Up to 32 sensors are supported by 1 logger on 2 cable runs of up to 16 sensors with up to 4 probes on each cable run in any configuration. The sensors utilise electrical capacitance to measure soil moisture. The capacitance field generated between the two metal plates of the sensor extends beyond the PVC access tube into the surrounding soil. The capacitance field is affected by the dielectric constant of the material the field passes through. The dielectric of soil, regardless of whether it is sand, loam or clay, is relatively constant ranging from 3 to 6 while air is 1 and water is 80.
Soil moisture is determined by the change in the amount of air and water in the soil structure.
This changes with rainfall, irrigation, drainage, evaporation and crop water use. Using capacitance the changing ratio of air and water at each soil depth can be measured very quickly and accurately. The sensor readings are converted to volumetric soil water content using a default calibration equation in the software that takes into account the varying soil types. This calibration was established using gravimetric sampling which is the international standard used to calibrate methods such as the Neutron Probe. Data can be logged at intervals as short as every one minute and is downloaded to a computer for display.