Campbell CS650, User Manual

Page: 36 / 56

CS650 and CS655 Water Content Reflectometers
24
A minimum of four data points are required for derivation of a 3
rd
degree
polynomial. Data points should be spaced as evenly as practical over the expected
range of water content and include the wettest and driest expected values.
8.4.3 Collecting Laboratory Data for Calibration
Water content reflectometer data needed for CS650 calibration are the CS650
permittivity reading and an independently determined volumetric water content.
From this data, the probe response to changing water content can be described by
a linear or polynomial function as described in Section
8.4.2 , User-Derived
Calibration Equation
(p. 23)
.
Required equipment:

CS650 connected to datalogger programmed to measure permittivity

Cylindrical sampling devices to determine sample volume for bulk
density, such as copper tubing with diameter of

2.5 cm (1 in) and
length at about 5 cm (2 ins).

Containers and scale to measure soil sample mass

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 permittivity output. The number of data sets needed to 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%).