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United States Department of Agriculture

Agricultural Research Service

Title: Analysis of coaxial soil cell in reflection and transmission

Authors
item Pelletier, Mathew
item Viera, Joe -
item Schwartz, Robert
item Lascano, Robert
item Evett, Steven
item Green, Timothy
item McMichael, Bobbie

Submitted to: Sensors
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 23, 2011
Publication Date: March 1, 2011
Citation: Pelletier, M.G., Schwartz, R.C., Evett, S.R., McMichael, B.L., Lascano, R.J., Gitz, D.C. 2011. Analysis of coaxial soil cell in reflection and transmission. Sensors. 11:2592-2610.

Interpretive Summary: This paper reports an accurate technique for measuring absolute permittivity of materials for cotton, soil-science, and geological community. The result is needed for accurate measurement of moisture content that presents a significant technological challenge in hydrological, geophysical, and biogeochemical research as well as for material characterization and process control. In particular, bound water content (in addition to accurate measurement of the surface area) is becoming increasingly important for providing answers to many fundamental questions such as (a) characterization of cotton fiber maturity, (b) characterization of soil water content in soil water conservation, (c) bio-plant water utilization as well as chemical reactions, (d) diffusions of ionic species across membranes in cells, and (e) dense suspensions that occur in surface films. The proposed technique of this paper improves the measurement techniques of apparent permittivity using time-domain-reflectometry (TDR). The TDR-based permittivity measurement has been used in many process control applications. Recent research, however, is indicating a need to increase the accuracy beyond what is currently available from traditional TDR. The most logical pathway then becomes a transition from TDR based techniques to Network Analyzer based techniques – in the frequency domain – to measure absolute permittivity as opposed to apparent permittivity. This technique removes the adverse effects that high surface area soils and conductivity impart onto the measurements of apparent permittivity in traditional TDR applications.

Technical Abstract: 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 are 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-reflectometery (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 the theoretical basis behind the coaxial probe, from which the modern TDR probe originated from, to provide a basis on which to perform absolute permittivity measurements. The research reveals currently utilized formulations in accepted techniques for permittivity measurements violate the underlying assumptions inherent in the basic models due to the TDR acting as an antenna by exhibiting radiation of energy off the end of the probe. To remove the effects of radiation from the experimental results obtained herein, this research utilized custom designed coaxial probes of various diameters and probe lengths by which to test the coaxial cell measurement technique for accuracy in determination of absolute permittivity. In doing so, the research reveals that all the basic models available in the literature omitted a key correction factor that is hypothesized by this research as being most likely due to fringe capacitance. To test this theory, a Poisson model of a coaxial cell was formulated to calculate the effective extra length provided 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 for an augmented measurement technique for measurement of absolute permittivity.

Last Modified: 10/1/2014
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