Campbell Scientific 229 Water Matric Potential Sensor and CE4/CE8 - Manual (Page 25)

229 Heat Dissipation Matric Water Potential Sensor
The
dry
T Δ
value requires that the ceramic be as dry as possible. Sensors c
be dried with desiccant or in an oven at temperature no
an
greater than 60 °C.
eratu
s a
t shown
significant advantage for this method over normalization as described by
Temp res greater than 60 °C may damage the sensor cable.
Reece (1996) suggested that inverse thermal conductivity can also be used a
normalization technique but work by Campbell Scientific has no
equation [3].
A calibration equation using
norm
T Δ
and having a form similar to equatio
is
n [2]
β α ψ + Δ =
norm
* T |) ln(| . [4]
Th e slope of equation [4] will
b
e slope of equation [2] will be positive while th
e negative.
6.2.2 Correction fo
n method of matric potential measurement is sensitive to
r the temperature dependence may be necessary
easurement. If the soil temperature when the
e
onductivity of the
ceramic. The thermal conductivity of the ceramic depends on the combination
rts
t (soil
-2
r Soil Temperature
The heat dissipatio
temperature and correction fo
to maintain accuracy of the m
matric potential measurements are made is close to the temperature at the tim
of sensor calibration there is no need for correction.
The 229 measurement method uses heat transfer away from a heated line
source and the heat transfer depends on the thermal c
of the conductivities of water, vapor and solid constituents. The vapor
component has a strong temperature dependence and consequently impa
sensitivity of the measurement to temperature. The sensitivity is related to the
difference between the temperature of the sensor at time of measuremen
temperature) and the temperature of the sensor during calibration. Figures 6
and 6-3 show the response of the matric potential measurement for a range of
temperatures when the calibration temperature is 20 C.
21