Easidew PRO I.S. User Manual
Michell Instruments
13
GOOD MEASUREMENT PRACTICE
•
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 protection from liquid droplets and can even stop flow to the
analyser completely when a large slug of liquid is encountered, saving the sensor from
potentially irreparable damage.
