Apparatus for Density and Dielectric Constant Measurements to 35 MPa on Fluids of Cryogenic Interest

An apparatus has been developed for simultaneous measurements of fluid densities and dielectric constants at temperatures from 70 to 320 K and at pressures to 35 MPa. A magnetic suspension technique, based on an application of Archimedes' principle, is employed in the density determination, while a concentric cylinder capacitor is used for obtaining the dielectric constant data. The apparatus can be used not only for determining densities and dielectric constants of compressed gases and liquids (including mixtures), but for saturated liquid and vapor properties as well. Also included is the capability for acquiring liquid-vapor equilibrium data for mixtures. The total uncertainty of a single density measurement is estimated to be approximately 0.1% for densities as low as 50 kg/m³; at lower densities, the uncertainty increases. The imprecision of the density data is typically less than 0.02%. The total uncertainty in the dielectric constants is approximately 0.01%. Experimental data for a 0.85 CH₄ + 0.15 C₂H₆ mixture are given here to demonstrate the performance of the apparatus.