Highly Sensitive Liquid-Level Sensing Based on Microwave Frequency Domain Reflectometry and Interferometry

The frequency domain reflectometry (FDR) and interferometry are two widely used investigative techniques that have been instrumented using optical and microwave devices for a diverse array of sensing applications. In this article, we present a highly sensitive liquid-level sensor using a custom hollow coaxial cable transmission line. Measurements of liquid levels with high sensitivity and resolution can be achieved by both FDR and interferometry-based measurements using the same sensor device. We show that FDR-based measurements are more straightforward for liquid-level sensing and are suitable for both small and large variations of liquid level, while interferometry-based measurements provide slightly higher measurement resolution than the FDR-based measurements but require additional data processing and are only suitable for sensing small variations of liquid level. We experimentally demonstrate that the FDR-based measurements exhibit a linear response over a large dynamic range of 122 mm with an estimated resolution of $5~\mu $ m. The capability of the proposed sensing system for the measurements of multiple levels of a stratified liquid (oil+water) is also investigated based on FDR measurements. The presented work suggests that the two different measurement techniques, namely, FDR and interferometry, can be flexibly applied to the proposed coaxial probe for liquid-level sensing based on different applications.