Michell Instruments MDM300 User Manual page 16

MDM300 Sampling System User's Manual
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.
MDM300 I.S. fl ow rate
MDM300 fl ow rate
!
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.
An excessively high fl ow rate can:
• Introduce back pressure, causing slower response times and unpredictable
effects on equipment such as humidity generators.
• Result in a reduction in heating capabilities of the sensor tile during the
initialization period. This is most apparent with gases that have a high
thermal conductivity such as hydrogen and helium.
Michell Instruments
0.2 to 0.5 Nl/min (0.5 to 1 scfh)
0.2 to 1.2 Nl/min (0.5 to 1.2 scfh)
The fl owmeter should never be pressurized.
Always expand a pressurized sample to atmospheric
pressure before it enters the fl ow meter.
WARNING
OPERATION
9