CS650 and CS655 Water Content Reflectometers
26
13.
Calculate gravimetric water content from wet mass of sample (m
wet
) and
dry mass of sample (m
dry
):
θ
g
=
m
wet
−
m
dry
m
dry
For the bulk density
ρ
bulk
=
m
dry
volume
cylinder
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
v
=
θ
g
•
ρ
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.
8.4.4 Collecting Field Data for Calibration
Required equipment:
•
CS650 connected to datalogger programmed to measure sensor
permittivity
•
Cylindrical sampling devices to determine sample volume for bulk
density, such as copper tubing with diameter of
≥
1 inch and length
about 2 inches.
•
Oven or microwave safe container of known weight
•
Scale to measure soil sample mass
•
Oven or microwave to dry samples
Data needed for CS650 calibration are the CS650 permittivity output and an
independently determined volumetric water content. From this data, the sensor
response to changing water content can be described by a function as described
in Section
User-Derived Calibration Equation
(p. 23)
.
The calibration coefficients are derived from a curve fit of known water
content and sensor 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 determine whether the
function is a quadratic or third order polynomial. Accuracy requirements may
need 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 sensor is used provides the best on-site soil-specific
