The Design And Application Of Microwave Transducers In Acoustic Gas Thermometry

The gas acoustic thermometer is a measurement carried out through the physical relationship between the sound velocity of an ideal gas and the thermodynamic temperature.The acoustic temperature measurement method is currently one of the most accurate methods for measuring the thermodynamic temperature.The sound velocity of gas is generally measured by the acoustic resonance method,and the accurate measurement of the cavity size is a necessary prerequisite for realizing the sound velocity measurement.The microwave resonance method is one of the best methods to measure the average size of the cavity.It has the advantages of small measurement uncertainty and real-time,in-situ measurement.At present,the thermodynamic temperature measurement in the high temperature zone is one of the key issues in the delineation of the international temperature scale.However,the existing commercial microwave sensors cannot meet the requirements of acoustic thermometers above 400K.Therefore,it is necessary to design a microwave sensor suitable for high-temperature environments to better transmit microwave signals into and out of the cavity.The optimized design of microwave cables is of great significance for the stable transmission of microwave signals and the accurate measurement of cavity dimensions.Based on the basic theory of electromagnetics,this paper derives the calculation formulas for the characteristic parameters of microwave cables,and establishes the physical model of microwave cables;compares and studies the properties of different high-temperature dielectric materials,proposes methods for optimizing the structure and materials of microwave cables,and designs them separately Several microwave cables with different dielectric materials are presented;through theoretical calculations and cable experimental measurements,an optimized microwave cable structure is obtained.The microwave cable is applied to the measurement of the microwave resonant frequency of the cylindrical resonant cavity,and the microwave resonant frequency and half-width of the two cylindrical cavities of different sizes and materials,and four transverse magnetic field microwave modes TM001,TM101,TM201 and TM301 are measured.Comparative analysis of the random deviation of the microwave frequency measurement.Using the microwave cable developed in this paper and the improved frequency measurement system,the standard deviation of the measurement of the microwave resonance frequency is reduced to the level of 1×10^-7,which is an order of magnitude higher than the original measurement system in the laboratory.Further carried out the 323 K,373 K,423K,473 K and 505 K to the cylindrical cavity internal size and thermal expansion experimental measurement research,the temperature stability is better than ±3mK/20min,through the microwave resonance frequency of high signal-to-noise ratio The size change of the resonant cavity can be obtained in real time,the fitting method of obtaining the length and inner diameter of the cylindrical cavity through the microwave resonance frequency is studied,and the relative standard deviation of the measurement is analyzed to meet the accuracy requirements of acoustic temperature measurement.The research in this paper lays the technical foundation for the future gas acoustic thermometer to measure the thermodynamic temperature of 400 K to 1358 K.It can also be used for the measurement of the refractive index of gas medium in high temperature environment and the precise determination of thermal expansion coefficient of high temperature cavity.