Measurements of virial coefficients of Helium, Argon and Nitrogen for the needs of static expansion method
Generally, there are three primary methods in vacuum metrology: mercury manometer, static expansion method, and continuous expansion method. For the pressure below 10 Pa, the idea of the primary standard is that the gas is measured precisely at a pressure as high as possible, and then the gas is expanded to the bigger volumes; this allows to calculate the expanded pressure. An important parameter that needs to be taken care of in primary vacuum calibration methods is the compressibility factor of the working gas. The influence of virial coefficients on the realization of primary standards in vacuum metrology, especially in the realization of the static expansion method is very important. In this paper we will present the measured data for virial coefficients of three gases Helium, Argon and Nitrogen measured at room temperature and a pressure range from 3 kPa to 130 kPa. The dominating term due to real gas properties arises from the second virial coefficient. The influence of higher orders of virial coefficients drops rapidly with lower pressure, particularly for gas pressures values lower than one atmosphere. Hence, in our calculation, the series of real gas was used for the first and second virial coefficients but not for higher-order virial coefficients.
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