Michell Instruments SF82 Series, User Manual

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SF82 Transmitter User’s Manual
97576 Issue 1, June 2019 10
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.
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.
Temperature and Pressure effects
As the temperature or pressure of the environment fluctuates, water molecules are adsorbed and
desorbed from the internal surfaces of the sample tubing, causing small fluctuations in the
measured dew point.
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 almost indefinitely.
However, as the temperature rises, so does the likelihood of desorption occurring.
Ensuring the temperature of the sampling components is kept at consistent levels is important
to prevent temperature fluctuation (i.e. through diurnal changes) continually varying the rates of
adsorption and desorption. This effect will manifest through a measured value which increases
during the day (as desorption peaks), then decreasing at night as more moisture is adsorbed into
the sampling equipment.
If temperatures drop below the sample dew point, water may condense in sample tubing and
affect the accuracy of measurements.