Campbell Scientific CS650 and CS655 Water Content Reflectometers - Manual (Page 10)

CS650 and CS655 Water Content Reflectometers
FIGURE 4-1. CS650 Water Content Reflectometer
Volumetric water content information is derived from the probe’s sensitivity to
the dielectric permittivity of the medium surrounding the probe 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.
Electrical conductivity is determined by exciting the rods with a known non-
polarizing waveform and measuring the signal attenuation.
Temperature is measured with a thermistor in contact with one of the rods.
It is well known that transmission line oscillators used for water content
measurements suffer from unwanted increases in oscillation period as
increasing electrical conductivity causes transmission line signal attenuation.
The CS650 handles this problem by making an electrical conductivity
measurement and then correcting the oscillator period accordingly. On-board
processing within the sensor head calculates electrical conductivity from the
signal attenuation measurement and combines the result with the oscillation
period measurement to calculate the dielectric permittivity of the media and
finally applies the Topp equation (Topp et al. 1980) to estimate volumetric
water content.
Probe electronics are encapsulated in the rugged epoxy probe head.
A five conductor cable including the drain or shield wire is used to provide
power and ground as well as serial communication with the CS650. The CS650
is intended to communicate with SDI-12 recorders, including Campbell
Scientific dataloggers. The orange Rx wire can be used to communicate by
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