Reference Values of the Dielectric Constant of Natural Gas Components Determined with a Cross Capacitor

A novel toroidal cross capacitor was used to measure accurately the dielectric polarizability =( p) (i.e., the dielectric constant as a function of the pressure) of helium, argon, nitrogen, methane, and carbon dioxide at T=50%C. The data extend up to 7 MPa (5 MPa for CO2) and may be useful for calibrating on-line, capacitance-based systems that are designed to measure the heating value of natural gas. The uncertainties of = and p are 4_10&6 and (3.0_10&5p+84 Pa), respectively. The properties of helium that had been calculated ab initio from quantum mechanics were used to verify that the cross capacitor deformed in a predictable manner under hydrostatic (gas) pressure. Thus, a common cause of systematic errors in measuring the dielectric constant of gases was avoided. For helium, the rms deviation of =( p) from the calculations was only 2.7_10&7. This suggests that the estimated uncertainty is very conservative.