Dielectric Constant
Dielectric moisture sensors are of two types. One measures the dielectric constant of a medium by finding the time taken for an electromagnetic pulse to traverse a transmission line buried in the medium. This type of sensor is called a time domain reflectometer (TDR). The second type of sensor (capacitance) measures the dielectric constant of a medium by finding the rate of change of voltage on a sensor that is embedded in the medium. Any sensor which accurately measures permittivity can be used to determine volumetric water content. Confounding factors typically are temperature and salinity.
Campbell Scientific TDR sensor
Acclima TDT sensor
Decagon EC-5 Capacitance Sensor
Baseline Systems TDT sensor