A lot of dive shops are comfortable overfilling tanks, often substantially, although it often depends on how will they know you and like you and where you live.
In Florida for example cave divers will routinely fill low pressure tanks to 3500 psi or more. One of the inside jokes among them is that the tank makers stamp the turnaround pressure right on the neck of the tanks. Myself, I wouldn't feel comfortable exceeding recommended pressures the way they do.
My own local dive shop routinely fills my 3442 tanks to 4k for me, and this isn't uncommon at all. Since these tanks start out with less air in them than if they were used for airgunning, they heat up quite a bit during the fill process, and when they cool they end up around 3600 or so.
If I know the shop tech well though and I ask, I can leave those tanks there and get a real 4000 psi fill. Unfortunately however, their banks and compressor really aren't set up to deliver much more than that, so that's about tops for my cf tank as well.
The standard hydro test procedure is to fill the tank to 5/3's of it's recommended pressure and then compare it's dimensions before during and after the test. Of course this is all done in a very controlled environment and using liquid so that if the tank blew there would be no explosion.
If you can get 4500 psi fills and you can afford the CF tank, then that's the route I would go. CF tanks really are superior for airgunning. If you are like me and can only get 4000 psi fills and you can get those with a steel tank too, then I'd save the money and get a steel tank. An HP119 at 4000 psi provides a whole lot more fills than a CF88 at 4000 psi, plus it costs less and will last forever. If you can only get the steel tank filled to 3500 though, then it's probably a wash and depends on how you're set for cash and how comfortable you are dealing with the extra weight. A full HP119 weighs about 50 lbs.
|This message has been edited by mr.fudd on May 6, 2012 12:47 PM|