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Dielectric Permittivity of Eight Gases Measured with Cross Capacitors



James W. Schmidt, Michael R. Moldover


A 4-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 C to 50 C and, in many cases, extend up to 7 MPa. The data for ε(p,T) may be useful for calibrating on-line, capacitance-based instruments that monitor the heating value of natural gas. The data also reveal minor imperfections in certain equations of state. The accurate measurement of ε(p,T) requires a good understanding of the deformation of the gas-filled capacitors with applied pressure. We tested our understanding in two ways. First, we compared the experimental values of ε(p,T) for helium with theoretical values. The average difference was within the noise, expt{-epsilon}theory> = (-0.05 0.21) 10-6, demonstrating that the 4-ring cross capacitor deformed as predicted. Second, we simultaneously measured {epsilion}(p,T) of argon on three isotherms using two capacitors: the 4-ring capacitor and a 16-rod cross capacitor made using different materials and a different geometry. The results for the two capacitors were slightly inconsistent: rod {-epsilon}ring> = (0.161 0.025) 10-6 (p/MPa). The inconsistency is equivalent to 1 10-6 of the measured capacitances, or, for argon, 3 10-5 Aε, where Aε is the zero-pressure limit of the molar polarizability P{identical with} (ε - 1)/[(ε + 2)p].
International Journal of Thermophysics
No. 2


carbon dioxide, cross capacitor, dielectric constant, ethane, methane, molar polarizability, natural gas, nitrogen, propane


Schmidt, J. and Moldover, M. (2003), Dielectric Permittivity of Eight Gases Measured with Cross Capacitors, International Journal of Thermophysics (Accessed May 18, 2024)


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Created March 1, 2003, Updated June 2, 2021