Michell Instruments MDM300 I.S, Руководство пользователя

Страница: 49 / 72

MDM300 I.S. User’s Manual
Michell Instruments 41
GOOD MEASUREMENT PRACTICE
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 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.
!
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 MDM300 I.S. from the gas line
it is essential to vent the system to atmospheric pressure,
otherwise severe injury could result.