Hopefully I can help out a little here. I work part-time in a dive shop doing sales, but my main job is equipment repair. I've filled my fair share of tanks, trust me.
First, I would not suggest draining a tank. In the case of aluminum and steel (especially steel), this puts you at risk of internal corrosion/rust. The tanks will be drained, internally inspected, o-ring replaced, and refilled at the time of your yearly VIP (inspection required by the DOT). Our shop will not refill your tank if it does not have a current VIP sticker on it.
Second, DOT standards state that tanks must be hydro'd every five years. At that time they will filled with water and pressurized to 5/3rds the tanks working pressure. They are held at that pressure for a predetermined amount of time. What they are looking for is tank wall expansion. Should it exceed the maximum limitations, the tank will be deemed condemned and marked as such. In this case, the tank can never be used for anything other than a doorstop or cut up to be used as a planter in the garden. If they pass hydro, our shop will also VIP the tank.
Third, if filled at a scuba shop, and that shop keeps current with compressor/filter maintenance, you should get clean breathable moisture-free air in your tank. In your case, the moisture-free part is important. You should never ever need to worry about water accumulation in your tank, thus you should never need to "drain" it. If you ever need to drain water from inside your tank, you should be immediately calling whom ever is filling your tank and let them know. In addition, you should also have your tank hydro'd before it is filled again. Our air is hyper filtered 4 times. In fact, I just changed our filters today.
As to air getting stale in a tank: If air is getting "stale", that means there is a major problem with their fill system, and there is no way on God's green Earth that I would be breathing from a tank filled at that station. Period.
As far as filling a tank to above working pressure: There are three ways of filling a tank (if not more). One way is to partially submerge the tank in water whilst filling it. This helps to dissipate heat, thus reducing PSI loss when the tank temperature cools to room temperature. Another way is to "hot fill" the tank and let it sit over night so as to return to room temperature. The tank is then topped off. The last way, and the way I fill the 3000 psi shop rental tanks, is to fill them slowly to 3400 psi. Once they return to room temperature, they typically settle out to 3000 psi.
I will say this, and in no way am I suggesting anyone ever do this......I'm simply relaying this to possibly alleviate any angst about over filling a tank by a few PSI: There are numerous tech divers and instructors associated with our shop. They will routinely double burst disc tanks and fill them to well above and beyond the tanks working pressure. In the past 20 years, they have never had an issue with any of the tanks. Like I said, I do not condone this whatsoever.
DOT does not allow for overfills on any tank that does not have a "+" rating stamped on the side of it. To my knowledge only steel tanks are plus rated. I know this might appear to be a contradiction to my above statement however, an overfill in this case is one in which the tank is brought to, and kept at, a working pressure that is 10% above the working pressure as indicated on the tank being filled.
Hope this helps in some way.
Cheers
