A microwave instrument for the continuous monitoring of the water content of crude oil

Since the effective permittivities of water and oil differ by an order of magnitude, it is shown that it is possible to measure their relative concentrations in crude oil by measuring the effective permittivities of the mixture at microwave frequencies. Dielectric mixture theory is used to show the relationship between the permittivity and the relative concentrations of two materials in a mixture. It is shown that equivalent results are obtained for the case of small concentrations of water in oil by using a perturbation theory analysis for a liquid cylindrical sample flowing through a resonant cavity. Results are presented for a sample enclosed in a thin-walled Pyrex tube with measurements taken for water concentrations in oil of 0-10 percent by volume. A practical instrument is described which uses a comparative technique for continuously comparing two samples of oil, one "dry" and one "Wet." Results show that this type of instrumentation can obtain sensitivities of 0.05-percent water content. The instrument is very simple, and has an output that can be readily adapted to automatic control applications.