Sampling Hints; Figure 22 Material Permeability Comparison - Michell Instruments Pura User Manual

Pura Advanced Online Hygrometer User Manual
5.2

Sampling Hints

Measurement of moisture content is a complex subject, but does not need to be difficult.
This section aims to explain the common mistakes made in measurement situations, the
causes of the problem, and how to avoid them. Mistakes and bad practices can cause the
measurement to vary from the expectation; therefore a good sampling technique is crucial
for accurate and reliable results.
All materials are permeable to water vapor, as the water molecule is extremely small
compared to the structure of solids, even when compared to the crystalline structure of
metals. The graph above shows the dew point inside tubing of different materials when
purged with very dry gas, where the exterior of the tubing is in the ambient environment.
Many materials contain moisture as part of their structure, particularly organic materials
(natural or synthetic), salts (or anything which contains them) and anything which
has small pores. It is important to ensure that the materials used are suitable for the
application.
If the partial water vapor pressure exerted on the outside of a compressed air line is
higher than on the inside, the atmospheric water vapor will naturally push through the
porous medium causing water to migrate into the pressurised air line. This effect is called
transpiration.
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 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.
Michell Instruments
Figure 22 Material Permeability Comparison
MEASUREMENT GUIDE
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