Precise determination of the compressibility factor of a gas from

Abstract

A method and an apparatus for the precise determination of the compressibility factor of a gas sample are disclosed. Two grating interferometers are coupled together with one interferometer defining a refractive index interferometer adapted to provide a signal of information related to the refractive index of the gas sample and the other interferometer defining a pressure interferometer adapted to provide another signal of information related to the pressure of the gas sample whereby to permit measurement of the refractive index as a function of pressure. Each interferometer is capable of dividing a linearly polarized monochromatic laser beam into a measuring beam and a reference beam and causing the measuring beam and reference beam to travel along respective optical paths extending in close parallel relationship over predetermined optical path lengths. The refractive index interferometer and pressure interferometer comprise respectively two optical cells in tandem alignment and a single optical cell with each cell having elongated measuring and reference compartments arranged in close parallel relationship along the parallel optical paths of the measuring beam and reference beam to receive same therethrough, the measuring compartments of both cells of the refractive index interferometer being interconnected to permit gas expansion therebetween with the measuring compartment of one of the cells being connected to the measuring compartment of the single cell of the pressure interferometer via a pressure equilibrium chamber to provide the interferometer coupling. The expansion technique enables one to obtain accurate values for higher order terms of the Lorentz-Lorenz equation. This technique combined with the absolute measurements of the refractive index as a function of pressure leads to values of the compressibility factor comparable to the best PVT values, in a much shorter time as compared to other methods.