Originally Posted by
JerryD
I should have said some moisture.
The moisture content in diving air is regulated to ensure that the demand regulator valve doesn't freeze up as the air expands through it. As such, those dive compressors do dry the air, but leave a small amount of water in the air. Stirrup pumps don't dry the air at all, and the moisture content can be fairly high. As such, a high pressure dessicant filter is recommended so that the moisture can condensate and collect in the filter before the dried air goes into the gun's cylinder.
T[URL="http://www.airgunbbs.com/showthread.php?706526-Has-anyone-actually-got-a-pump-corroded-PCP-cylinder/page1"]his old thread[/URL talks about some of the issues and fixes for using pumps.
.