Michell Instruments SF82 Series, Руководство пользователя

Страница: 18 / 38

SF82 Transmitter User’s Manual
11 Michell Instruments
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 reduces
the water vapour content of the gas being measured. Condensed liquid can also alter the humidity
elsewhere by dripping or running to other locations where it may re-evaporate.
Although ambient pressure does not change drastically in a single location, the gas sample
pressure does need to be kept constant to avoid inconsistencies introduced by adsorption or
desorption. 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, however, vortices at the leak point and a negative vapour pressure differential
will also allow water vapour to contaminate the flow.
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.