Good Measurement Practice; Sampling Hints - Michell Instruments Optidew User Manual

Optidew User's Manual
4.3

Good Measurement Practice

4.3.1

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.
Transpiration and Sampling Materials
All materials are permeable to water vapour, 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 vapour pressure exerted on the outside of a compressed air line is
higher than on the inside, the atmospheric water vapour 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
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Material permeability comparison
Figure 27
nylon
copper
polyethylene
PTFE
nickel
stainless steel
2 3 4 5
Time (hours)
OPERATION
33