Reflective-Mode Phase-Variation Submersible Sensor for Liquid Characterization

A one-port reflective-mode microwave submersible sensor based on an open-ended step-impedance microstrip line configuration, operating at a single frequency, useful for the dielectric characterization of liquids and the measurement of liquid composition in binary mixtures, is proposed in this article. The sensor constitutes two parts, namely, the microwave module, including the sensitive region, and a 3-D-printed case (made of polylactic acid—PLA), necessary for sealing, thus preventing from any liquid leakage to the nonsensing regions of the microwave module. To enhance the sensitivity, the open-ended line Section of the microwave module, i.e., the sensing region in contact with the liquid under test (LUT), is a quarter wavelength microstrip line with high impedance. Thus, the phase of the reflection coefficient is the output variable, highly dependent on the input variable—the dielectric constant of the LUT placed directly above the sensing region. In order to validate the sensing structure, a prototype device has been designed and fabricated. The average sensitivity to variations in the concentration of ethanol in solutions of deionized (DI) water is 1.22°/%, and sensor resolution is 2%. The fabricated sensor is resistant to liquid leakage, and therefore, it can be completely submerged for measuring purposes, acting as an effective measuring probe.