Dielectric Permittivity of Eight Gases Measured with Cross Capacitors

A four-ring, toroidal cross capacitor was used to measure accurately the relative dielectric permittivity ε(p,T) of He, Ar, N2, O2, CH4, C2H6, C3H8, and CO2. (ε is often called the “dielectric constant.”) The data are in the range from 0 to 50°C and, in many cases, extend up to 7 MPa. The accurate measurement of ε(p,T) required a good understanding of the deformation of the gas-filled capacitors with applied pressure. This understanding was tested in two ways. First, the experimental values of ε(p,T) for helium were compared with theoretical values. The average difference was within the noise, 〈εexpt−εtheory〉=(−0.05±0.21)×10−6, demonstrating that the four-ring cross capacitor deformed as predicted. Second, ε(p,T) of argon was measured simultaneously on three isotherms using two capacitors: the four-ring capacitor, and a 16-rod cross capacitor made using different materials and a different geometry. The results for the two capacitors are completely consistent, within the specifications of the capacitance bridge. There was a small inconsistency that was equivalent to 1×10−6 of the measured capacitances, or, for argon, 3×10−5Aε, where Aε is the zero-density limit of the molar polarizability ℘≡(ε−1)/[(ε+2)ρ].