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ORIGINAL RESEARCH

Laboratory Characterization of a Commercial Capacitance Sensor for Estimating Permittivity and Inferring Soil Water Content

^a Institute of Terrestrial Ecosystems (ITES), Swiss Federal Institute of Technology (ETH), CHN E29, Universitätstr. 16, CH-8092 Zürich, Switzerland
^b USDA-ARS, Great Plains Systems Research Unit, Fort Collins, CO, USA
^c Institute of Applied Physics, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland
^d Laboratory for Electromagnetic Fields and Microwave Electronics, ETHZ, ETZ K 88, Gloriastrasse 35, CH-8092 Zürich, Switzerland
^e Institute of Terrestrial Ecosystems (ITES), Swiss Federal Institute of Technology (ETH), CHN F 28.1, Universitätstr. 16, CH-8092 Zürich, Switzerland
^* Corresponding author (mike.schwank{at}env.ethz.ch)

Received 13 January 2006.

Ring-capacitor sensors are used widely for real-time estimation of volumetric soil water content from measured resonant frequency f_r, which is directly affected by the bulk soil permittivity . However, the relationship f_r() requires improved quantification. We conducted laboratory experiments to characterize the response of the Sentek EnviroSMART sensor system for a full range of values from air to water and a range of temperatures. Water–dioxane mixtures were placed into a solvent-resistant container equipped with custom tools for heating and mixing the fluid, removing air bubbles from sensitive surfaces, measuring permittivity in situ, and creating an axisymmetric metal disturbance to the electric field. Total capacitance C was measured using a vector network analyzer (VNA) connected to one sensor, while four other sensors provided replicated f_r readings. The measured temperature response of free water permittivity was linear with a negative slope, which is qualitatively consistent with theory. A precise nonlinear relationship between and normalized f_r was derived. The instrumental error in was RMSE = 0.226 (for 3 < < 43), which corresponds to a measurement precision in () derived from Topp's equation of RMSE= 0.0034 m³m^–3. Axisymmetric numerical simulations of the electric field supplemented the experimental results. The characteristic length scale for the distance measured radially from the access tube is 12.5 mm, meaning that 80 and 95% of the signal are sensed within approximately 20 and 37 mm of the access tube, respectively. The results are crucial for scientific applications of the investigated sensor type to environmental media.

Abbreviations: FEP, fluorinated ethylene-propylene • SMD, surface mounted device

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