Optidew User’s Manual
Michell Instruments 35
OPERATION
Maintaining the temperature of the sample system tubing above the dew point of the
sample is vital to prevent condensation. Any condensation invalidates the sampling
process as it changes the water vapour content of the gas being measured. Condensed
liquid can alter the humidity elsewhere by dripping or running to other locations where
it may re-evaporate.
The integrity of all connections is also an important consideration, especially when
sampling low dew points at an elevated pressure. If a small leak occurs in a high
pressure line, gas will leak out but vortices at the leak point and a negative vapour
pressure differential will also allow water vapour to contaminate the flow.
Flow Rates
Theoretically flow rate has no direct effect on the measured moisture content, but in
practice it can have unanticipated effects on response speed and accuracy. The optimal
flow rate varies depending on the measurement technology, and can always be found
in the instrument or sensor manual.
An inadequate flow rate can:
• Accentuate adsorption and desorption effects on the gas passing through
the sampling system.
• Allow pockets of wet gas to remain undisturbed in a complex sampling
system, which will then gradually be released into the sample flow.
• Increase the chance of contamination from back diffusion: ambient air
that is wetter than the sample can flow from the exhaust back into the
system. A longer exhaust (sometimes called a pigtail) can also help
alleviate this problem.
• Slow the response of the sensor to changes in moisture content.
An excessively high flow rate can:
• Introduce back pressure, causing slower response times and unpredictable
effects on equipment such as humidity generators.
• Result in a reduction in depression capabilities in chilled mirror instruments
by having a cooling effect on the mirror. This is most apparent with gases
that have a high thermal conductivity such as hydrogen and helium.
POSSIBLE INJURY! The tubing, valves and other
apparatus attached to this instrument must be
adequate for the maximum pressure which will be
applied, otherwise physical injury to the operator or
bystander is possible.
Before disconnecting the instrument from the gas
line it is essential to vent the system to atmospheric
pressure, otherwise severe injury could result.
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