Page 155 - Biosystems Engineering

134 Cha pte r F o u r

combination of low plate thermal conductivity, thermal contact resis-
tance, and latent heat-transfer effects (Ochsner et al. 2006). The heat
flux plate–derived heat fluxes has shown errors of up to 26 percent in
coarse porous substrates due to poor contact between plate and
substrate (Weber et al. 2007).
Developing reliable low-cost research material for laboratory and
field tests is still a priority for many authors who develop research on
soil, or in the area of environmental sciences. For this reason, many
research tasks concern the development of temperature measurement
probes that are suited to the procedures for indirect determination of
soil thermal properties. Manohar et al. (2000) designed an experiment
to measure effective thermal conductivity in soils. They constructed
three thermal probes in accordance with the American Society for
Testing and Materials method ASTM D 5334. The temperature–time
response was logged at 1-s intervals for 1000 s, which allowed for the
estimation of soil thermal conductivities. The relationship between
temperature variations and time was used also in the single-probe
heat-pulse method developed by Kaminsky (1994). Bristow et al.
(2001) developed a small multineedle probe for measuring soil ther-
mal properties that improved cost, robustness, and easy automation
of soil instrumentation. Later, Saito et al. (2007) evaluated the effects
of sensor locations and thermal properties of the heat-pulse probe-
sensor body material, heater diameter and heat-pulse intensity, and
vapor flow on system of measurement performance. Their results
showed that significantly different temperature responses are
obtained depending on the axial location of the thermistors.
Abu-Hamdeh and Reeder (2000) used the single-probe heat-pulse
technique to determine the thermal conductivity of soils repacked in
the laboratory under different density, moisture, salt concentration,
and organic matter conditions. According to their results, effective
thermal conductivity increased with increasing bulk density at a
given moisture content and with increasing moisture at a given bulk
density. Clay soils showed lower effective thermal conductivities
than sandy soils, which verified the results obtained by Noborio and
McInnes (1993). Abu-Hamdeh (2001) analyzed the influence of min-
eral salt concentration (NaCl, MgCl , CaCl ) on the conductivity of
2 2
soils altered under laboratory conditions and concluded that effective
thermal conductivity decreased with the increase in salt contents.
Finally, it has been suggested that indirect methods for determining
soil thermal properties from temperature may allow for simultaneous
estimation of soil water retention (when combined with matric poten-
tial measurements) and unsaturated hydraulic conductivity. Simultane-
ous estimation of both properties is possible if numerical solutions to
the heat-flow equation are used, and heat and mass flows are taken into
consideration (Hopmans et al. 2002). In fact, the heat-pulse probe has
received more attention as it allows in situ, simultaneous, and auto-
mated measurements of soil hydraulic and thermal properties, as well