Manual

229 Heat Dissipation Matric Water Potential Sensor

3. Measurements of sensor temperature response are made periodically to

determine if equilibration is attained. This will require depressurization of

the pressure vessel if a pressure-tight feedthrough is not used. Prior to

depressurization, it is important that the effluent hose be blocked by

clamping or other method to prevent solution from re-entering the soil and

sensors.

4. When equilibration is attained, the effluent hose is blocked, the vessel is

depressurized and 229 measurements are recorded.

5. The next calibration pressure is then applied and the process repeated.

Pressure-tight bulkhead connectors are available for some pressure vessels.

Determining whether equilibrium has been reached is simplified when using a

feedthrough connector since the pressure vessel doesn’t have to be

depressurized and opened for each reading. A temporary connector can be

used to disconnect the datalogger from the other components installed in the

pressure vessel.

Pressurized readings can be used for determining equilibrium but should not be

used for calibration data. Thermal properties are affected by pressure and

calibration data should be collected at the same pressure the sensors will be

used—in most cases this is atmospheric pressure.

6.4.1 Wiring for Calibration using Pressure Plate Extractor

The wiring arrangement of Figure 6-4 depicts a datalogger with an AM16/32

multiplexer, a CE4 current source and a thermistor being used in a typical 229

calibration arrangement. This is similar to the wiring for program example #2

and #4 (see Section 5). All components except the datalogger can be placed

inside a 5 bar pressure vessel. If this method is used, the electronic

components in the pressure vessel should be protected against moisture

damage. Place the components in a container such as plastic bag with

desiccant.