CS650 and CS655 Water Content Reflectometers
23
In these cases, the user may develop a calibration equation to convert CS650
permittivity to volumetric water content over the range of water contents the
sensor is expected to measure.
8.4.2 User-Derived Calibration Equation
A quadratic equation or third order polynomial can describe the relationship
between soil permittivity and volumetric water content. In many applications, a
linear equation similar to Ledieu et al (1986) gives required accuracy.
Quadratic form:
θ
v
(K
a
) = C
0
+ C
1
•K
a
+ C
2
•K
a
2
with
θ
v
the volumetric water content, K
a
the bulk dielectric permittivity of the
soil, and Cn, the calibration coefficient.
Third-degree polynomial form:
θ
v
(K
a
) = C
0
+ C
1
•K
a
+ C
2
•K
a
2
+ C
3
•K
a
3
with
θ
v
the volumetric water content, K
a
the bulk dielectric permittivity of the
soil, and Cn, the calibration coefficient.
Linear form:
θ
v
(K
a
) = C
0
+ C
1
•K
a
0.5
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 Section
User-Derived Calibration Equation
(p. 23)
.
