Campbell Scientific Wireless Sensor Network (CWB100, CWS220, and CWS900) - Manual (Page 12)

Wireless Sensor Network
1.4 CWS655 Wireless Water Content Reflectometer
The CWS655 is based on Campbell Scientific’s CS655 water content
reflectometer. It measures volumetric soil water content, electrical
conductivity (EC), dielectric permittivity, and ambient temperature of soils or
other porous media.
The water content information is derived from the probe’s sensitivity to the
dielectric permittivity of the medium surrounding its stainless-steel rods.
Dielectric permittivity increases with volumetric water content in a predictable
fashion. During water content measurements, the CWS655 is configured as a
water content reflectometer with the two parallel rods forming an open-ended
transmission line. A differential oscillator circuit is connected to the rods. An
oscillator state change is triggered by the return of a reflected signal from one
of the rods. The two-way travel time of the oscillator-induced electromagnetic
waves on the rod increases with increasing volumetric water content of the
surrounding media, hence the name water content reflectometer.
The electrical conductivity of the surrounding medium is derived from signal
attenuation information. This is accomplished by exciting the rods with a
known non-polarizing waveform and determining the signal attenuation due to
EC. The EC measurement is used to correct the oscillator period and is also
available to the user.
Temperature is measured with a thermistor that is in contact with one of the
rods. The thermistor makes a point measurement near the top of the sensor
rods. In a typical application this will correspond to a soil temperature reading
near the soil surface so care should be taken in interpreting soil temperature
data.
Because the CWS655 is wireless, it can be mounted in many locations that
would be problematic for a cabled sensor.
The CWS655 is generally not intended for burial below the soil surface in
typical installations as this will attenuate the radio signal and make it difficult
to change the batteries. Submersion in water or saturated soil may cause
damage to sensor electronics.
FIGURE 1.4-1. CWS655 Wireless Water Content Reflectometer
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