Research on Frequency-Tunable Microwave Resonant Cavity Sensor to Overcome the Influence of Salinity in Ultralow Water Content Detection

Accurate measurement of ultralow water volume fraction (WVF) (ranging from 0% to 3%) is critically important during the production process and trade settlement at crude oil joint stations. Traditional techniques utilizing electrical, microwave, and radiation measurements are invariably affected by salinity. This study employs microwave resonant cavity technology to target ultralow WVF. Further design of the resonant cavity sensor structure is carried out, and a salinity compensation method based on a mechanical tuner is proposed. Theoretical analysis, simulations, and experimental results show that the tuner inserted into the microwave resonant cavity can effectively influence the resonance mode frequency, with simulation and experimental results showing good agreement. A prototype based on the design principle was developed, and by adjusting the insertion depth of the tuner, the influence of salinity within the 30 parts per thousand (ppt) range was mitigated. The measurement deviation of WVF is reduced from 2.5‰ to 0.6‰. A comparison between experimental and simulation results reveals a systematic deviation between the tuner depth h and the frequency shift. After calibration, the deviation is reduced to only ±0.2%.