Campbell CS650, User Manual

Page: 17 / 56

User Manual
5
Figure 5-1. 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 means of RS-
232 Tx/Rx. The A200 USB-to-Serial Module allows RS-232 serial
communication between a computer and the CS650 by means of Campbell
Scientific’s
Device Configuration Utility ( DevConfig ) software.