Campbell Scientific CS616 CS616 and CS625 Water Content Reflectometers - Page 39

The User-Derived Calibration Equation

Get Campbell Scientific CS616 PDF manuals and user guides View all Campbell Scientific CS616 manuals
Add to My Manuals
Save this manual to your list of manuals

Page 39 highlights

CS616 and CS625 Water Content Reflectometers Some clays are very polar and/or conductive and will also attenuate the applied signal. Additionally, if the clayey soil is compacted, increased bulk density, the conductivity is increased and the response is affected. Given the water content reflectometer response to changing water content in attenuating media changes as described above, the accuracy of the volumetric water content measurement can be optimized by characterizing the probe response in the specific medium to be measured. The result is a specific calibration equation for a particular medium. The precision and the resolution of the water content reflectometer measurement are not affected by attenuating media. Both precision and resolution are better than 0.1% volumetric water content. 8.2 The User-Derived Calibration Equation The probe output response to changing water content is well described by a quadratic equation, and, in many applications, a linear calibration gives required accuracy. Quadratic form: θv (τ) = C0 + C1 * τ + C2 * τ2 with θv , the volumetric water content (m3 m-3); τ, the CS616/CS625 period (microseconds); and Cn , the calibration coefficient. The standard calibration coefficients are derived from factory laboratory measurements using curve fitting of known volumetric water content to probe output period. Linear form: θv (τ) = C0 + C1 * τ with θv , the volumetric water content (m3 m-3); τ, the water content reflectometer period (microseconds); Co , the intercept; and C1 , the slope. Two data points from careful measurements can be enough to derive a linear calibration. A minimum of 3 data points is needed for a quadratic. With 3 evenly spaced water contents covering the expected range, the middle water content data point will indicate whether a linear or quadratic calibration equation is needed. Note from FIGURE 7-1 and FIGURE 7-2 that the calibration function describing the CS616/CS625 response to changing water content is always concave up. If calibration data suggests a different shape, there may be a problem with the data or method. 33

  • 1
  • 2
  • 3
  • 4
  • 5
  • 6
  • 7
  • 8
  • 9
  • 10
  • 11
  • 12
  • 13
  • 14
  • 15
  • 16
  • 17
  • 18
  • 19
  • 20
  • 21
  • 22
  • 23
  • 24
  • 25
  • 26
  • 27
  • 28
  • 29
  • 30
  • 31
  • 32
  • 33
  • 34
  • 35
  • 36
  • 37
  • 38
  • 39
  • 40
  • 41
  • 42
  • 43
  • 44
  • 45
  • 46
  • 47
  • 48
  • 49
  • 50
CS616 and CS625 Water Content Reflectometers
Some clays are very polar and/or conductive and will also attenuate the applied
signal.
Additionally, if the clayey soil is compacted, increased bulk density,
the conductivity is increased and the response is affected.
Given
the water content reflectometer response to changing water content in
attenuating media changes as described above, the accuracy of the volumetric
water content measurement can be optimized by characterizing the probe
response in the specific medium to be measured.
The result is a specific
calibration equation for a particular medium.
The precision and the resolution of the water content reflectometer
measurement are not affected by attenuating media.
Both precision and
resolution are better than 0.1% volumetric water content.
8.2
The User-Derived Calibration Equation
The probe output response to changing water content is well described by a
quadratic equation, and, in many applications, a linear calibration gives
required accuracy.
Quadratic form:
(
)
θ
τ
τ
τ
v
C
C
C
=
+
+
0
1
2
2
*
*
with
θ
v
, the volumetric water content (m
3
m
-3
);
τ
,
the CS616/CS625 period
(microseconds); and C
n
, the calibration coefficient.
The standard calibration
coefficients are derived from factory laboratory measurements using curve
fitting of known volumetric water content to probe output period.
Linear form:
(
)
θ
τ
τ
v
C
C
=
+
0
1
*
with
θ
v
, the volumetric water content (m
3
m
-3
);
τ
,
the water content
reflectometer period (microseconds); C
o
, the intercept; and C
1
, the slope.
Two data points from careful measurements can be enough to derive a linear
calibration.
A minimum of 3 data points is needed for a quadratic.
With 3
evenly spaced water contents covering the expected range, the middle water
content data point will indicate whether a linear or quadratic calibration
equation is needed.
Note from FIGURE 7-1 and FIGURE 7-2 that the calibration function
describing the CS616/CS625 response to changing water content is always
concave up.
If calibration data suggests a different shape, there may be a
problem with the data or method.
33