CS616 and CS625 Water Content Reflectometers
( )
θ τ
τ
τ
v
C
C
C
=
+
+
0
1
2
2
*
*
with
θ
v
being the volumetric water content (m
3
m
-3
);
τ
, the CS616 period
(microseconds); and C
n
, the calibration coefficient (n = 0..2).
The linear form is
( )
θ τ
τ
v
C
C
=
+
0
1
*
with
θ
v
, the volumetric water content (m
3
m
-3
);
τ
,
the CS616 period
(microseconds); C
o
, the intercept; and C
1
, the slope.
Required equipment:
1. CS616/CS625 connected to datalogger programmed to measure output
period
2. Cylindrical sampling devices to determine sample volume for bulk density;
for example, copper tubing of diameter
≥
1 in. and length about 2 in.
3. Containers and scale to measure soil sample weight
4. Oven to dry samples (microwave oven can also be used)
The calibration coefficients are derived from a curve fit of known water
content and probe output period. The number of data sets needed to derive a
calibration depends on whether the linear or quadratic form is being used and
the accuracy requirement. Consider the expected range of soil water content
while viewing FIGURE 7-1 and FIGURE 7-2. If the expected response is
nearly linear, fewer laboratory measurements are needed to derive the
calibration. A linear response is best described by data taken near the driest
and wettest expected water contents.
The measurement sensitive volume around the probe rods must be completely
occupied by the calibration soil. Only soil should be in the region within 4
inches of the rod surface. The probe rods can be buried in a tray of soil that is
dry or nearly dry. The soil will be homogeneous around the probe rods if it is
poured around the rods while dry. Also, a 10 cm diameter PVC pipe with
length about 35 cm can be closed at one end and used as the container.
It is important that the bulk density of the soil used for calibration be similar to
the bulk density of the undisturbed soil. Using dry soil without compaction will
give a typical bulk density, 1.1 - 1.4 g cm
-3
. This is especially important when
bulk density is greater than 1.55 g cm
-3
. Compaction of the calibration soil to
similar bulk density may be necessary.
The typically used method for packing a container of soil to uniform bulk
density is to roughly separate the soil into three or more equal portions and add
one portion to the container with compaction. Evenly place the first loose soil
layer in the bottom of the container. Compact by tamping the surface to a level
in the container that is correct for the target bulk density. Repeat for the
remaining layers. Prior to placing successive layers, scarify the top of the
existing compacted layer.
36
Summary of Contents for CS616
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Page 8: ...Table of Contents iv ...
Page 50: ...CS616 and CS625 Water Content Reflectometers 42 ...
Page 52: ...Appendix A Discussion of Soil Water Content A 2 ...
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