Campbell Scientific CS650 and CS655 Water Content Reflectometers - Manual (Page 39)

CS650 and CS655 Water Content Reflectometers
Gravimetric water content is calculated after the container mass is accounted
for.
θ
g
wet dry
dry
m m
m
=
−
For the bulk density
ρ
bulk
dry
cylinder
m
volume
=
the dry mass of the sample is divided by the sample tube volume.
The volumetric water content is the product of the gravimetric water content
and the bulk density
θ θ ρ
v g bulk
= *
The average water content for the replicates and the recorded CS650
permittivity are one datum pair to be used for the calibration curve fit.
8.4 Collecting Field Data for Calibration
Required equipment
1. CS650 connected to datalogger programmed to measure probe 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)
Data needed for CS650 calibration are the CS650 permittivity output and an
independently determined volumetric water content. From this data, the probe
response to changing water content can be described by a function as described
in Section 8.2, The User-Derived Calibration Equation.
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.
Collecting measurements of CS650 permittivity and core samples from the
location where the probe is to be used will provide the best on-site soil-specific
calibration. However, intentionally changing water content in soil profiles can
be difficult.
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