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Why Not Pre Dry High Pressure Air When Filling Your PCP?

Most moisture-removal systems used with HPA compressors work on the compressed air side of the unit. An alternative is to pre dry High Pressure Air, as described in this interesting post.

It’s written by Alan Applegate and Steve Dampier, co-owners of the Airgun Guild forum. Thanks to these gentlemen for their contribution on this important topic.


Compressed air and water are egregious cousins, especially so for us PCP airgun users. When we compress air, the moisture in that air condenses. If we don’t remove the moisture, it can, and does cause corrosion.

Air reservoirs (aluminum or steel), valves, seals, and other internal parts are all susceptible to corrosion. In fact, if the corrosion gets severe enough, a literal explosion of compressed air is possible. In other words, we need to dry our air!

A typical method is to use a water trap. They’re effective to a point, but the result is still too much moisture. A better method perhaps, it is use a high-pressure air dryer.

Inside the dryer are filters, sometimes activated carbon, and a desiccant, usually calcium chloride. These are held in sealed cartridge, and somewhat costly to replace. And, since their internal volume is typically larger than the reservoir we’re filling, we add significant wear and tear on our compressors.

Worse, we can’t tell when they’re no longer effective. Fortunately, there is an alternative—A pre-dryer!

The photo below shows a means to pre-dry High Pressure Air mounted on an Omega Air Charger compressor. It’s made from easily-obtained materials.

Why Not Pre Dry High Pressure Air When Filling Your PCP?

The YOEDAF filter housing, the drying medium, the plastic fittings, and vinyl hose are all available from Amazon.

The medium shown (orange substance inside the filter housing) is one of a variety of color-indicating, silica gel-based desiccants. We use the blue variety ourselves, but other desiccants and colors can be used with equal success.

One end of the filter housing is left open to the air (there are internal screens supplied on both ends of the filter housing to keep the medium inside). The other end is connected to the compressor’s inlet.

In the case shown (Omega Air Charger), the inlet is lower housing-mounted, and factory-fitted with a brass-fritted filter, with standard 1/2 pipe threads. It is simple to remove, and replaced with a suitable PVC pipe fitting.

The second photo shows the air inlet (1/8” BPT fitting with blue tape around it) on a Hill EC-3000. Other compressors are similar, but may require different techniques of attachment.

From experience, these pre-dryers do a very good job or removing moisture from the incoming air. In fact, the air pressure bleed valve, on the bottom of this Hill EC-3000, never has any visible moisture spray when opening, says Steve.

Without the pre-dryer it does! QED.

And the best part? It is really easy to see when the desiccant needs to be recharged, simply by noting its color!


Thanks Alan and Steve! If you have questions or comments on their suggestions, you can post them at the Airgun Guild.