I mean think about it, shop air compressors collect liquid water. Even in the desert. The physics of it is at any given temperature the air (no matter how dense or what pressure) can only hold so much water. At normal pressure (like where we live) this number is something like a few percent of the total. Any more than that and you have fog, clouds, dew, rain and all that stuff. Double that pressure (to say 1500 mm of Mercury), two atmospheres, 15 psig, and you have twice the problem (you can still only 'hold' a small weight of water in a given volume tube). Go to 150 psi and the problem is ten times worse. Pump your Discovery to 100 atmospheres (1500 psi) and unless your relative humidity was under one percent (in which case you're probably dead) you'll have condensation in there.
Rust on surfaces at normal pressure is another matter of course. Local humidity counts big time.
I think Tim's right. Unless stuff is 'rust proof', or otherwise protected (he recommends a thin coat of grease, also traps debris) you could be in for trouble. It's wet in there.
I have a Career Ultra that died from rust due to occasional hand pumping (long storage after a hunting trip). Tim saved it.
I think tank air is a good start, but there is a risk even then. One of the reasons SCUBA tanks are inspected (valve removed and a survey for cracks and rust inside) every year. Some of those tanks see a lot of use in a year. Coating them with grease or anything else a diver could breathe is clearly out of the question. Many years ago makers tried to get around this with glass liners on the inside of SCUBA tanks. It backfired, the rust would work under the liner......
Doug Owen
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