Coaxial probe and apparatus for measuring the dielectric spectra of high pressure liquids and supercritical fluid mixtures

A probe and apparatus were developed for measuring the dielectric spectra (complex permittivity) of high pressure liquids and supercritical fluid mixtures. The probe consisted a 2.2 mm semirigid coaxial cable that was cut off flat and mounted into a high pressure tube. The apparatus for measuring complex permittivity consisted of the dielectric probe, cell, densimeter, piston for varying the system density at constant composition, and magnetic pump for agitation and recirculation, all of which were housed in a constant temperature air bath. The probe is simple, robust, inexpensive, and further, its design allows for quick connection to high pressure systems. Probe accuracy is estimated to be ±0.5 in ${\epsilon }^{\prime }$ and ±0.5 in ${\epsilon }^{″}$ from 200 MHz to 18 GHz based on replicate measurements of calibration and 2σ deviations over the interval. Dielectric spectra were measured over the 200 MHz–20 GHz range for $\mathrm{methanol+carbon}$ dioxide mixture at 323.2 K and a pressures up to 18 MPa.