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Optidew User's Manual
4.2
Operating Guide
4.2.1
Description
Once the Optidew has been powered on and has carried out its' initial DCC, it will attempt
to find the dewpoint. In order to measure the dewpoint a Chilled Mirror hygrometer
must control a thin film of condensed water or ice on the mirror.
To initially form the condensate layer the mirror must be cooled past the actual dew
or frost point. The control system will then gradually heat the mirror to reduce the
thickness of this condensate layer. It typically takes several heating/cooling cycles
until the instrument has achieved the optimal film thickness where evaporation and
condensation are occurring in equilibrium. This is the true dew/frost point of the sample.
After finding the true dewpoint, the control system will continue to maintain the film
thickness at a constant level. Any decrease in actual sample dewpoint will cause
evaporation from the condensate film to increase – reducing its thickness and causing
the control system to cool the mirror to compensate. Likewise if the dewpoint increases
then condensation on the mirror will increase, and the control system will heat to
compensate.
In extreme cases where the dewpoint decreases very abruptly, then the condensate will
be completely evaporated from the mirror. In these scenarios the system will 'search'
for the dewpoint again by cooling, resulting in cooing past the dewpoint as described
above. A similar situation occurs when the dewpoint increases abruptly, however the
condensate film can be lost here by the control system heating to compensate and
exceeding the new dewpoint.
4.2.2
Operating Practice
There are two basic methods of measuring with the Optidew:
In-situ measurements are made by placing the sensor(s) inside the environment to be
measured.
Extractive measurements are made by installing the sensor into a block within a sample
handling system, and flowing the sample outside of the environment to be measured
through this system
Extractive measurements are recommended when the conditions in the environment
to be measured are not conducive to making reliable measurements with the product.
Examples of such conditional limitations are:
•
Excessive flow rate
•
Presence of particulates matter
•
Presence of entrained liquids
•
Excessive sample temperature
•
Dew point is beyond depression capability at sample temperature
The basic considerations for each measurement type are as follows:
In-Situ
Dewpoint Sensor position – will the sensor see an area of the
1.
environment that is representative of what you want to measure? For
Kahn Instruments
OPERATION
31
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