CS615 WATER CONTENT REFLECTOMETER
3
electrical conductivity values greater than
5 dS m
-1
the probe output can become unstable.
4.3.2 Soil Organic Matter and Clay Content
The amount of organic matter and clay in a soil
can alter the response of dielectric-dependent
methods to changes in water content. This is
apparent when mechanistic models are used to
describe this measurement methodology.
The electromagnetic energy introduced by the
probe acts to re-orientate or polarize the water
molecules which are polar. If other forces are
acting on the polar water molecules, the force
exerted by the applied signal will be less likely to
polarize it. This has the net effect of ‘hiding’
some of the water from the probe.
Organic matter and most clays are highly polar.
Additionally, some clays sorb water interstitially
and thus inhibit polarization by the applied field.
It would be convenient if the calibration of water
content to CS615 output period could be
adjusted according to some parameter of the
soil which reflects the affect of the intrinsic
forces. However, identification of such a
parameter has not been done, and it is likely
that measurement of the correlation parameter
would be more difficult than calibrating the
CS615 for a given soil.
4.3.3 Cable Length
Probe cable length is not a limitation under
typical applications. Laboratory measurements
show no degradation in measurement quality
with cable lengths up to 100 meters. Cable
lengths greater than 50 m may increase the
potential for damage from electrostatic
discharge (lightning). The performance may be
degraded if a cable type other than that
provided with the probe is used.
4.3.4 Temperature Dependence
The CS615 output is sensitive to temperature,
and compensation can be applied to enhance
accuracy. The magnitude of the temperature
coefficient varies with water content.
Laboratory measurements were performed at
various water contents and over the
temperature range from 10
°
C to 30
°
C. The
calibration information presented in Section 9 is
for a temperature of 20
°
C. The following
equation can be used to interpolate the
temperature coefficient for a range of volumetric
water content (
θ
v
) values.
Coef
temperature
v
v
= −
+
−
−
3 46 10
0 019
0 045
4
2
.
*
.
.
θ
θ
To apply this correction, the following equation
can be used.
θ
θ
vcorrected
vuncorrected
temperature
T
Coef
=
−
−
(
) *
20
Application of this correction yields a maximum
difference between corrected and uncorrected
water content of approximately 1.6%.
Considering the accuracy of the measurement
and the potential spatial variability of soil
temperature along the length of the probe rods,
the correction is not necessary in most cases.
An example for using the temperature correction is a
measurement taken on a soil at a water content of
about 0.23 and a temperature of 25
°
C. The
temperature coefficient value is 0.00164 m
3
m
-3
°
C
-1
which means that the measured water content is
5
°
C *(0.00164 m
3
m
-3
°
C
-1
) or 0.8% high.
5. INSTALLATION
5.1 ORIENTATION
The probe rods can be inserted vertically into
the soil surface or buried at any orientation to
the surface. A probe inserted vertically into a
soil surface will give an indication of the water
content in the upper 30 cm of soil. The probe
can be installed horizontal to the surface to
detect the passing of wetting fronts or other
vertical water fluxes. A probe installed at an
angle of 30 degrees with the surface will give an
indication of the water content of the upper 15
cm of soil.
5.2 POTENTIAL PROBLEMS WITH IMPROPER
INSERTION
The method used for probe installation can
affect the accuracy of the measurement. The
probe rods should be kept as close to parallel
as possible when installed to maintain the
design wave guide geometry. The sensitivity of
this measurement is greater in the regions
closest to the rod surface than at distances
away from the surface. Probes inserted in a
manner which generates air voids around the
rods will reduce the measurement accuracy. In
