Campbell Scientific CS616 CS616 and CS625 Water Content Reflectometers - Page 38

Water Content Reflectometer User-Calibration

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CS616 and CS625 Water Content Reflectometers Water Content Error with Temperature 8 6 Water Content Error (%VWC) 4 2 0 2 4 10 15 20 25 30 35 40 Soil Temperature (C) Water Content = 30% Water Content = 12% 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 SiteSpecific 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. 32

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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)
Water Content Error (%VWC)
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.
32