Campbell Scientific CS616 and CS625 Water Content Reflectometers - Manual (Page 39)

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.
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