Campbell Scientific 229 Water Matric Potential Sensor and CE4/CE8 - Manual (Page 7)

229 Heat Dissipation Matric Water Potential Sensor
A change in the water potential and water content of the ceramic matr
a corresponding change in the thermal conductivity of the ceramic/water
complex. As the water content in the ceramic increases, the thermal
conductivity of the complex also increases. At very low water contents, th
ceramic material controls the thermal conductivity. As water content in t
ceramic increases, water films are established between the solid particles
resulting in a rapid increase in thermal conductivity. As the pores in the
ceramic continue to fill, the thermal conductivity becomes ix causes
e
he
,
increasingly
controlled by the continuous water and the increase in thermal conductivity of
ture
the
d by
gy
e
se ranges from
approximately 0.7ºC under wet conditions to 3.0ºC when dry. Figure 1-3
presents a typical temperature response in a silt loam.
the ceramic/water complex approaches a constant value.
When a constant power is dissipated from the line heat source, the tempera
increase near the heat source will depend on the thermal conductivity of
ceramic/water complex surrounding the heater. A temperature increase is
caused by heat that is not dissipated. As the water content and thermal
conductivity of the ceramic increases, the temperature increase as measure
the thermocouple will be reduced because conduction of the thermal ener
from the heat source is greater. A drier sensor will have a lower thermal
conductivity, so the thermal energy will not dissipate as quickly and th
temperature rise will be greater. When 50 milliamps is passed through the
heating element for 30 seconds, the temperature increa
0.5
1
1.5
2
2.5
3
0 5 10 15 20 25 30
200 kPa
100 kPa
50 kPa
heating time (s)
10 kPa
Temperature Response of 229 Sensor in Silt Loam Soil
tem
p
er
atu
re i
n
cr
eas
e (
C
)
FIGURE 1-3. Typical
3