with θ
v
the volumetric water content, K
a
the bulk dielectric permittivity of the soil, and Cn, the
calibration coefficient.
Two data points from careful measurements sometimes are enough to derive a linear calibration.
Use at least three data points for a quadratic calibration. With three evenly spaced water contents
covering the expected range, the middle water content data point indicates whether a linear or
polynomial calibration equation is needed.
Use at least four data points for the derivation of a third-degree polynomial. Space the data
points 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, a linear or
polynomial function can describe the sensor response to changing water content. For more
information, refer to
User-derived calibration equation
Required equipment:
l
CS650 connected to data logger programmed to measure permittivity
l
Cylindrical sampling devices to determine sample volume for bulk density, such as copper
tubing with diameter of ≥ 1 inch and length at about 2 inches.
l
Non-metal container such as a PVC pipe (20 cm diameter, 35 cm length) with one end
closed. The container should be large enough to ensure that only soil is within 10 cm (4 in)
of the CS650 rod surface.
l
Oven or microwave safe container of known weight
l
Scale to measure soil sample mass
l
Oven or microwave to dry samples
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.
CS650 and CS655 Water Content Reflectometers
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