Campbell CS616, Instruction Manual

Page: 41 / 54

CS616 and CS625 Water Content Reflectometers
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,
Water Content Reflectomer
User-Calibration , for calibration information.
7.6 Temperature Dependence and Correction
The error in measured volumetric water content caused by the temperature
dependence of the CS616/CS625 is shown in FIGURE 7-4. 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
°
C 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,
τ
uncorrected
, to 20
°
C knowing the soil
erature,
T
soil
. See sample datalogger programs. The temperature
correction assumes that both the water content and temperature do not vary
over the length
temp
of the probes rods.
( ) ( )
( )
τ τ τ τ
corrected soil uncorrected soil uncorrected uncorrected
T T
= + − − +
20 0 526 0 052 0 00136
2
* . . * . *
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