Thermodynamic Temperature Measurements Near The Triple Point Of Water Temperature Using Double Cylindrical Acoustic Resonantor

The thermodynamic temperature stably and consistently reproducing the Kelvin is the base of the consistence and accuracy of temperature measurement in the world.With the continuous deepening of scientific research,it has been found that there is a certain deviation between the international temperature scale T₉₀ and the new thermodynamic temperature T.Therefore,it is necessary to obtain accurate thermodynamic temperatures and then revise the international temperature scale.The research group of the Chinese National Institute of Metrology,where the author of this article is located,used the cylindrical resonator to determine the Boltzmann constant with an uncertainty of 2×10^-6 in 2017,which contribution to International Scientific Data Committee（Committee On Data For Science And Technology,CODATA）adjustments.The author of this article participated in the work of the Boltzmann constant measurement of the research group.Based on the experience gained from the Boltzmann constant measurement,and first using thermodynamic temperature experiment system for measuring near triple point of the water using double cylinder acoustic resonance method.The main research contents are as follows:（1）Build the thermodynamic temperature experiment system for measuring the triple point of water near the resonant cavity of double cylinders.The resonance cavities can simultaneously measure the acoustic resonance frequency and the microwave resonance frequency.Improves a high-purity gas path and pressure system,and gas flow is used to ensure the gas purity,measure the fluctuation of pressure in the cavities within 10 hours within±3 Pa.Improves resonant cavity active temperature control method,and the temperature stability of resonance cavities less than±0.2 mK over 10 hours.（2）The measurement of acoustic frequencies of longitudinal modes in double cylindrical cavities,which filled with high purity argon.The acoustic resonance frequency of temperature（270²80）K and pressure（550¹00）kPa was measured respectively.Improve low pressure measurement method,analyzed the influence of the piezoelectric ceramic sensor on the acoustic measurements,and analyzed the influence of the gas flow on the acoustic resonance frequency of the gas non-flowing state;Corrected the perturbation of ideal factors,the relative uncertainty of the acoustic resonance frequency is less than 2×10^-6.（3）The method for measuring the size of double-cylindrical cavity using microwave resonance is studied.The microwave resonant frequencies were respectively measured at temperatures（270²80）K and pressures（550⁵0）kPa.The small effects of piezoelectric ceramic sensors on cavity size measurement and the influence of refractive index on cavity size measurement during acoustic measurements were analyzed.Using multi-mode fitting method to obtain the actual length of the cavity,and the fitted value of the cavity length is analyzed,the relative deviation from the measured value is less than±0.05×10^-6.（4）Calculation obtained the deviation between the thermodynamic temperature and the international temperature standard ITS-90 temperature value.According to the acoustic resonance frequency and cavity length under different working conditions,the speed of sound of ideal gas was obtained by weighted fitting by the least squares method.Using the acoustic gas thermometer’s relative method,the triple point of water as reference temperature,obtained three thermometer temperature.The deiation(T-T₉₀)agrees well with the internationally published high-precision measurement results and should provide independent data sources for the revision of the ITS-90international temperature scale.