FDR and capacitance probes measure the soil dieletric constant using two or more plates or rods which are embedded into the soil. In FDR the dielectric constant is determined by applying an oscillating charge to the circuit and measuring its resonant frequency. In capacitance measurements, the plates complete an oscillator circuit (using the soil as a dieletric) with the frequency determined by the soils permittivity. The capacitor plates are usually arranged concentrically for easy borehole measurement in access tubes [1].
Used for measuring soil water content.
Measurements are affected by the soils bulk density, texture, etc.
Commercial.
1. Dean, T.J., Bell, J.P., Baty, A.J.B., 1987. Soil moisture measurement by an improved capacitance technique. Part I. Sensor design and performance. Journal of Hydrology 93 (1–2), 67–78