Liquid Level Sensor Based on Phase-Shifting of Radio-Frequency Wave

Abstract

The presence of liquid slightly changes the local properties of submerged transmission line. A dedicated sensor is presented in this paper to estimate the level of liquid, through two different determinations: measurement of the overall capacitance of a bi-wires and time of flight estimation of electromagnetic radiation. Since two measurement methods are physically independent, the proposed sensor has an intrinsic redundancy. The level of liquid is measured through phase shift: the signals are sampled at a sufficiently high frequency and then phase differences is evaluated by calculating the coefficients of the digital Fourier transform. Both measurements are performed directly in baseband without requiring frequency conversions. In contrast with typical TDR system where the end of the cable is open or short-circuited, the bi-wires fold and return to the measurement electronics. The characterization of the proposed sensor is determined experimentally for water and diesel fuel, in good agreement with simulation results. As results show, both phase-shifting measurements are linear with the liquid level, providing a resolution in the order of 0.1 mm through a compact and low-cost realization.