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A Closer Look
Atmospheric Dewpoint vs. Pressure Dewpoint
At Fluid Energy, a major portion of our business revolves around the sale and service of dry, oil-free compressed air and gas systems. As such, we are often
asked to install hygrometers (dewpoint measuring instruments) to report the performance of regenerative dryers. Often we find that hygrometers are set-up
to read "atmospheric" dewpoint as opposed to "pressure" dewpoint. This in itself is not a problem as long as operators of compressed air drying equipment
know the difference between "atmospheric" and "pressure" dewpoint.
One of the golden rules of thermodynamics is that "the amount of water vapor that air and gases can hold is a function of its temperature only." In other
words, one actual cubic foot of air at atmospheric pressure can hold the same amount of water vapor as one actual cubic foot of air at 100 PSIG pressure,
provided that temperatures remain constant.
However, within the dynamics of a compressed air system, dewpoint can fluctuate as pressure changes. This is especially evident with the installation of
hygrometers for dewpoint measurement. Figure 1 shows a typical example of a regenerative air dryer controlled by a downstream hygrometer. While Fluid
Energy does not condone this type of control system, this is none the less a typical set up to control some brands of regenerative dryers. In the example
below, a sample line is run from the dryer discharge to the hygrometer sensor for dewpoint measurement. Depending on how the throttle valve is adjusted,
actual air pressure in the sample cell can vary all the way from line pressure to atmospheric. At Fluid Energy it has been our experience that most
hygrometers are set-up to read atmospheric dewpoint, not pressure dewpoint.
Figure 1. Typical Regenerative Air Dryer Controlled by a Downstream Hygrometer
For example, if the hygrometer output display showed
-40°F and the sample cell was metering atmospheric pressure, the result would be a -40°F "atmospheric" dewpoint. If in this same example the
throttle valve was adjusted such that 100 PSIG line pressure was now passing across the sample cell, the dewpoint display would change to 0°F.
This would be termed a 0°F "pressure" dewpoint.
In this example there is a 40°F difference between atmospheric and line pressure dewpoint. This by itself can often make a tremendous difference in
the success of an air or gas system, since compressed air is, in fact, what is being consumed.
For further information on this subject, contact Fluid Energy for:
Dewpoint conversion charts
Information on hygrometers
Portable dewpoint verification
Dryer control systems
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