Shaw SDHmini - Pressure and Flow Control; Particulate Contamination; Liquid Contamination

3.9 Pressure and Flow Control
In most cases the gas to be sampled will be at a posive pressure, which will need
to be reduced to atmospheric pressure. At low pressures, up to around 8 barg, this
can be done by including a simple needle valve (5) to control the ow and thereby
the pressure dropping across it. Ideally this would be the isolaon valve (1) on the
sample point, thus keeping the number of joints and glands, which are points of
potenal moisture ingress to a minimum.
At higher pressures it is advisable to reduce the pressure across a pressure
regulator (4). Any regulator used should be stainless steel and of a low swept
volume. Typical laboratory or welding type regulators are not suitable because
they have elastomeric diaphragms which are permeable to water vapour and the
high swept volume will be slow to purge.
3.10 Parculate Contaminaon
If the sample gas is likely to contain parculate material this should be kept away
from the sensor by including a small inline parcle lter (2) in the sample system.
If the surface of the sensor becomes partly coated with parcles it will not aect
the calibraon of the sensor but will restrict the movement of water molecules
across the surface so increasing the me it takes for the sensor to come to
equilibrium with the water vapour concentraon around it. If the solid material is
hygroscopic, such as desiccant dust, whenever there is a change in the moisture
concentraon in the sample, this material must come to equilibrium with the
moisture content before the sensor can respond to the change, which will cause
delays in obtaining accurate results.
3.11 Liquid Contaminaon
If the sample gas may contain liquid contaminaon, such as oil mist, this should
be kept away from the sensor by including a coalescing lter or an appropriate
membrane lter (2) in the sample system.
If the surface of the sensor becomes partly coated with liquid, again, it will
not aect the calibraon of the sensor. It will restrict the movement of water
molecules across the surface so increasing the me it takes for the sensor to
come to equilibrium with the water vapour concentraon around it. If the liquid is
hygroscopic, such as glycol, the buering eect described for desiccant dust may
be so great that the sensor becomes too slow to use.
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