CS650 and CS655 Water Content Reflectometers
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5. Overview
The CS650 measures volumetric water content, electrical conductivity,
dielectric permittivity, and temperature of soils or other porous media. These
values are reported through SDI-12 communications.
FIGURE 5-1. CS650 (left) and CS655 Water Content Reflectometers
Volumetric water content (VWC) is derived from the sensor sensitivity to the
dielectric permittivity of the medium surrounding the sensor stainless-steel
rods. The CS650 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, with an oscillator state change triggered by the
return of a reflected signal from one of the rods. The two-way travel time of the
electromagnetic waves that are induced by the oscillator on the rod varies with
changing dielectric permittivity. Water is the main contributor to the bulk
dielectric permittivity of the soil or porous media, so the travel time of the
reflected wave increases with increasing water content and decreases with
decreasing water content, hence the name water content reflectometer. The
average travel time of the reflected wave multiplied by a scaling factor of 128
is called the period average. Period average is reported in microseconds and is
considered to be the raw output of a water content reflectometer.
Electrical conductivity is determined by exciting the rods with a known non-
polarizing waveform and measuring the signal attenuation. Signal attenuation
is reported as a dimensionless voltage ratio, which is the ratio of the excitation
voltage to the measured voltage along the sensor rods when they are excited at
a fixed 100 kHz frequency. Voltage ratio ranges from 1 in non-conductive
media to about 17 in highly conductive media. Values greater than 17 are
highly unstable and indicate that the soil conditions are outside of the specified
operating range of the sensor.
Temperature is measured with a thermistor in contact with one of the rods.
