Gas Refractive Index Primary Thermometry Based On Microwave Resonance

Kelvin（K）,the unit of the thermodynamic temperature is one of the seven basic units of international system（SI）of units.Accurately measuring,stably and consistently reproducing the Kelvin is the base of the consistence and accuracy of temperature measurement in the world.Thermodynamic temperature is the real physical temperature of a system,but because of the difficulty in measuring results,according to the thermodynamic temperature measurement of the international temperature organizations to develop a practical protocol scale,also known as the international scale.The international temperature scale is approaching to the thermodynamic temperature.However,the research and the key comparison lately showed the triple point of water was not stable because of the isotopic abundance in the water.In 2005,International Committee for Weights and Measures（CIPM）proposed that thermodynamic temperature and the rest of the SI units should be fixed on the related fundamental physical constants.This will change the history that the standards rely on the properties of materials.At present,microwave resonance frequency method is one of the most accurate methods to measure the thermodynamic temperature.This work is based on the experimental platform of the acoustic gas thermometry in the Chinese institute of metrology.The measurement of the thermodynamic temperature by using the properties for argon from“ab initio”quantum mechanics calculation and experimental results and the microwave resonant method.Based on the microwave resonance method,the refractive index of gas at different temperatures is measured,and the thermodynamic temperature is obtained by the relationship between the temperature and the refractive index of the gas.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 13 h was better than 10 Pa.The thermodynamic temperature and the differences between the thermodynamic temperature and ITS-90,(T-T₉₀)at five isotherms in the range from the triple point of mercury and the melting point of gallium was obtained.Four transverse magnetic microwave resonance frequencies were measured to obtain the refractive index in argon at 700 kPa,the consistence of the refractive index from different microwave modes was better than 1×10^-8.The thermodynamic temperature then was determined by combining the refractive index measurement results and the viral equation of state for argon.The uncertainty of the difference of the thermodynamic temperature T and the ITS-90 international temperature was evaluated to be 11.6 mK and the results agreed with the Consultative Committee for Thermometry（CCT）recommendation values well.The uncertainty from this method will be decreased with the development of the theoretical calculation for argon and the improvement of the pressure measurement in the next future.