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• W2781798876 abstract "This thesis details the design and testing of several soil moisture sensorsfor use in agricultural applications. The thesis concentrates on capacitivesensors measuring water content by the change in capacitance of twoelectrodes in direct contact with the soil water, although several other sensortypes are investigated.Two electrodes placed in the soil will have a characteristic compleximpedance that depends on the sensor configuration, soil structure and soilcomponents. The impedance measured is dependant also on the frequency ofthe signal used for the measurement. As the frequency is lowered the effect dueto capacitance and inductance become quite small and the impedancebecomes mainly resistive. Conversely at high frequencies(>100MHz) theresistive component becomes less significant. A sensor designed to operate at10kHz needs to sense small changes in an already small capacitance (pF)while ignoring the very significant resistive component. The use of insulatedelectrodes allows the separation of resistive and capacitive components but atthe expense of placing a capacitance in series with the small soil capacitance. Amethod was developed to compensate for variations due to manufacturingtolerances in the insulating layer which would have made this solutionimpractical. Reference electrodes constructed on the same substrate as thesensor electrodes correct errors introduced by environmental andmanufacturing effects on the electrode insulation.A model is presented describing the relation between water content andmeasured capacitance and conductivity. In experimental tests the model issuccessful in describing some damp to wet soils.It is shown that the small soil capacitance due to the water content isswamped by conduction in the soil. This would seem to prevent practical watercontent measurement at low frequencies. Experimental work demonstrated thatin an insulated electrode sensor operating at low frequencies, quite largevariations in measured capacitance still occur with changes in water content.Soil water forms conductive paths terminating on the electrode insulationsurface. This capacitance is shown to be unaffected by soil conductivity andvaries with water content and so can be used to estimate soil water.An insulated electrode sensor incorporating reference electrodes andoperating at 10KHz was designed and tested. It was shown that the referenceelectrodes enabled correction of manufacturing and environmental variables. Itis further demonstrated that contamination of the insulation coating withconductive material from within the soil prevents the practical use of this design.A method is shown for deriving both soil water content and waterconductivity by measurement of the complex impedance of the sensor in soil.Solder-less construction of sensor circuitry and electronics using screenprinted circuitry on plastic films was demonstrated.A design for a sensor using un-insulated electrodes and operating at10MHz was constructed and tested. This sensor was not affected bycontamination of the sensor surface. Measurements in samples of sand rangingfrom dry to saturated (20% water by mass) were linearly related( < 6%) to thelog of water content. The sensor used only 2 active components and 23 passivecomponents." @default.
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• W2781798876 date "2007-01-01" @default.
• W2781798876 modified "2023-09-24" @default.
• W2781798876 title "The Design of an Integrated Soil Moisture Sensor for Agriculture" @default.
• W2781798876 doi "https://doi.org/10.25904/1912/3681" @default.
• W2781798876 hasPublicationYear "2007" @default.
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