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229 Heat Dissipation Matric Water Potential Sensor

correspond to the water potential expected during sensor use should be

included in the calibration

2.5

3.5

4.5

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(|matric water potential|) (kPa)

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

, described by

wetdry

dry

norm

Δ−Δ

−

=Δ

[3]

where is the change in temperature during measurement when the 229

sensor is dry,

dry

TΔ

wet

is the change in temperature during measurement when

the 229 sensor is fully saturated and

is the change in temperature during

the measurement. The range of

norm

will be 0 to 1 with equal 0 for

dry soil and 1 for saturated soil.

norm

TΔ

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

TΔ

movement of air from the ceramic. When tapping or jarring the container

shows no release of bubbles, the ceramic is saturated.

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