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A Capacitance Sensor with Planar Sensitivity for Monitoring Soil Water Content

GSF National Research Center for Environment and Health, Institute of Soil Ecology, 85764 Neuherberg, Germany

*Corresponding author (ruth{at}gsf.de).

ABSTRACT

A pair of isolated parallel wires in a frame were used as a capacitance sensor for soil water content determination. Due to the high dielectric constant of water, rising water content of th esoil between the wires increases the capacitance. As the frame and the wires have a horizontal position in the soil, no preferential vertical water flow is induced by the sensor. The total wire diameter of 1.1 mm and the separation of 2.5 mm leave enough space for undisturbed natural water flow. The sensitive volume has a thickness given by the wire separation of 2.5 mm and the open frame area of 5 by 5 cm. Measurements in two soil types (loamy sand and loamy silt) showed a rise of the capacitance from 15 to 65 pF when the water content increases from 0 to 0.45. A theoretical consideration exhibits the relationship between the measured dielectric constant _M and the volumetric water content given by _M = A( + Z)/( + N) with three parameters A, Z, and N depending only on the sensor geometry and dielectric constants. The corresponding fit function determined the parameters and approximated the calibration curve with a root mean square (RMS) deviation of 0.024, indicating the accuracy of the method. No significant difference in the _M- relationship could be obtained between the two soils. Measurements with eight sensors at depths between 1 and 15 cm yield the time-dependent water content profile with a resolution of 1 cm during the watering and drying of the soil.

Received for publication October 29, 1997.