Michell Instruments Optidew, Руководство пользователя

Страница: 34 / 66

Optidew Series User’s Manual
26 97430 Issue 2, March 2017
OPERATION
4 GOOD MEASUREMENT PRACTICE
4.1 Sampling Hints
Measurement of moisture content is a complex subject, but does not need to be dif fi 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.
Transpiration and Sampling Materials
- 20
- 30
- 40
- 50
- 60
- 70
1 2 3 4 5
Time (hours)
Dew point (ºC
)
nylon
copper
polyethylene
PTFE
nickel
stainless steel
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 pressurized air line. This effect is
called transpiration.