This Is AuburnElectronic Theses and Dissertations

Development of Passive Wireless Humidity Sensors and Dielectric Methodology for A Real-time In-situ Sensing of Disinfectant




Liu, Jiachen

Type of Degree

PhD Dissertation


Materials Engineering

Restriction Status


Restriction Type

Auburn University Users

Date Available



Many agricultural sensors, which play a critical role in smart and precision agriculture, are used to provide real-time sensing information for better control of agricultural production and saving of freshwater. There are preharvest and postharvest processes in agricultural production. In preharvest, soil water sensors are heavily used to detect soil water in vast crop farmland, which requires a wireless sensing feature and large numbers of distributions of the sensors. In postharvest, a huge amount of fresh produce is disinfected by disinfection and wash processes to ensure food safety before being sold in the supermarket, which requires an in-situ real-time monitor of the disinfectant. Based on these two needs, we developed two passive wireless sensor platforms to detect soil water for the preharvest process and new dielectric sensing and analyzing methodology to monitor the disinfectant for the postharvest process. First, a wireless sensor platform, i.e., a magnetostrictive particle (MSP) sensor coated by a layer of the water-sensitive polymer was developed. The polymer-MSP sensor was a low-cost passive wireless freestanding sensor to detect humidity, which had the potential to be used to in-situ wirelessly monitor soil water in the soil. Moreover, two water-sensitive polymers were coated on the MSP sensor platform to measure their overall humidity sensing performances. Cellulose nanofiber (CNF), which was an environmental-friendly material, was used to develop the polymer-MSP humidity sensor. Another water-sensitive polymer, polyvinyl alcohol (PVA), which was originally vulnerable to water, by crosslinking, turned out to be water-resistant without losing its water-sensitive capability. An issue came out that, as the polymer gained water resistance by crosslinking, the water-sensitive ability was gradually decreased. To obtain water-resistance capability while retaining its high water sensitivity, a fundamental study to optimize the crosslinking ratio for the PVA was conducted. Second, a high dielectric permittivity ceramic was discovered to make a humidity capacitance sensor, due to that the sensor had a unique humidity sensing feature and a potential to be used to develop an inductance-capacitance (LC) resonant sensor to in-situ wirelessly monitor soil water in the soil. Third, dielectric sensing and analyzing methodology to in-situ real-time monitor the disinfectant was developed. This research determined a characteristic frequency, which was changing with the disinfectant concentration in the water. As this dielectric methodology had many advantages, such as in-situ real-time detection, low-cost, and simple handling, over other sensing technologies, it had the potential to be used in the disinfection and wash processes of fresh produce production. Water quality influence on sensing of the disinfectant concentration was studied. Moreover, the influence of contamination from sand/soil on dielectric sensing was also studied.