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.
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
CS650 and CS655 Water Content Reflectometers
5
