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3. Principle of Operation
Dewpoint sensors, like all hygroscopic materials, adsorb much faster than they desorb.
Therefore, an accurate measurement will be obtained much faster, if at the beginning of
the measurement, the sensor is at a dewpoint drier than the gas to be measured. The
XPDM design allows the sensor to be moved directly between dry-storage and the sample
cell, without any exposure to ambient air.
Sensor in Sensor
In dry-storage position measuring position
1. Sample Gas In 7. Sensor Actuator
2. Sample Gas Out 8. Desiccant Chamber
3. Piston
9. Filter Mesh
4. Sample Cell 10.Electronic Board
5. Sensor 11.Instrument Case
6. Spring loaded PTFE seals
The drawing on the left shows the XPDM with the sensor in dry-storage position. The sen-
sor is enclosed in a gas-tight container and surrounded by desiccant. The sample gas
passes through the sample chamber and prepares the environment for measurement.
Blocking of the outlet port will pressurize the sample chamber and move the piston into the
position shown on the right. The sensor is now exposed to the sample gas without ever
having been exposed to ambient air. After the measurement is taken, pushing in the sen-
sor actuator will return the sensor into dry-storage, where it will dry down and ready itself
for another fast measurement.
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