Dielectric properties of standard liquids at hyperbaric pressures

Abstract

Knowledge of the dielectric properties of a given material is essential for the characterization of its interaction with an electromagnetic field. To date, all dielectric property measurements on liquids have been reported at atmospheric pressure. Water-based liquid solutions contain dissolved gases (mostly oxygen and nitrogen), the concentration of which increases with the pressure of the overlying air. This is bound to affect the dielectric properties. The aim of this project is to devise a method for carrying out dielectric measurements over a wide frequency range as a function of pressure. This study could potentially lead to dielectric measurements on human blood under pressure, and hopefully result in establishing a direct relationship of the complex permittivity of blood with the concentration of dissolved nitrogen. An in-house designed and constructed high pressure vessel was used and measurements were carried out on distilled water, 0.1M NaCl, Propan-2-ol, Acetone and Triton X-100. A Vector Network Analyser (VNA) connected with an open-ended coaxial probe was used to carry out measurements on the liquid under test inside the air tight vessel. A three step calibration (open, short, load) was used with 0.1M NaCl used as a reference liquid for validation. The results were compared to published dielectric parameters obtained at atmospheric pressure. The most significant change was observed for relative permittivity of distilled water which exhibited an average increase of around 4%. The loss factor did not exhibit any significant change. Similar behaviour was observed in 0.1M NaCl. Solutions with relatively low dielectric properties (Propan-2-ol, Acetone and TX-100) did not undergo any significant changes as the overlying air pressure was increased. However, experimental limitations due to the limited surface area in the pressure vessel might have limited any significant changes. Thus, a modified pressure system could be used to obtain better characterisation of the dielectric properties as a function of pressure.