the cavity.Microwave resonance method is a technical route for real-time and in-situ measurement of cavity size and thermal expansion at high-temperature.We optimized the high-temperature acoustic gas thermometry system to improve the temperature and pressure stability.Homemade high-temperature microwave cable and sensor were used for the measurement of microwave resonant frequencies in a cylindrical cavity from 335 K to 493 K with a relative standard deviation of(2～13)×10-8.The variation of the cavity size with temperature was obtained through microwave resonance frequencies.The stability of geometric of the cylindrical cavity was acceptable.Then the refractive index and pressure of the gas inside the cylindrical cavity were determined for analyzing the pressure difference from flowing gas.These results are helpful for future determination of T-T90 above 335 K with low uncertainties.
temperature measurementacoustic gas thermometrythermodynamic temperaturemicrowave resonance methodthermal expansionrefractive indexpressure
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