The Environscan is a soil moisture measuring instrument developed in Australia. It works on the capacitance principle ie it measures the change in capacitance of the soil depending on the moisture level, as there is a large difference in the dielectric constant of soil, air and water. The Environscan measures the change in frequency response of the soil's capacitance due to its soil moisture status. The calibration curve is supposed to be universal, but there is some evidence that this is not so.
Care has to be taken in installation as air gaps can dramatically alter the response.
There is a similar instrument manufactured in the States. Hope this is a simple enough explanation. This equation is supplied as the default equation in the software and is more than adequate for the majority of users that are using the data for irrigations scheduling purposes. Can set refill points by assessing the point of inflection on the soil moisture depletion curve. Can assess water use as vines come out of dormancy, extraction patterns and root development.