Campbell Scientific CS616 and CS625 Water Content Reflectometers - Manual (Page 38)

CS616 and CS625 Water Content Reflectometers
10 15 20 25 30 35 40
4
2
0
2
4
6
8
Water Content = 30%
Water Content = 12%
Water Content Error with Temperature
Soil Temperature (C)
W
at
er
Co
nt
en
t
Erro
r
(%
VW
C)
FIGURE 7-4. Percent volumetric water content error corrected for by
temperature correction equation
8. Water Content Reflectometer User-Calibration
8.1 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.
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