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United States Department of Agriculture

Agricultural Research Service

Title: Laboratory Characterization of a 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 Schulin, Rainer - ETHZ
item Fluehler, Hannes - ETHZ

Submitted to: Vadose Zone Journal
Publication Type: Book / Chapter
Publication Acceptance Date: April 15, 2006
Publication Date: August 24, 2006
Citation: Schwank, M., Green, T.R., Matzler, C., Benedickter, H., Schulin, R., Fluehler, H. 2006. Laboratory Characterization of a Commercial Capacitance Sensor for Estimating Permittivity and Inferring Soil Water Content. Vadose Zone Journal.

Interpretive Summary: Volumetric soil water content ''can be estimated from the bulk soil dielectric constant ' measured using ring-capacitor sensors inserted into a plastic access tube augured into soil. The present laboratory experiments were designed to characterize the sensor response over a full range of environmental conditions including ' 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. The measured temperature response of free water permittivity was linear with a negative slope, which is consistent with theory. A precise nonlinear relationship between ' and normalized sensor reading was derived, where the instrumental error in ' (RMSE' = 0.226 for 3 < ' < 43) corresponds to a measurement precision of 0.0034 m3m-3 for ''('). Axisymmetric numerical simulations of the electric field were used to extend the experimental results. The characteristic length scale for the distance measured from the access tube is approximately 12 mm, meaning that 95% of the signal is sensed within 36 mm of the access tube. The results are crucial for scientific applications of the sensor to environmental media including future investigations of physically based dielectric mixing models.

Technical Abstract: Volumetric soil water content ''can be estimated from the bulk soil dielectric constant ' measured using ring-capacitor sensors inserted into a plastic access tube augured into soil. The present laboratory experiments were designed to characterize the sensor response over a full range of environmental conditions including ' values from air to water and over a range of temperatures. First, resonant frequency fr values were recorded for surface-mounted devices of known capacitance C soldered to the sensor electronics, confirming a linear relationship between fr-2 and C. Next, 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 C was measured using a Vector Network Analyzer connected to one sensor, while four other sensors provided replicated fr readings. The measured temperature response of free water permittivity was linear with a negative slope, which is consistent with theory. A precise nonlinear relationship between ' and normalized sensor reading is derived, where the instrumental error in ' (RMSE' = 0.226 for 3 < ' < 43) corresponds to an error (i.e., precision) in soil water content ''(') derived from Topp’s equation'of RMSE' = 0.0034 m3m-3. Different models for ''(')'were compared with the manufacturer’s default calibration equation, which tends to underestimate ''by approximately 0.066 m3m-3. Axisymmetric numerical simulations of the electric field were used to extend the experimental results. The characteristic length scale of an exponential model for the distance measured from the access tube is approximately 12 mm over a range of ' values typical of soils, meaning that 95% of the signal is sensed within 36 mm of the access tube. The results are crucial for scientific applications of the sensor to environmental media including future investigations of physically based dielectric mixing models.

Last Modified: 12/28/2014
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