Environmental Engineering Reference
In-Depth Information
Although other moisture sensing techniques could arguably provide as much
measurement accuracy, their use is often hindered by practical limitations
1
. Con-
versely, BMR holds considerable potential for practical implementation; in particu-
lar, time domain reflectometry (TDR)-based systems have become competitive with
respect to other techniques, such as capacitive methods and ultrasonic sensors.
TDR-based systems can be customized according to the operating conditions,
thus providing high adaptability to the specific application. Such a feature, com-
bined with low-cost instrumentation and with an adequate measurement accuracy,
has contributed to the spread of TDR in soil science, not only for moisture content
evaluation but also for determining the moisture content profile [23, 29]. On such
bases, relevant research effort has been dedicated to developing innovative TDR
probes, enhanced measurement procedures, and sound theoretical modeling [38].
The typical TDR-based instrumental setup for in situ moisture sensing includes a
portable reflectometer and a three-rod probe that can be easily inserted in the soil (or
granular material in general) under investigation, thus acting as measurement cell.
As mentioned in the previous chapters, despite having some shortcomings with re-
gards to calibration, this probe configuration is widely used for TDR-based moisture
content measurements.
The present chapter is structured as follows. First, the evaluation of
θ
through
TDR-measurements of the apparent dielectric constant,
ε
app
, is considered. In par-
ticular, the adoption of the so-called
calibration curves
, which relate
,is
discussed: the metrological analysis of this method demonstrates that it can ensure
accurate results, provided that for each different material a specific calibration curve
is individuated.
Secondly, the adoption of an enhanced approach that also takes into account
the frequency-dependence of the dielectric permittivity of the investigated material
is fully described. Similarly to the previously reported applications, the proposed
method relies on the combination of TDR, frequency domain reflectometry (FDR),
and a transmission line (TL) modeling of the measurement setup. Most importantly,
this application was also used as a case-study for testing an innovative FD-based
calibration technique that relies on three short-circuits applied at different probe
sections. This calibration procedure is particularly important when multi-rod probes
are used; in fact, in this case, traditional short-open-load (SOL) calibration is often
practically unfeasible, due to the impossibility of connecting standard loads to the
electrodes.
Successively, a noninvasive approach for moisture monitoring is also assessed:
this method focuses on the possibility of adopting antennas as sensing elements.
More specifically, the antenna is placed on the top surface of the material under test
(MUT), and the shift in the resonant frequency of the antenna is associated to the
corresponding moisture content of the sample.
ε
app
to
θ
1
As an example, the gamma ray attenuation method is accurate but highly costly, and the
presence of a radioactive source requires competent handling thus making the method
unappealing for routine applications [45].
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