Steel tank volume question

[trigfunctions]

Hi all - on Sunday I dove with a local operator. I saw that they offered Steel 100's for rent so I thought I would try them to help balance the air air consumption between me and my diminutive girlfriend. At the shop I noticed that the steel tanks were the same size as the aluminum tanks, so I asked the operator if they contained more air. he said sure, since they are filled to a higher pressure they contain the equivalent of 100 cf in an aluminum 80. That sounded reasonable, but it wasn't until later that I realized the steel tanks were only filled to around 2900 psi, just like the aluminum tanks were.

A little research on the net shows that the steels have to be filled to a higher pressure to have more air in them. Makes sense. It sounds like I paid extra for the steel tanks for nothing. Correct?

 

[Johnw...ski]

I calculate 96.6 cf at 2900 psi. Looks like you still did ok.

 

[Scuba_Steve]

you need a trig function to get the math right

Why does my Steel 104 @ 2640PSI contain more than your ALU80 @ 3000 PSI?

 

[Doc]

you need a trig function to get the math right

Why does my Steel 104 @ 2640PSI contain more than your ALU80 @ 3000 PSI?

Steel s-t-r-e-t-c-h-e-s more. Elasticity, you know.

If you want to "stay even" with your girl friends air consumption, just put a little nick in her l.p. hose.

 

[Scuba_Steve]

Ahhh, OK, thanks

I though I was getting ripped.

 

[spectrum]

A little research on the net shows that the steels have to be filled to a higher pressure to have more air in them. Makes sense. It sounds like I paid extra for the steel tanks for nothing. Correct?

You are correct that the external dimensions of an AL80 and HP 100 are darned close.

In your case the upshot is that the aluminum tank has a wall thickness of about 1/2 inch. The steel cylinder is closer to 3/16. This difference adds up to considerable volume.

So while your tank pressures were probably very close there was more volume in yours.

For an HP 100 at 2900 PSI I get 84.25 Cubic Feet.

Pete

 

[Blackwood]

For an HP 100 at 2900 PSI I get 84.25 Cubic Feet.
Pete

Agree (assuming a rated pressure of 3,442 psi)

 

[BrianV]

It's also probable that although the dive shop was renting steel 100's, their fill equipment is only rated to 3000 PSI. If that's the case, then all they're really renting you is the added benefit of a steel tank, not more air. Remember, steel tanks also don't have the buoyancy changes as you assume air that the aluminum cylinders have.

 

[Codyjp]

Where can i get those calcs for tank pressures/volume etc? or what is the equation?

 

[Firefyter]

Remember, steel tanks also don't have the buoyancy changes as you assume air that the aluminum cylinders have.

Wrong. The air consumed weighs the same whether it's in a steel tank or an AL one. The bouyancy swing will still be there, it's just that steel tanks will still be negative at the end of the dive instead of positive.