Capacitance

Soil moisture measurement using the capacitance principle
The 10HS and EC5 have a high frequency moisture detector which uses the capacitance principle to measure the dielectric permittivity of soil.

The moisture measurement of the material is based upon the fact that in a water: soil: air matrix, the dielectric constant is dominated by the amount of water present. The dielectric constant of water is approximately equal to 80 whereas the dielectric constant of soil is approximately equal to 3 or 4 and air is equal to 1. Therefore any changes in the volume matrix ratio of water will result in a substantial change in the dielectric constant of the matrix. Then the soil water content can be measured exactly because changes in water content of the soil result in changes in the dielectric constant of the soil.

The capacitance technique determines the dielectric permittivity of a medium by measuring the charge time of a capacitor, which uses that medium as a dielectric. The capacitance sensor forms a pair of electrodes and the soil acts as a dielectric. The capacitor charge time is a linear function of the dielectric permittivity of the soil.

An article explaining capacitance from first principles and the effects of variables such as temperature and material as explained   -   Capacitance First Principles

The 10HS and EC5 have a high frequency of measurement (>70MHz) and can therefore measure the dielectric permittivity of the soil accurately.

The Soil Moisture Meter (SMM) is a stand-alone logging instrument used to convert raw signals from the 10HS and EC5 into calibrated volumetric moisture contents. ICT International provides calibration curves or the user can easily add their own calibration curves.