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Theory of TDR and ECH2O Probes' Operation
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The ECH2O soil moisture probes are capacitance-type sensors that measure the dielectric constant or permittivity of the material in which they are buried. 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), and is expensive and relatively complex. 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. The ECH2O and other low-cost moisture probes are of this type. Water has a permittivity of about 80, while the value for soil minerals is around 4, and air is 1. This high value for water results in relatively large changes in the permittivity of soil when the water content changes. Any sensor which accurately measures permittivity can be used to determine volumetric water content. Confounding factors typically are temperature and salinity. The ECH2O probe circuitry minimizes effects due to temperature variation, and for the ECH2O models EC-10 and EC-20, its probe coating somewhat minimizes salinity effects. Soil texture affects the calibration of the EC-10 and EC-20 to about the same extent that it does any other dielectric sensor. However, The EC-5 and ECHO TE sensor operate at a higher frequency (70 MHz) than the EC-10 and EC-20, and therefore the influence of textural and salinity effects of these probes is greatly minimized.https://blog.sciencenet.cn/blog-3628-9618.html
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