The composition of a thin emulsion layer backed by air can be determined from permittivity measurements with an open-ended coaxial probe. In this work, techniques for probe design and determination of liquid permittivity are studied. Measurements on thin liquid layers backed by a low-permittivity material show that the measured apparent permittivity varies exponentially with the layer thickness, with a probe constant that is independent of measurement frequency and liquid permittivity. It is found that an electrostatic model is applicable for design of probes with regard to the probe's sensitivity depth. Models of different complexity for determination of permittivity are compared and it is shown that a bilinear model in combination with an empirical exponential model estimates the permittivity of thin liquid layers with high sensitivity provided that the thickness of the layer is known. The bilinear model uses calibration measurements on three samples with known permittivities. A somewhat lower precision is obtained with a full-wave model of the probe, but this model has the advantage that no calibration except a simple reference plane rotation is needed. However, the computation time of the full-wave model is high.

Several parameters describing the quality of petrochemical products can be revealed from the permittivity spectrum of the product. This requires that the permittivity be measured with high sensitivity, so the calibration of measurement cells is of great importance. In this work calibration techniques for the transmission/reflection method for complex permittivity determination of low-loss liquids are studied. It is shown that a bilinear transformation can be used to determine the scattering parameters at the reference planes from the measured scattering parameters of a coaxial transmission/reflection cell. The permittivity of a sample in the cell can then be calculated with high precision from the reference plane parameters. The de-embedding procedure consists of determining three calibration coefficients by measuring three samples with known permittivities. Measurements on low-loss liquids and crude oils confirm the performance of the calibration procedure. The technique is compared to a simpler calibration method and the choice of calibration fluids to ensure broad-band measurements is discussed.

The formation of gas hydrates in multiphase flow from oil-producing wells can cause severe problems such as agglomeration and clogging of pipelines. The formation of gas hydrates in emulsions gives rise to a change in the bulk permittivity, and hence dielectric spectroscopy is a potential technique for the detection of gas hydrates in emulsified systems. This paper presents a time domain system for permittivity measurements of emulsions. The permittivity was calculated by using a simple admittance model from reflection measurements from 10 MHz to 10 GHz with two different open-ended coaxial probes. Measurements on liquids with known permittivity were used to verify the sensitivity and reproducibility of the measurements. The formation of gas hydrates in a 60% water-in-oil emulsion was also monitored with the dielectric measurement system. During the formation of gas hydrates decreases in the relaxation time and the static and high-frequency permittivities were observed.