Detection of Soil Moisture, Humidity, and Liquid Level Using CPW-Based Interdigital Capacitive Sensor

The measurement of soil moisture and air humidity is essential to many technological and environmental applications. Sensors provide information that is useful in order to effectively manage and control various processes. This study presents an energy-efficient, highly reliable, ultra-high frequency (UHF) capacitive sensor that can measure moisture/humidity, and liquid levels. As a sensitive part, an interdigital capacitor (IDC) is fed via a coplanar waveguide (CPW) for efficient power transfer. Aside from the sensitive part of the IDC, the structure of the sensor is insensitive to variations in surrounding permittivity, which reduces uncertainty in results and thus improves accuracy. The principle of sensing is based on a reactive phase variation of the input signal upon reflection, whereas its amplitude and phase are actively changing with variations in the dielectric constant of the test medium. Practical testing was conducted on the sensor to determine soil moisture, air humidity, and liquid level measurement. At a fixed UHF frequency of 915 MHz the sensor offers a capacitive sensitivity of 7.5 fF/%WC, 4.5 fF/%RH, and 13.4 fF/mm for soil moisture, air humidity, and water level detection respectively. This new sensor provides high reliability, good sensitivity, low power consumption, and can be implemented in a number of applications, including agriculture, oil, and gas industry, land and water treatment, medical equipment, and biotechnology.