Campbell Scientific 229 Water Matric Potential Sensor and CE4/CE8 - Manual (Page 24)

229 Heat Dissipation Matric Water Potential Sensor
correspond to the water potential expected during sensor use should be
included in the calibration
2
2.5
3
3.5
4
4.5
5
5.5
1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6
Temperature rise,T(30s) - T(1s), deg-C
ln(
|m
at
ric
w
at
er
pot
ent
ia
l|)
(
kP
a)
FIGURE 6-1. Data Points (x) and Regression for Typical Calibration
6.2 Normalized Temperature Change and Correction for Soil
Temperature
6.2.1 Normalized Temperature Change
The effect of sensor-to-sensor variability can be reduced by using normalized
temperature increase,
norm
T Δ
, described by
wet dry
dry
norm T T
T T
T
Δ − Δ
Δ − Δ
= Δ
[3]
where is the change in temperature during measurement when the 229
sensor is dry,
dry
T Δ
wet
T Δ
is the change in temperature during measurement when
the 229 sensor is fully saturated and T Δ
is the change in temperature during
the measurement. The range of
norm
T Δ
will be 0 to 1 with equal 0 for
dry soil and 1 for saturated soil.
norm
T Δ
The value requires full saturation of the ceramic. The ceramic portion
of the sensor must be immersed in water while under vacuum to remove all air
from the ceramic. Measurements at the factory show that complete saturation
will occur after 3-4 hours at 0.7 atmospheres (70 kPa) vacuum. However, the
arrangement of the sensors in the water during vacuum extraction may hamper
wet
T Δ
movement of air from the ceramic. When tapping or jarring the container
shows no release of bubbles, the ceramic is saturated.
20