HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Figures Only Full Text Free Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (19) Citing Articles via Google Scholar Google Scholar Articles by Kelleners, T. J. Articles by Skaggs, T. H. Search for Related Content PubMed Articles by Kelleners, T. J. Articles by Skaggs, T. H. Agricola Articles by Kelleners, T. J. Articles by Skaggs, T. H. Related Collections Water Content Soil Hydrology Soil Methods/Instrumentation

DIVISION S-1—SOIL PHYSICS

Calibration of Capacitance Probe Sensors using Electric Circuit Theory

^a USDA-ARS, George E. Brown, Jr. Salinity Lab., 450 W. Big Springs Rd, Riverside, CA 92507
^b USDA-ARS, Water Management Research Lab., 9611 S. Riverbend Ave., Parlier, CA 93648
^c Alterra-ILRI, P.O. Box 47, 6700 AA Wageningen, The Netherlands
^d Dep. of Plants, Soils, and Biometeorology, Utah State Univ., Logan, UT 84322-4820

^* Corresponding author (tkelleners{at}ussl.ars.usda.gov).

Capacitance probe sensors are an attractive electromagnetic technique for estimating soil water content. There is concern, however, about the influence of soil salinity and soil temperature on the sensors. We present an electric circuit model that relates the sensor frequency to the permittivity of the medium and that is able to correct for dielectric losses due to ionic conductivity and relaxation. The circuit inductance L is optimized using sensor readings in a modified setup where ceramic capacitors replace the sensor's capacitance plates. The three other parameters in the model are optimized using sensor readings in a range of nonconductive media with different permittivities. The geometric factor for the plastic access tube g_p is higher than the geometric factor for the medium g_m, indicating that most of the electromagnetic field does not go beyond the access tube. The effect of ionic conductivity on the sensor readings is assessed by mixing salts in three of the media. The influence is profound. The sensor frequency decreases with increasing conductivity. The effect is most pronounced for the medium with the lowest permittivity. The circuit model is able to correct for the conductivity effect on the sensors. However, as the dielectric losses increase, the frequency becomes relatively insensitive to permittivity and small inaccuracies in the measured frequency or in the sensor constants result in large errors in the calculated permittivity. Calibration of the capacitance sensors can be simplified by fixing two of the constants and calculating the other two using sensor readings in air and water.

Abbreviations: TDR, time domain reflectometry

This article has been cited by other articles:

				R. B. Thompson, M. Gallardo, M. D. Fernandez, L. C. Valdez, and C. Martinez-Gaitan Salinity Effects on Soil Moisture Measurement Made with a Capacitance Sensor Soil Sci. Soc. Am. J., September 28, 2007; 71(6): 1647 - 1657. [Abstract] [Full Text] [PDF]

				A. Fares, H. Hamdhani, and D. M. Jenkins Temperature-Dependent Scaled Frequency: Improved Accuracy of Multisensor Capacitance Probes Soil Sci. Soc. Am. J., May 16, 2007; 71(3): 894 - 900. [Abstract] [Full Text] [PDF]

				M. Schwank, T. R. Green, C. Matzler, H. Benedickter, and H. Fluhler Laboratory Characterization of a Commercial Capacitance Sensor for Estimating Permittivity and Inferring Soil Water Content Vadose Zone J., August 24, 2006; 5(3): 1048 - 1064. [Abstract] [Full Text] [PDF]

				S. R. Evett, J. A. Tolk, and T. A. Howell Soil Profile Water Content Determination: Sensor Accuracy, Axial Response, Calibration, Temperature Dependence, and Precision Vadose Zone J., July 26, 2006; 5(3): 894 - 907. [Abstract] [Full Text] [PDF]

				T. J. Kelleners, M. S. Seyfried, J. M. Blonquist Jr., J. Bilskie, and D. G. Chandler Improved Interpretation of Water Content Reflectometer Measurements in Soils Soil Sci. Soc. Am. J., September 29, 2005; 69(6): 1684 - 1690. [Abstract] [Full Text] [PDF]

				S. D. Logsdon Soil Dielectric Spectra from Vector Network Analyzer Data Soil Sci. Soc. Am. J., June 2, 2005; 69(4): 983 - 989. [Abstract] [Full Text] [PDF]

				T. J. Kelleners, D. A. Robinson, P. J. Shouse, J. E. Ayars, and T. H. Skaggs Frequency Dependence of the Complex Permittivity and Its Impact on Dielectric Sensor Calibration in Soils Soil Sci. Soc. Am. J., January 1, 2005; 69(1): 67 - 76. [Abstract] [Full Text] [PDF]