Development and Application of the Single-Spiral Inductive-Capacitive Resonant Circuit Sensor for Wireless, Real-Time Characterization of Moisture in Sand
A wireless, passive embedded sensor was designed and fabricated for monitoring moisture in sand. The sensor, consisted of an inductive-capacitive (LC) resonant circuit, was made of a printed spiral inductor embedded inside sand. When exposed to an electromagnetic field, the sensor resonated at a specific frequency dependent on the inductance of the inductor and its parasitic capacitance. Since the permittivity of water was much higher than dry sand, moisture in sample increased the parasitic capacitance, thus decreasing the sensor’s resonant frequency. Therefore, the internal moisture level of the sample could be easily measured through tracking the resonant frequency using a detection coil. The fabrication process of this sensor is much simpler compared to LC sensors that contain both capacitive and inductive elements, giving it an economical advantage. A study was conducted to investigate the drying rate of sand samples of different grain sizes. The experimental data showed a strong correlation with the actual moisture content in the samples. The described sensor technology can be applied for long term monitoring of localized water content inside soils and sands to understand the environmental health in these media, or monitoring moisture levels within concrete supports and road pavement. 1. Introduction Monitoring of water content in porous mechanical structures is important in ensuring and predicting their mechanical integrity. For example, monitoring water content in asphalt is critical for assessing its structural stability because the bond between asphalt and aggregate particles can be broken or reduced in strength due to the presence of unwanted moisture. Similarly, it is important in concrete mixing to take into consideration the natural moisture of aggregates during the mix design to ensure proper slump and compressive strength of the resulting concrete structure [1]. Several methods currently exist for measuring water content in civil constructs. A common method involves determining the mass of a sample before and after drying. The disadvantages of this method include the need to remove the material (or a sample), the length of the drying process, and the possibility of degrading the sample as a result of heating. Additional complications can arise when moisture-sensitive environments must be breeched to collect the sample for drying. A more sophisticated method for measuring water content consists of two insulated conductor probes. The permittivity of the medium will depend on water content between the probes and can be determined by
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