Michell Instruments Optidew User Manual page 36

GOOD MEASUREMENT PRACTICE
Condensation and Leaks
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 vapor 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 vapor
pressure differential will also allow water vapor to contaminate the fl ow.
Flow Rates
Theoretically fl ow 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
fl ow rate varies depending on the measurement technology, and can always be found
in the instrument or sensor manual.
An inadequate fl ow 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 fl ow.
• Increase the chance of contamination from back diffusion: ambient air
that is wetter than the sample can fl ow 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.
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Optidew Series User's Manual
Dewpoint > T Dewpoint < T
97430 Issue 2, March 2017