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

Published in : IEEE Sensors Journal (Volume: 22, Issue: 11, June 2022)
Authors : Shaheen Ahmad, Nabil Khalid,Rashid Mirzavand
DOI : https://doi.org/10.1109/JSEN.2022.3167337
Summary Contributed by:  Saurabh Dubey

Accurate measurement of soil moisture and air humidity is crucial for technology and environmental applications. Advanced sensor technology is employed to meet the demands of energy-efficient production and smart agriculture.

The high permittivity of water, with a dielectric constant of 80 at 20°C, leads to increased sample permittivity corresponding to higher water content. Measurements of humidity and soil moisture utilize capacitive sensing techniques instead of low responsive resistive sensors.

Capacitive sensors, known for their energy efficiency and sensitivity to permittivity changes, provide a stable, linear, and swift response across diverse dielectric constants for soil moisture monitoring. These sensors include an extra sensing layer composed of organics, polymers, oxides, and biodegradable materials to enhance sensitivity. This allows real-time environmental tracking without internal power sources, improving sensor longevity and reliability.

Interdigital capacitive sensors (IDC) are widely used for their simplicity, high sensitivity, and cost-effectiveness in estimating material dielectric properties. The integration of coplanar waveguide (CPW) feeding enhances the sensing capabilities of this sensor. Consisting of interdigital electrodes on a PCB, it operates by detecting variations in reactive capacitance through penetrating fringing electric fields in the test medium (MUT).

The sensing layer's type and thickness are crucial factors affecting sensor capacitance, sensitivity, and penetration depth. Ansys full-wave FEM simulations, along with calibrated LPKF U3, ensure accurate substrate and copper thickness for IDC sensor design and testing.

Polyvinyl alcohol (PVA) serves as the sensing layer, enhancing humidity detection sensitivity. The PVA solution is created at a 1:50 ratio by dissolving PVA powder in deionized water, facilitating water particle accumulation on sensing electrodes.

Practical testing of the prototype for soil moisture, air humidity, and liquid level detection employs ROHDE & SCHWARZ Vector Network Analyzer (VNA) for phase and capacitance variation measurements to assess the sensor’s efficacy.

Laboratory experiments used clay soil with an estimated 20% water content, evaluating sensor response at 22°C and 44%RH through water-to-clay weight ratio calculations. A linear phase change was recorded across 20-100% soil moisture, with nine samples prepared at regular intervals using a fixed 915 MHz frequency VNA.

Results demonstrated good detection sensitivity with a reasonably linear curve fitting model with R2 equal to 0.978, with minor non-linearity due to uneven water content distribution and incomplete gap filling between electrodes.

Liquid level detection showed no phase change up to 20 mm, initiating at 20 mm above ground vias. For humidity detection, the PVA spray-coated sensor was tested in a special humidity chamber designed with a Repti Fogger RF-10 humidifier and monitored by an HT-10 reference humidity sensor.

The IDC sensor maintains linearity between 65-100% RH, excelling in soil moisture, air humidity, and liquid level detection at room temperature, with a linear response for soil moisture (20-100%) and reliable detection of liquid levels up to 40 mm.

Designed for a zero-power RFID sensing architecture, the sensor exhibits promising linear responses, and evaluations across varying temperatures suggest potential applications in diverse environmental monitoring. The use of polyamide spray coating can further enhance sensitivity, highlighting the practical deployment prospects of the sensor.

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