The Capacitance Measurement Principle
Capacitance sensors measure the dielectric permittivity of a surrounding medium.
The configuration is either like the neutron probe where an access tube, made of PVC, is installed in the soil or buried probes connected to a data logger. In either down hole or buried configuration, a pair of electrodes form the plates of the capacitor with the soil in between these plates, acting as the dielectric. The oscillating electrical field is generated between the two plates and extends into the soil medium either through the wall of the PVC access tube or conformal coating of the buried probe. Changes in dielectric constant of the surrounding media are detected by changes in the operating frequency. The output of the sensor is the frequency response of the soil’s capacitance due to its soil moisture level.
Capacitance sensors come in many configurations and have many shapes. Due to the low cost and low power consumption capacitance sensors are common. The impact of temperature and conductivity on the measurement of volumetric soil moisture means they are suited to monitor relative changes of soil water content and require in-situ calibration for accurate measurement of volumetric soil water content (VSW%).
Capacitance sensors have a small volume of measurement and usually a limited life of several years in situ. Capacitance sensors are widely used for irrigation scheduling.