Good Measurement Practice; Sampling Hints; Figure 29 Material Permeability Comparison - Michell Instruments Optidew User Manual

Optidew User's Manual

4.3. Good Measurement Practice

4.3.1. Sampling Hints

Ensuring reliable and accurate moisture measurements requires the correct sampling techniques,
and a basic understanding of how water vapour 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 vapour since water molecules are extremely small compared
to the structure of solids, even including the crystalline structure of metals. The graph below
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.

Figure 29 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 vapour to ingress
into the sample line from outside. This effect is called diffusion and occurs when the partial water
vapour 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.
36 97551 Issue 4.1, October 2019