Instruction manual

CS650 and CS655 Water Content Reflectometers
derive a calibration depends on the form of the calibration equation. At least
three data sets should be generated to determine whether the linear form is
valid. If a polynomial is to be used, four data sets will determine whether the
function is a quadratic or third order polynomial. Accuracy requirements may
require additional data sets. Consider the expected range of soil water content
and include data sets from the highest and lowest 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
10 cm (4 in) 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 20 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 to 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 at the field site is necessary for an accurate
calibration.
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 (loosen) the top of
the existing compacted layer.
The container to hold the soil during calibration should be non-metal and large
enough that the rods of the probe are no closer than about 10 cm 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
surrounded by soil. Movement of rods from side-to-side during insertion can
form air voids around rod surface and lead to measurement error.
Collect the probe permittivity output. Repeat previous step and this step three
or four times.
Determine volumetric water content by subsampling soil column after
removing probe or using mass of column. If subsampling is used, remove soil
from column and remix with samples used for water content measurement.
Repack column.
Water can then be added to the top of the container. It must be allowed to
equilibrate. Cover the container during equilibration to prevent evaporation.
The time required for equilibration depends on the amount of water added and
the hydraulic properties of the soil. Equilibration can be verified by frequently
observing the CS650 permittivity output. When permittivity is constant,
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