A Low Cost, Wireless Embedded Sensor for Moisture Monitoring in Hard-to-Access Places

A wireless, passive embedded sensor based on an inductive-capacitive (LC) resonant circuit comprised of a planar spiral inductor is presented. The sensor consists of a layer of electrical conductor in the form of a square spiral that resonates due to parasitic capacitance between the conductive traces. Therefore, the fabrication process is simpler than previous planar LC sensors that require multiple conductor layers. To monitor moisture at hard-to-access locations such as behind building drywall, the sensor is first adhered to the enclosed side of the wall during building construction. The detection system can then wirelessly measure the resonant frequency of the sensor through the wall. Since water vapor has a high electrical permittivity compared to air, increasing moisture content around the sensor increases the parasitic capacitance of the spiral inductor, which in turn decreases the sensor's resonant frequency. Unlike other wireless, passive sensors like the RFID sensors, the presented sensor is very simple in design and fabrication, making it easy to install and use and keeping the sensor's unit cost low. Compared to infrared heat mapping, which is commonly used to indirectly detect moisture in inaccessible places, the presented embedded sensor is more accurate and less susceptible to interference from the surrounding environment. This paper presents the design and fabrication of the sensor, and demonstrates its application in monitoring moisture content inside drywall walls. Using a drywall enclosure, the performance of the sensor was characterized and its strengths and limitations were determined.