ARS | Publication request: CALIBRATION OF CAPACITANCE PROBE SENSORS USING ELECTRIC CIRCUIT THEORY

Submitted to: Agronomy Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: August 1, 2003
Publication Date: November 1, 2003
Citation: Kelleners, T.J., Soppe, R., Robinson, D., Schaap, M., Ayars, J.E., Skaggs, T.H. 2003. Calibration of capacitance probe sensors using electric circuit theory. CD-ROM. Agronomy Abstracts. Denver, CO.

Technical Abstract: 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 is able to correct for dielectric losses due to ionic conductivity and relaxation. The parameter describing circuit inductance is optimized using sensor readings in a modified setup where ceramic capacitors replace the sensor's electrodes. The three other parameters in the model are optimized using sensor readings in a range of non-conductive media with different permittivities. 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.