Calibration of FDR sensor for irrigation automation
Mohamed Fahmy Hussein
The theoretical basis of FDR sensors is often obscure for many scientists and field users. The present work introduces the concept and gives a first local calibration of two affordable VH400 Frequency Domain Reflectometry (FDR) sensors (running at 80 MG Hz) from Vegetronics, Utah, USA, enhanced by local electronic components. The two calibrations were carried out in August 2013 (35°C) and January 2014 (15°C) at Cairo University, Egypt. The full scale relationships of moisture content to mV and soil dielectric, among others, were all non-linear. However, for the narrower scale useful for soil moisture contents, from wilting point to saturation, were acceptably linear. The observed strong linearity in this narrower range makes the sensor practical for irrigation automated control. A famous power dielectric mixing-formula was used to estimate soil dielectric from three components mixing (minerals, water and air). Temperature decrease significantly increased soil dielectric values of the used sand sediments of the Nile bank that were adjusted to given moisture contents. Temperature dependence urges that the calibration process must be run at least four times per year for each soil material. Irrigation automation is particularly needed in for the partial fulfillment of irrigation control in order to tackle Egypt's augmenting water scarcity problems.