Environmental Engineering Reference
In-Depth Information
conductivity sensors. Reece (1996) suggested the use of a
normalization procedure to reduce the influence of varying
ambient temperature. Flint et al., (2002) suggested using the
following normalization equation to give a dimensionless
temperature rise,
T
*:
16
No temperature correction
14
12
Shuai et al., (2002)
10
Nichol et al., (2003)
8
T
d
−
T
T
d
−
T
w
6
T
∗
=
(4.11)
4
2
where:
0
a
1
b
1
c
1
Slope
T
=
temperature rise of the source in the ceramic in a
specified elapsed time,
Fitting parameters
Figure 4.56
Comparison of coefficients of variation in fitting
parameters when using different temperature correction equations
(after Hu et al., 2007).
T
d
=
temperature rise for a dry ceramic, and
T
w
=
temperature rise for a fully saturated ceramic.
Equation 4.11 normalizes the sensor output reading
between completely dry and fully saturated ceramic condi-
tions. The normalized temperature change would be equal to
zero for a dry sensor and unity for a water-saturated ceramic.
It was reasoned that the most significant effect of ambient
temperature would be realized when the ceramic was dry.
In other words, the largest temperature corrections would
occur when the ceramics contain considerable air. (Note that
this is opposite to the observations made with the GCTS
thermal conductivity sensors.) It was reasoned that the gas
phase thermal conductivity changed because of changes
in the latent heat of distillation across the pores of the
ceramic.
4.2.9.6
Influence of Temperature on the CS-229 Suction
Sensors
Campbell Scientific manufactures a thermal conductivity
soil suction sensor referred to as the CS-229 Water Matric
Potential Sensor (Fig. 4.57). The thermal conductivity
suction sensor measures soil water potential from 10 to
1000 kPa. The sensor consists of a heating element and
thermocouple placed in epoxy in a hypodermic needle
which is encased in a porous ceramic matrix.
The effect of the ambient temperature on the suction mea-
surement has also been realized for the CS 229 thermal
200
30
°
C
100
24
°
C
22
°
C
0
18
°
C
16
°
C
-100
10
°
C
-200
0
100
200
300
400
500
Matric suction (kPa)
Figure 4.57
Possible measurement error for range of ambient temperatures and soil suctions
up to 500 kPa (after Campbell Scientific Instruction Manual, 2009).
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