Development of a Capacitive Sensor for Monitoring Soil Moisture
Ilda Silva, Adunias dos Santos Teixeira, Francisco José Firmino Canafístula, Raimundo Leão · 2005 Tampa, FL July 17-20, 2005 · 2005
A capacitive sensor for soil-water monitoring was developed using a two-plate capacitorand an oscillator. The capacitor was built by placing two parallel fiberglass circuit boards coated withby-component automotive varnish 5 mm apart. The resistance produced by the varnish coating wasof the order of 106 .. The capacitor, together with resistors the IC 74LS122N (Retriggerable Monostable Multivibrators) was designed as an RC oscillator. The output of the 74LS122 wasdirected to a 12 Stage Ripple Carry Binary Counter/Divider (TC4040BP) In order to remove anyaddictive effect from wires, the circuit was installed directly at the capacitor boards. Two sensorswere installed on a block of soil installed at one side of a suspended scale and reading from thesensors, from a load cell and from a temperature sensor were taken daily at 8:00, 11:00, 13:00 and15:00 Oclock for 63 days continuously. Results from calibration displayed a third degree polynomialcorrelation between frequency output and volumetric moisture content with R2 above 0.985 for bothcapacitive sensors. The frequency output of the sensors presented a sensibility to room temperaturevariation as the temperature produced a reduction on the output frequency of the sensors. However,a regression analysis in a temperature dependant model of the data demonstrated no statisticalsignificance of temperature on the response of the sensors.