Standing Wave

Soil moisture measurement using the standing wave principle
The MP406 and MP306 have a high frequency moisture detector which uses the standing wave principle to indicate the ratio of two or more substances forming a body of material, each substance having a different electric constant (Ka).

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 standing wave technique uses an oscillator to generate an AC field in order to detect the dielectric properties of a soil or substrate of interest. The parallel needles of the MP406 and MP306 act as a coaxial transmission line which is used to generate a signal. The amplitude of the signal which is generated by a reflected AC signal is measured and converted to moisture content.

The material that can be measured by the MP406 and MP306 is often soil but can be any composition of non-metallic powdered, liquid or solid phase substance into which the needles are inserted.

The Soil Moisture Meter (SMM) is a stand-alone logging instrument used to convert raw signals from the MP406 and MP306 into calibrated volumetric moisture contents. ICT International provides calibration curves or the user can easily add their own calibration curves.