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A Capacitance Sensor with Planar Sensitivity for Monitoring Soil Water Content
Author(s) -
Ruth B.
Publication year - 1999
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1999.03615995006300010008x
Subject(s) - loam , capacitance probe , water content , soil water , capacitance , dielectric , materials science , analytical chemistry (journal) , water flow , soil science , environmental science , chemistry , geotechnical engineering , geology , optoelectronics , electrode , chromatography
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 ε M and the volumetric water content θ given by ε M = A (θ + Z )/(θ + 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 ε M ‐θ 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.