Posted by Eric Frasco on April 30, 2001 at 16:20:57:
In Reply to: best to store your tanks with 150 psi in them posted by RaiderKarl on April 30, 2001 at 13:17:24:
Folks:
1) If there is saltwater in your steel tanks, oxidation (rust in this case) will occur and some of the oxygen will get consumed. The higher the pressure, the faster the reaction will occur and more oxygen will be consumed. So storing tanks with a minimal amount of pressure is recommended. A similar reaction will occur with aluminum, but aluminum oxide corrosion is somewhat self passivating, in other words it covers itself with an oxide layer that tends to inhibit further corrosion. So if corrosion is occurring inside your tank, you will be breathing air with a reduced % of oxygen. This could be significant over a period (months, years) of time or if there is a lot of moisture in your tank.
2) There is a very important reason to store your tank at low PSI (say 100 psi or so): If there was a fire where you store your tanks. Of course, as the tank heats up, pressure increases at about 5 psi per degree F and eventually the burst disk will rupture. But if the burst disk is clogged by debris and corrosion, it may not burst before catastrophic sidewall rupture occurs. It was explained to me that members of the Fire Department consider pressurized scuba cylinders to be extremely dangerous in fires, because when the sidewall ruptures the tank basically explodes - which makes it very dangerous to be around during fire fighting. If only 100 psi was in the tank, then the tank will essentially soften & melt before it can explode. The worst case is at an intermediate pressure, say a tank stored with half its working pressure. In fact, some firemen will refuse to enter a burning house if they know or suspect that scuba tanks are present.
Bottom line: Store your tank with minimal pressure and get a fresh fill right before you go diving.
-Eric-