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United States Department of Agriculture

Agricultural Research Service

Title: Laboratory Characterization of Capacitance Sensors for Measuring Soil Water Content

Authors
item Green, Timothy
item Schwank, M. - ETHZ, ZURICH
item Fluehler, H. - ETHZ, ZURICH

Submitted to: Trans American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: September 5, 2005
Publication Date: December 5, 2005
Citation: Green, T.R., Schwank, M., Fluehler, H. 2005. Laboratory Characterization of Capacitance Sensors for Measuring Soil Water Content. Fall Meeting of theTrans American Geophysical Union, San Francisco, CA. December 5-9, 2005.

Interpretive Summary: Automated sensing of soil water content with capacitance methods is common due to the relative ease of installation and monitoring at multiple sites. The resonant frequency of an inductance-capacitance circuit is a function of the dielectric permittivity of the material surrounding the ring-capacitor sensor. However, limited relationships between resonant frequency and permittivity in the soil water range have been reported. Furthermore, sensor readings and apparent water contents have been shown to vary with temperature in laboratory and field studies in the opposite direction of that expected for free water. We designed laboratory equipment and experiments to improve our fundamental knowledge about the behavior and characterization of such a capacitance sensor (Sentek EnviroSMART). Four sensors are used for resonant frequency readings, while a fifth measures total capacitance directly on an HP Network Analyzer. A solvent-resistant container was designed for water-dioxane mixtures, which provide a complete range of permittivity values from 2.2 to over 80. The sensor readings are sensitive to permittivity changes in the range expected for soils from very dry to fully saturated. Variability between four sensors is reduced by normalization to readings in air and water. Frequency-based permittivity estimates in free water decreased linearly with temperature, as expected, substantiating the field-measured temperature dependence. Next, an expanding metal cylinder was used to interfere with the electrical field in air, water and two dioxane-water mixtures. The change in normalized readings with distance to the metal boundary is approximated by a negative exponential function with a characteristic length of 11 mm. The laboratory results are confirmed with numerical experiments assuming axisymmetric materials. Using the improved capacitance sensor characterization, water content can be estimated directly from permittivity using a universal calibration, and there is now a stronger basis for addressing the temperature-dependence of measurements in soil-air-water systems.

Technical Abstract: Automated sensing of soil water content with capacitance methods is common due to the relative ease of installation and monitoring at multiple sites. The resonant frequency of an inductance-capacitance circuit is a function of the dielectric permittivity of the material surrounding the ring-capacitor sensor. However, limited relationships between resonant frequency and permittivity in the soil water range have been reported. Furthermore, sensor readings and apparent water contents have been shown to vary with temperature in laboratory and field studies in the opposite direction of that expected for free water. We designed laboratory equipment and experiments to improve our fundamental knowledge about the behavior and characterization of such a capacitance sensor (Sentek EnviroSMART). Four sensors are used for resonant frequency readings, while a fifth measures total capacitance directly on an HP Network Analyzer. A solvent-resistant container was designed for water-dioxane mixtures, which provide a complete range of permittivity values from 2.2 to over 80. The sensor readings are sensitive to permittivity changes in the range expected for soils from very dry to fully saturated. Variability between four sensors is reduced by normalization to readings in air and water. Frequency-based permittivity estimates in free water decreased linearly with temperature, as expected, substantiating the field-measured temperature dependence. Next, an expanding metal cylinder was used to interfere with the electrical field in air, water and two dioxane-water mixtures. The change in normalized readings with distance to the metal boundary is approximated by a negative exponential function with a characteristic length of 11 mm. The laboratory results are confirmed with numerical experiments assuming axisymmetric materials. Using the improved capacitance sensor characterization, water content can be estimated directly from permittivity using a universal calibration, and there is now a stronger basis for addressing the temperature-dependence of measurements in soil-air-water systems.

Last Modified: 4/19/2014
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