Michell Instruments Optidew User Manual page 35

Optidew Series User's Manual
GOOD MEASUREMENT PRACTICE
Adsorption and Desorption
Adsorption is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved
solid to the surface of a material, creating a fi lm. The rate of adsorption is increased at
higher pressures and lower temperatures.
Desorption is the release of a substance from or through the surface of a material. In
constant environmental conditions, an adsorbed substance will remain on a surface
almost indefi nitely. However, as the temperature rises, so does the likelihood of
desorption occurring.
In practical terms, as the temperature of the environment fl uctuates, water molecules
are adsorbed and desorbed from the internal surfaces of the sample tubing, causing
small fl uctuations in the measured dew point.
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.
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