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United States Department of Agriculture

Agricultural Research Service

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

Authors
item Schwank, Mike - ETHZ
item Green, Timothy
item Matzler, Christian - UNIVERSITY OF BERN
item Benedickter, Hansruedi - ETHZ
item Fluehler, Hannes - ETHZ

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 1, 2006
Publication Date: August 24, 2006
Citation: Schwank, M., Green, T.R., Matzler, C., Benedickter, H., Fluehler, H. 2006. Laboratory Characterization of Commercial Capacitance Sensor for Estimating Permittivity and Inferring Soil Water Content. Vadose Zone Journal.

Interpretive Summary: Ring-capacitor sensors are used widely for real-time estimation of volumetric soil water content from measured resonant frequency, which is directly affected by the bulk soil permittivity. The present laboratory experiments characterize the sensor response over a full range of permittivity values from air to water and over 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 a coaxial metal disturbance to the electric field. A precise nonlinear relationship between permittivity and normalized resonant frequency was derived. The instrumental error in permittivity is 0.226, which corresponds to a measurement precision of 0.34 percent volumetric water content. Numerical simulations of the electric field supplemented the experimental results. The characteristic length scale for the distance measured from the access tube is 12.5 mm, meaning that 80% of the signal is sensed within approximately 20 mm of the access tube. The results are crucial for scientific applications of the sensor to environmental media.

Technical Abstract: Ring-capacitor sensors are used widely for real-time estimation of volumetric soil water content from measured resonant frequency f which is directly affected by the bulk soil permittivity. However, the permittivity-frequency relationship requires improved quantification. The present laboratory experiments characterize the sensor response over a full range of permittivity values from air to water and over 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 connected to one sensor, while four other sensors provided replicated f 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 permittivity and normalized f was derived. The instrumental error in permittivity is 0.226 (for 3 < permittivity < 43) which corresponds to a measurement precision in water content derived from the Topp equation of 0.0034 volumetric water content. Axisymmetric numerical simulations of the electric field supplemented the experimental results. The characteristic length scale for the distance measured from the access tube is 12.5 mm, meaning that 80% and 95% of the signal are sensed within approximately 20 mm and 37 mm of the access tube, respectively. The results are crucial for scientific applications of the sensor to environmental media.

Last Modified: 10/20/2014
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