A 18-ms Measurement-Time MLS-Based System for Moisture Assessment in Lubricant Oil

In this article, a system and a measurement approach to reduce the measurement time in the assessment of moisture contamination in lubricant oils is presented. The system's sensing principle leverages the permittivity change of a miniaturized interdigital capacitor (IDC) while immersed in oil. The time-domain impedance concept, that is, the impulse response (IR), is exploited by using maximum length sequences (MLSs) as efficient broadband signals for the sensor's excitation in a wide range of frequencies. Different from conventional impedance spectroscopy (IS), MLS-based measurements are performed with simpler hardware, higher computational efficiency, lower power consumption, and lower measurement time. As a novelty with respect to the state-of-the-art, this article introduces a linear model to relate a single measured quantity from the IR to water concentration in oil. This permits to reduce the digital processing operations, leading to low measurement time and, thus, to low energy-per-measurement parameters with respect to other works which rely on laboratory instrumentation. The validity of the linear model, for the detection of small concentrations of water in lubricant oil, has been verified through experimental measurements. Water-oil samples have been prepared with 0.2 vol%, 0.5 vol%, 1 vol%, 2 vol%, and 3 vol% water concentrations at room temperature, obtaining an estimated limit of detection (LOD) of 6.3 ppm. A low measurement time of 18 ms has been achieved which advances the state-of-the-art.