ECHO measures the dielectric
constant or permittivity of the material in which it is embedded. Dielectric
moisture sensors, or aquameters, 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
measures the dielectric constant of a medium by finding the rate of change
of voltage on a sensor that is embedded in the medium. ECHO 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. ECHO's circuitry
minimizes effects due to temperature variation, and its probe coating minimizes
salinity effects. Soil texture affects the ECHO calibration to about the
same extent that it does any other dielectric sensor.