Sampling Hints - Michell Instruments Easidew User Manual

Easidew Transmitter User's Manual
2.5.2

Sampling Hints

Measurement of moisture content is a complex subject, but does not need to be diffi cult.
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 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
Michell Instruments
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Figure 11 Transmitter Mounting - Sensor Block
Figure 12
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Material Permeability Comparison
OPERATION
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