PST MICHELL Instruments MDM300, Руководство пользователя

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MDM300 User Manual
Michell Instruments 39
GOOD MEASUREMENT PRACTICE
4 GOOD MEASUREMENT PRACTICE
Ensuring reliable and accurate moisture measurements requires the correct sampling
techniques, and a basic understanding of how water vapor behaves. This section aims
to explain the common mistakes and how to avoid them.
Sampling Materials – Permeation and Diffusion
All materials are permeable to water vapor since water molecules are extremely small
compared to the structure of solids, even including the crystalline structure of metals.
The graph above demonstrates this effect by showing the increase in dew point
temperature seen when passing very dry gas through tubing of different materials,
where the exterior of the tubing is in the ambient environment.
- 20
- 30
- 40
- 50
- 60
- 70
1 2 3 4 5
Time (hours)
D
ew poin
t (
ºC)
nylon
copper
polyethylene
PTFE
nickel
stainless steel
Figure 31 Material Permeability Comparison
What this demonstrates is the dramatic effect that different tubing materials have on
the humidity levels of a gas passed through them. Many materials contain moisture
as part of their structure and when these are used as tubing for a dry gas the gas
will absorb some of the moisture. Always avoid using organic materials (e.g. rubber),
materials containing salts and anything which has small pores which can easily trap
moisture (e.g. nylon).
As well as trapping moisture, porous sampling materials will also allow moisture vapor
to ingress into the sample line from outside. This effect is called diffusion and occurs
when the partial water vapor pressure exerted on the outside of a sample tube is
higher than on the inside. Remember that water molecules are very small so in this
case the term ‘porous’ applies to materials that would be considered impermeable in
an everyday sense – such as polyethylene or PTFE. Stainless steel and other metals
can be considered as practically impermeable and it is surface finish of pipework that
becomes the dominant factor. Electropolished stainless steel gives the best results over
the shortest time period.
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.