Michell Instruments Optidew User Manual page 48

Optidew User's Manual
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. An inadequate flow rate may:
• 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 tube can 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 changes in dew
point
• 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.
System design for fastest response times
The more complicated the sample system, the more areas there are for trapped moisture to hide.
The key pitfalls to look out for here are the length of the sample tubing and dead volumes.
The sample point should always be as close as possible to the critical measurement point to
obtain a truly representative measurement. The length of the sample line to the sensor or
instrument should be as short as possible. Interconnection points and valves trap moisture, so
using the simplest sampling arrangement possible will reduce the time it takes for the sample
system to dry out when purged with dry gas.
Over a long tubing run, water will inevitably migrate into any line, and the effects of adsorption
and desorption will become more apparent.
Dead volumes (areas which are not in a direct flow path) in sample lines, hold onto water
molecules which are slowly released into the passing gas. This results in increased purge and
response times, and wetter than expected readings. Hygroscopic materials in filters, valves (e.g.
rubber from pressure regulators) or any other parts of the system can also trap moisture. Plan
your sampling system to ensure that the sample tap point and the measurement point are as
close as possible to avoid long runs of tubing and dead volumes.
Filtration
All trace moisture measurement instruments and sensors are by their nature sensitive
devices. Many processes contain dust, dirt or liquid droplets. Particulate filters are used for
removing dirt, rust, scale and any other solids that may be in a sample stream. For protection
against liquids, a coalescing or membrane filter should be used. The membrane provides
38 97551 Issue 4.1, October 2019