Michell Instruments MDM300 User Manual page 15

OPERATION
Sample Tubing Length
The sample point should always be as close to the critical measurement point as possible,
in order 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. It is clear from the graph shown
above that the best materials to resist transpiration are stainless steel and PTFE.
Trapped Moisture
Dead volumes (areas which are not in a direct fl ow 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 fi lters, valves (e.g. rubber from pressure regulators) or any other parts of the system
can also trap moisture.
Sample Conditioning
Sample conditioning is often necessary to avoid exposure of sensitive measuring
components to liquids and other contaminants which may cause damage or affect the
accuracy over time, depending on the measurement technology.
Particulate fi lters are used for removing dirt, rust, scale and any other solids that may
be in a sample stream. For protection against liquids, a coalescing fi lter should be used.
The membrane fi lter is a more expensive but highly effective alternative to a coalescing
fi lter. It provides protection from liquid droplets, and can even stop fl ow to the analyzer
completely when a large slug of liquid is encountered.
Condensation and Leaks
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 changes the water vapor content of the gas being measured. Condensed
liquid can alter the humidity elsewhere by dripping or running to other locations where
it may re-evaporate.
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 but vortices at the leak point and a negative vapor
pressure differential will also allow water vapor to contaminate the fl ow.
8
MDM300 Sampling System User's Manual
Dewpoint > T Dewpoint < T
97232 Issue 1.4, October 2016