Technical information

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2.4 EMBEDDED INSTRUMENTATION
2.4.1 Environmental Monitoring
Instrumentation devoted to monitoring environmental changes included moisture probes
and temperature gauges. Campbell Scientific moisture probes (model CS615) were
installed at NCAT during the phase I experiment (Freeman et al., 2001). This type of
gauge was also selected to be installed in the ‘structural study’ (Timm et al., 2004).
These probes indicate changes in volumetric moisture content (i.e., volume of water per
unit bulk volume of soil) by detecting changes in the dielectric constant of the
surrounding material. The dielectric constant of soils is a composition of the dielectric
constants of its individual constituents. Solid soil particles like sand and clay have
dielectric constants in the range of 2 to 4. Water, however, has a much higher dielectric
constant of about 80. Thus, increases in the moisture content of soil can be identified by
measured increases in the soil’s dielectric constant. The CS615 probes, sometimes
referred to as water content reflectometers, use time-domain measurement methods.
They consist of two parallel stainless steel rods, spaced 2 in. (51 mm) apart, connected
to a printed circuit board which is encapsulated in epoxy. Each rod is 12 in. (305 mm)
long and 1/8 in. (3.2 mm) in diameter. The circuit board transmits electrical waves that
travel along the rods with travel times that depend primarily on the dielectric constant of
the surrounding material. When such probes are calibrated for a specific soil, their
accuracy is typically ±2% moisture by volume. The readings from these probes were
not considered reliable enough for the ‘structural study’ (personal communication, D.
H. Timm, 2007).
Temperatures were monitored using passive elements based on thermistor
technology (Model 108 temperature probes manufactured by Campbell Scientific, Inc.).
A thermistor is essentially a resistor whose resistance varies according to temperature.
For each test section in the ‘structural study,’ four thermistors were bundled together to
provide temperature information near the surface and at the following depths: 2, 4 and
10 in. (51, 102 and 254 mm). These probes were installed after paving had been
completed given their survival temperature range is -50°C to +100°C (-58°F to 212°F).
Over the range of -3°C to 90°C (26.6°F to 194°F) the measurement accuracy of these