Development of an apparatus for high-resolution measurement of permittivity of fluids near the critical point and detection of the critical anomaly for supercritical ethylene

We developed an apparatus to conduct relative permittivity measurements for supercritical fluids. It was designed to measure the values with sufficient accuracy by a convenient and simple treatment. The use of pressure-tight coaxial cables decreased the electrical noise and stray capacitance, and made it possible to use a small-sized ordinary capacitor with a multi-layered structure of parallel plates. As a result, permittivity of supercritical fluids was measured within an uncertainty of +/-0.1% even for the thermodynamic state in the vicinity of the critical point. Using the apparatus, the relative permittivity of C2H4 was measured at six temperatures near the critical temperature in the pressure range of 0.1-10 MPa. It was proved that there definitely exists a critical anomaly of molar polarization P-m with a temperature dependence.