Combined thickness and permittivity measurement of layers using an open-ended coaxial probe

Abstract

The purpose of this thesis is to develop a method for estimating both the thickness and permittivity of layers simultaneously utilizing an open-ended coaxial probe. One possible application of this method is the detection and characterization of deposits inside pipelines; examples are gas hydrate deposits in multiphase petroleum transportation. The hydrates forming can result in deposits on the interior surface of the piping and may restrict the flow of the production, it is thus necessary to monitor the layer thickness of the deposits to prevent any obstruction of flow, and the permittivity can tell us something about the composition of the deposits. The open-ended coaxial probe is a coaxial line that is a cut-off section of the transmission line. Permittivity measurements with the open-ended coaxial probe rely on analyzing the reflection of the electromagnetic wave from the probe-sample boundary. The open-ended coaxial probe is known to become radiating at high frequencies when the probe dimensions are comparable to the wavelength in the material under test. When measuring on samples with a finite thickness, this may result in additional reflections from the sample boundary interfering with the main reflection. If the applied permittivity model assumes an infinite thickness, the additional reflection may result in artifacts in the measured permittivity. Typically, this effect will be stronger at some frequencies due to resonance effects, which is seen as an unwanted measurement error. The resonance amplitude and frequency depend on the layer thickness, the permittivity of the layer/backing material and the probe dimensions. This thesis shows that by comparing the measured permittivity spectra with a matrix of finite element simulation, we can estimate both the layer thickness and material by comparing the measured data against the simulations. The simulations were verified by measuring liquids with known dielectric properties. The unwanted resonances manifest as artifacts in the permittivity spectra and increase the accuracy of the comparison. With the methods proposed in the thesis, we can also determine when the resonances occur for a given material, layer thickness, and frequency.