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Sensors 2011, 11(1), 757-770; doi:10.3390/s110100757

Fringe Capacitance Correction for a Coaxial Soil Cell

1,* , 2, 3, 4, 3, 4, 1 and 1
1 Cotton Production and Processing Unit, USDA-ARS, Lubbock, TX 79403, USA 2 Sensors Group Microsemi Corporation Lowell, MA 01851, USA 3 Soil and Water Management Research Unit, USDA-ARS, Bushland, TX 79012, USA 4 Agricultural Systems Research Unit, USDA-ARS, Fort Collins, CO 80526, USA 5 Wind Erosion and Water Conservation Unit, USDA-ARS, Lubbock, TX 79403, USA
* Author to whom correspondence should be addressed.
Received: 10 November 2010 / Revised: 27 December 2010 / Accepted: 28 December 2010 / Published: 12 January 2011
(This article belongs to the Section Physical Sensors)
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Accurate measurement of moisture content is a prime requirement in hydrological, geophysical and biogeochemical research as well as for material characterization and process control. Within these areas, accurate measurements of the surface area and bound water content is becoming increasingly important for providing answers to many fundamental questions ranging from characterization of cotton fiber maturity, to accurate characterization of soil water content in soil water conservation research to bio-plant water utilization to chemical reactions and diffusions of ionic species across membranes in cells as well as in the dense suspensions that occur in surface films. One promising technique to address the increasing demands for higher accuracy water content measurements is utilization of electrical permittivity characterization of materials. This technique has enjoyed a strong following in the soil-science and geological community through measurements of apparent permittivity via time-domain-reflectometry (TDR) as well in many process control applications. Recent research however, is indicating a need to increase the accuracy beyond that available from traditional TDR. The most logical pathway then becomes a transition from TDR based measurements to network analyzer measurements of absolute permittivity that will remove the adverse effects that high surface area soils and conductivity impart onto the measurements of apparent permittivity in traditional TDR applications. This research examines an observed experimental error for the coaxial probe, from which the modern TDR probe originated, which is hypothesized to be due to fringe capacitance. The research provides an experimental and theoretical basis for the cause of the error and provides a technique by which to correct the system to remove this source of error. To test this theory, a Poisson model of a coaxial cell was formulated to calculate the effective theoretical extra length caused by the fringe capacitance which is then used to correct the experimental results such that experimental measurements utilizing differing coaxial cell diameters and probe lengths, upon correction with the Poisson model derived correction factor, all produce the same results thereby lending support and for an augmented measurement technique for measurement of absolute permittivity.
Keywords: TDR; cotton moisture; moisture sensing; permittivity; microwave sensing; microwave moisture TDR; cotton moisture; moisture sensing; permittivity; microwave sensing; microwave moisture
This is an open access article distributed under the Creative Commons Attribution License (CC BY) which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Pelletier, M.G.; Viera, J.A.; Schwartz, R.C.; Lascano, R.J.; Evett, S.R.; Green, T.R.; Wanjura, J.D.; Holt, G.A. Fringe Capacitance Correction for a Coaxial Soil Cell. Sensors 2011, 11, 757-770.

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