The Echo2 probes measure the dielectric constant of the soil in order to find its volumetric water content. They d o this by finding the rate of change of voltage applied to the sensor once it is buried in the soil. The Echo2 probe is the only one of its kind to have a comparatively low sensitivity to saline and temperature effects in the soil. Furthermore, it has a very low power requirement, ideal for operation with all types of Adcon RTUs. With the output signal of the probe being directly related to the input voltage it requires a voltage stabilizer interface when used with an Adcon RTU. Such an interface can handle up to three Echo2 probes.       

Echo2 Theory
The Echo2 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 Echo2 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 Echo2 probe circuitry minimizes effects due to temperature variation, and for the Echo2 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.

For a detailed analysis of the effects of texture, temperature, and salinity on the EC-10 and EC-20 ECHO probes, click here to read Decagon's paper (pdf) titled "Response of the Echo2 Soil Moisture Probe to Variation in Water Content, Soil Type, Solution Electrical Conductivity, and Temperature" by Dr. C.S. Campbell.

Technical information and download paper provided courtesy of Decagon Devices, Inc. Echo2 is a registered trademark of Decagon Devices, Inc., Washington, USA.