User Manual
19
The linear form is
v
C
C
0
1
*
with
v
, the volumetric water content (m
3
m
-3
);
,
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 inch and length
about 2 inches
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
8-1
and Figure
8-2
. 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 5 cm 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.
The container to hold the soil during calibration should be large enough
that the rods of the probe are no closer than about 4 inches from any
container surface.
Pack the container as uniformly as possible in bulk density with relatively
dry soil (volumetric water content <10%).
Probe rods can be buried in a tray or inserted into a column. When using a
column, insert the rods carefully through surface until rods are completely
