Campbell Scientific CS650 and CS655 Water Content Reflectometers - Manual (Page 37)

CS650 and CS655 Water Content Reflectometers
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.
8.3 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 8.2, The
User-Derived Calibration Equation.
Required equipment:
1. CS650 connected to datalogger programmed to measure permittivity
2. Cylindrical sampling devices to determine sample volume for bulk
density, e.g. copper tubing of diameter ≥ 1” and length about 2”
3. Containers and scale to measure soil sample mass
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 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 3 rd 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 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 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 - 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.
29