Environmental Engineering Reference
In-Depth Information
Fig. 5.5 Schematic representation of the experimental setup. The rectangle highlights all the
effects that are included in the error network (not to scale) [48]
beginning of the probe rods, section A in Fig. 5.5). Unfortunately, the structure of
commercial three-rod probes does not allow such a calibration because the open
and matched loads insertion would require detaching the rods from the probe head
and the exact knowledge of the value of the matching impedance. As a matter of
fact, SOL standards for three-rod probes are not commercially available, and even
in-house solutions for calibration fixtures mimicking open and load conditions are
difficult to be reproduced.
To overcome such a limitation, an innovative calibration procedure that does not
require either the open or the load conditions was specifically developed. In fact,
the proposed calibration method relies only on the short-circuit conditions obtained
at three different sections along the probe (triple short calibration - TSC) [48].
The TSC procedure was tested and validated through preliminary measurements
on well-referenced materials. Successively, it was applied to sandy soil brought
at different levels of moisture. Results show that the combination of a frequency-
dependent dielectric mixing modeling, of a minimization routine, and of the pro-
posed TSC procedure is the key strategy for obtaining accurate TDR moisture
content evaluations while reducing the complexity of the calibration phase. Another
possible alternative solution for allowing traditional SOL calibration could be to de-
sign and realize a custom-made probe, similarly to the case presented in Sect. 4.3.
However, the TSC procedure presented herein can be readily applied to commer-
cially available multi-rod probes.
As aforementioned, to effectively enhance the accuracy in the estimation of
soil moisture content, the reflection coefficient S 11 , MEAS (
at section A of the
probe (see Fig. 5.5) must be evaluated. In fact, it is worth mentioning that, simi-
larly to the previously-discussed cases, TDR intrinsically measures the reflection
f
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