Dual-load Hybrid Detection of Water Content Using Electromagnetic and Surface Acoustic Wave Sensors

This work focuses on water content detection both in materials and in gases, i.e., measurements of moisture and humidity. To detect the moisture, a cavity perturbation method (CPM) is introduced to extract the relationship between permittivity and water content in material under test (MUT), which, in this application is tobacco at a single frequency. The obtained information is used as pre-knowledge for a mass flow sensor design in tobacco production. The mass flow sensor is an open cylindrical resonator that consists of two circular waveguides with different diameters. An on-sight test of the mass flow sensor is performed, and the measured weights are within a 10% discrepancy compared to the supplied weights. For the humidity measurement, a material named polyvinyl alcohol (PVA) is employed as a sensitive layer. The PVA film involves two mechanisms for humidity sensing, which are variations of electrical loading and mechanical loading with relative humidity (RH). To apply the electrical loading property, the PVA film is characterized by an interdigital capacitor (IDC) in a wide frequency range at different RHs, and the results are used to create a comprehensive model of the PVA film for computer simulation tools to predict the performance of the sensor. A wireless humidity sensor is designed to verify the PVA film model, and a good agreement between the simulation and measurement results proves the accuracy of the model. To utilize the mechanical loading property, the PVA film is loaded on surface acoustic wave (SAW) resonators. A wet chemical method of patterning the PVA film is found, and the influence of the PVA film thickness is investigated. With different patterns and thicknesses of the PVA film, humidity sensors can achieve various performances in terms of operation range, sensitivity, and quality factor. To apply both, electrical and mechanical loading properties of the PVA film for humidity sensing at the same time, a novel dual-load SAW resonator is introduced by using additional IDCs as reflectors with the PVA film loaded on top. The humidity sensor based on this novel hybrid structure extends its operating range, and shows better sensitivity and quality factor with different sensing mechanisms within certain RH ranges. A figure of merit (FoM) is defined to evaluate the sensitivity and quality factor of the sensor at the same time, and the sensing mechanism with better FoM can be selected for humidity sensing in a defined RH range