Environmental Engineering Reference
In-Depth Information
σ
0
of soils has proved to be a difficult task; and
this is the case of granular materials in general [15]. The well-established four-
point technique for electrical conductivity/resistivity measurements
6
imposes strin-
gent operating requirements that are not always easily achievable, especially when
dealing with granular moistened materials
7
.
For this reason, there has been a growing interest toward alternative measurement
techniques, such as the Sigma probe [25], the electromagnetic induction method
[30], and TDR [46].
Indeed, the most attractive trait of the TDR-based approach is that the test-signal
is a broadband electromagnetic signal: the value of the static electrical conductivity
is extrapolated through appropriate analysis and processing of the TDR waveforms;
hence, the DC-related limitations are overcome. It goes without saying that the addi-
tional advantages of using TDR relate to the possibility to simultaneously estimate
several other material properties, such as dielectric permittivity [16], water content
[17], salinity [53], and solute transport [36]. Finally, the wide availability of portable
low-cost TDR instruments and the capability of obtaining accurate real-time, in situ
monitoring make the TDR technique definitely attractive also in view of practical
applications.
However, the accuracy of traditional TDR-based electrical conductivity measure-
ments is strongly influenced by the preliminary calibration technique. In this regard,
two innovative approaches for speeding up the preliminary calibration procedures
required for the TDR-based measurement of
However, the evaluation of the
σ
0
were assessed. The first method
relies on the combination of the TDR measurement with a transmission-line mod-
eling (TLM); the second method simply relies on a couple of independent capac-
itance measurements (ICM) performed through an LCR meter. The metrological
performance of this last method is compared with that of the traditional one, thus
validating its applicability. Experimental results and related uncertainty analysis on
various samples demonstrate that the proposed method is suitable for a simpler and
accurate estimation of the
σ
0
, also when dealing with moistened soils.
5.5.1
Traditional TDR-Based Static Electrical Conductivity
Measurement
Traditionally, TDR-based measurements of
σ
0
of granular materials are carried out
employing multi-rod probes.
6
In the four-point technique, a current is forced through the two amperometric electrodes,
placed at the ends of the sample, and the corresponding voltage value is measured at the
two voltmetric electrodes.
7
For example, good and electrically-stable contacts between the four electrodes and the
sample are essential [67]. Nevertheless, unless very expensive materials are used for the
electrodes (e.g., titanium, gold), electrochemical processes due, for example, to the pres-
ence of water in the soil, occur at the electrodes interfaces, thus rapidly degrading the
electric contacts. Indeed, this kind of measurements should be performed in static condi-
tions, therefore, as well known, the electrode polarization effect is an unavoidable limit
that practically impairs the measurement feasibility.
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