Environmental Engineering Reference
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are related to a common point along the calibration curve.
A typical value for the d 1 parameter was about 190 kPa/ C.
Figure 4.42 plots the response of the dry sensor to the
response of the wet sensor. The results show that whenever
the dry sensor response is a little higher than average, the wet
sensor response likewise seems to increase. Figure 4.43 com-
pares the difference between the wet and dry response of the
FTC sensors to the dry sensor reading. The difference between
the wet and dry response remains quite similar for all sensors,
with an average value of 4.02 C. The temperature change is
a function of the applied constant current. Figure 4.44 com-
pares the slope of the calibration curve at the inflection point
with the suction reading at the inflection point.
The results from the calibrations study suggest that in-
creased quality control in constructing the thermal conduc-
tivity sensors should lead to multiple sensors having a similar
response.
Thermal Conductivity Suction Measurements in the
Laboratory. Results of laboratory measurements of soil
suction using thermal conductivity suction sensors on highly
plastic clays are shown in Figs. 4.45, 4.46, and 4.47. The
soils were sampled in the field using Shelby tubes near Scep-
tre and Regina, Saskatchewan.
250
Average value
200
150
100
200
220
240
260
280
300
Suction at inflection point ( b 1 ), kPa
Figure 4.44 Slope of calibration curve at inflection point versus
inflection point suction for FTC-100 thermal conductivity suction
sensors (after Hu et al., 2007)
Several laboratory measurements were conducted using
two sensors inserted into each soil specimen. One sensor
was initially air dried, and the other was initially water sat-
urated. The sensors were inserted into predrilled holes at
either end of the Shelby tube sample. The specimen with
the inserted thermal conductivity sensors was wrapped in
aluminum foil to prevent moisture loss as the sensors came
14.0
13.5
13.0
12.5
12.0
8.0
8.5
9.0
9.5
10.0
Saturated reading ( a 1 ),
C
°
Figure 4.42 Relationship between wet sensor readings and dry sensor readings for 16 FTC-100
thermal conductivity suction sensors (after Hu et al., 2007).
4.5
4.0
3.5
3.0
12.5
13.0
13.5
14.0
Dry reading ( c 1 ),
°
C
Figure 4.43 Comparison of difference between wet and dry sensor readings with dry sensor
readings for FTC-100 suction sensors (after Hu et al., 2007).
 
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