Highly Sensitive Microwave Sensor for High Precision Sensing of Water Contamination in Mineral Oil

One of the most important characteristics of planar microwave resonator-based sensors is their sensitivity. Since their measuring parameter is resonance frequency shift, the sensitivity could be described as the shift in the resonance frequency of the microwave resonators versus variation in the dielectric constant of the material under the test. Although label-free microwave resonator-based sensors are capable of providing moderate levels of sensitivity sufficient for enabling numerous applications, detection of material variations in sub-percent or ppm (part per million) range is not easily possible with these structures. This paper investigates the reasons behind the limited sensitivity of planar microwave resonator-based sensors and suggests a measurement structure capable of measuring extremely low concentrations of water contamination in mineral oil samples. Simulation and experimental results verify the capability of the presented sensing structure in the detection of as low water concentrations as 50 ppm in mineral oil samples expressing extremely high sensitivity for the presented sensor with 30 kHz in the frequency shift of the resonator.

Automated summary

One of the key characteristics of planar microwave resonator-based sensors is their sensitivity, which is described by the shift in resonance frequency versus variation in dielectric constant. While these sensors offer moderate sensitivity levels for various applications, they face limitations in detecting material variations in sub-percent or ppm range. This study explores the reasons behind limited sensitivity in these sensors and proposes a measurement structure capable of detecting extremely low concentrations of water contamination in mineral oil samples, showing high sensitivity with 30 kHz frequency shift for detecting water concentrations as low as 50 ppm.