User Manual
17
10
15
20
25
30
35
40
4
2
0
2
4
6
8
Water Content = 30%
Water Content = 12%
Water Content Error w ith Temperature
Soil Temperature (C)
W
at
er
C
on
te
nt
E
rr
or
(%
VW
C)
Figure 8-4. Percent volumetric water content error adjusted with
temperature correction equation
8.2
Water Content Reflectometer User-Calibration
8.2.1
Signal Attenuation in Conductive Soils and Need for Site-
Specific Calibration
A shift in water content reflectometer response results if the applied signal
is attenuated significantly. There is a voltage potential between the probe
rods when a pulse is applied to them. If the material between the rods is
electrically conductive, a path for current flow exists and the applied signal
is attenuated. Since the parallel rod design in soil is inherently a lossy
medium and attenuation is frequency dependent, both the amplitude of the
reflection and the rise-time or bandwidth are affected. Instead of a
relatively short rise-time return pulse, the rise-time is greater and the
amplitude is less.
The reflected signal must exceed a set amplitude before the next pulse is
triggered. Reflections that are attenuated and have longer rise-times will
take longer to be detected and trigger the next pulse leading to decreased
frequency or increased period in conductive materials.
Some clays are very polar and/or conductive and will also attenuate the
applied signal. Additionally, if the clayey soil is compacted, increased
bulk density, the conductivity is increased and the response is affected.
Given the water content reflectometer response to changing water content
in attenuating media changes as described above, the accuracy of the
