Figure 30 Condensation In Sample Tubing - Michell Instruments Optidew User Manual

Optidew User's Manual
Take into consideration the gas you are measuring, and then choose materials appropriate to the
results you need. The effects of diffusion or moisture trapped in materials are more significant
when measuring very dry gases than when measuring a sample with a high level of humidity.
Temperature and Pressure effects
As the temperature or pressure of the environment fluctuates, water molecules are adsorbed and
desorbed from the internal surfaces of the sample tubing, causing small fluctuations in the
measured dew point.
Adsorption
is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to
the surface of a material, creating a film. 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 indefinitely.
However, as the temperature rises, so does the likelihood of desorption occurring.
Ensuring the temperature of the sampling components is kept at consistent levels is important
to prevent temperature fluctuation (i.e. through diurnal changes) continually varying the rates of
adsorption and desorption. This effect will manifest through a measured value which increases
during the day (as desorption peaks), then decreasing at night as more moisture is adsorbed into
the sampling equipment.

Figure 30 Condensation in Sample Tubing

I f tem peratures drop below the sam ple dew point, w ater m ay condense in sam ple tubing and affect the
accuracy of m easurem ents.
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.
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