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CS616 and CS625 Water Content Reflectometers
16
8.1.4.3 Signal Attenuation Error
Section 8.1 , Water Content Reflectometer Method for Measuring
Volumetric Water Content
(p. 10)
, presents a detailed description of
CS616/CS625 operation. In summary, the CS616/CS625 is primarily
sensitive to the dielectric permittivity of the material surrounding the probe
rods. The propagation of electromagnetic energy along the probe rods
depends on the dielectric properties of the medium. When the reflection of
the applied signal from the end of the rods is detected by the CS616/CS625
circuit, another pulse is applied. The time between pulses depends on the
propagation time, and the associated period is empirically related to
volumetric water content.
The applied signal is subject to attenuation from losses in the medium
being measured. While this does not directly affect propagation time, it
causes delays in detection of the reflected signal. Attenuation of the signal
will occur if there are free ions in soil solution, polar solid constituents
such as organic matter or some clay, or conductive mineral constituents.
The general calibration equation for the CS616/CS625 will provide good
results with attenuation equivalent to about 0.5 dS m
–1
bulk electrical
conductivity. Between 0.5 dS m
–1
and 5 dS m
–1
, the CS616/CS625 will
continue to give a well-behaved response to changes in water content but a
soil specific calibration is required. See Section
8.2 , Water Content
Reflectometer User-Calibration
(p. 17)
, for calibration information.
8.1.5 Temperature Dependence and Correction
The error in measured volumetric water content caused by the temperature
dependence of the CS616/CS625 is shown in Figure
8-4 . The magnitude
of the temperature sensitivity changes with water content. Laboratory
measurements were performed at various water contents and over the
temperature range from 10 to 40

C to derive a temperature correction for
probe output period. The following equation can be used to correct the
CS616/CS625 output period,

uncorrected
, to 20

C knowing the soil
temperature,
T
soil
. See Appendix C.2.2 , CR200(X) Program with
Temperature Correction
(p. C-6)
. The temperature correction assumes that
both the water content and temperature do not vary over the length of the
probes rods.
   
 
2
00136 . 0 052 . 0 526 . 0 20
d uncorrecte d uncorrecte soil d uncorrecte soil corrected
T T
   
       