This may be a dumb question...

[cstreu1026]

OK, so I am considering getting my own tanks sometime before the next dive season. I understand most the advantages and disadvantages of the steel and a luminum. The one thing I haven't quite got a grasp on is the volume vs. pressure thing. Am I correct in thinking that if you have a steel tanks filled to capicity (I am thinking something like 2650...if my memory serves me correctly) and a aluminum tank of equal volume fill to capacity at 3000psi then the amount pf air in either tank is the same right? In other words 80 cu. ft. @ 2650 is the same as 80 cu. ft. @ 3000 and should last an equal amount of time. Do I have things straight or am I totally confused?

 

[BILLB]

Basically your are right.

There are tank experts on the board (NetDoc being one) but for the steel tanks, the delivered new tank pressure is, say 2400 PSI for an 80 Cu.Ft. tank. As I understand this, it is not 80 Cu. Ft. until the (+) rating is applied (next Hydro) that the tank actually has 80 Cu. Ft. of air.

But as you may find, adding more pressure to an LP tank has not deterred anyone who wants to do this practice (and without physical problems).

Net Doc?

 

[WileEDiver]

You've got it right...

A tank holds its stated volume of gas when filled to its rated pressure.

 

[HarleyDiver]

Steel tanks come in two flavors:
LP & HP
LP - Low Pressure tanks are rated at 2400+ . The + means a 10% overfill is allowed, .
a typical LP85 is the same height as a AL80 but only 7" diam, and holds 85cf at 2640psi

An AL80 actually holds about 77cf at its rated pressure of 3000psi
I don't know why we call them 80's when they are not.

a HP80 (High Pressure steel) is much shorter than the AL80 & must be pressurized to 3500 (3442) to achive its rated fill of 80cf.

And yes, some people do overfill LP steels to pack more gas into the tank. That little LP85 can carry over 110cf of air at 3500psi.
This is comman practice in cave country, but NOT allowed in most other places.