Mineral oil moisture measurement with the use of impedance spectroscopy

Abstract

In this study, an attempt was made to evaluate very small water content (below 0.05% in volume) in the ‘20–30’ mineral oil using both electrochemical impedance spectroscopy (EIS) and dielectric constant calculation based on the measured impedance values. Measurements were performed with the use of laboratory EIS system and were designed to verify experimentally that it is possible to develop an IS-based measuring system that would be able to work in situ, be relatively cheap and fast alternative compared with systems widely used nowadays. Known amounts of water were added to the oil samples, which were not treated by any other factors, e.g. solvents, contrary to other researchers’ findings. Impedance of the samples was measured in the frequency range of 0.01–100 Hz. Equivalent circuit was proposed and used to reveal how electric properties of the oil change due to water addition. Both calibration curves based on the value of an equivalent circuit element and dielectric constant showed linear correlation of their values with the water amount. Thus, experimental results confirm that with some discussed limitations, EIS and dielectric constant calculation can be used to estimate even such small quantities of water in mineral oil.