Trying to understand cylinder sizes

[Kirk Bauer]

I was trying to conceptualize how much air is inside a standard Aluminum 80 cylinder at 3000psi.

Based on an old thread (Untangling cubic feet, litres, PSI, bar for scuba tanks and RMV / SAC calculations) it says that the actual volume inside of an Aluminum 80 is approximately 11 liters. But 11 liters is approximately 0.4 cubic feet. If that's all true then the tank holds air equal to about 27 times its actual volume, if the air was at normal atmospheric pressure. In other words, there is approximately 300 liters of air inside an 11 liter cylinder.

But that math must be incorrect, because atmospheric pressure is about 15psi, and the tanks has 3000psi, which is approximately 200 times greater. So by this math, the cylinder is holding 200 times its volume in air.

Where am I going wrong in my math and understanding?

 

[loosenit2]

Good explanation in simple terms
Art - PSI vs Cuft

Good calculator
DiveBuddy

 

[wKkaY]

Can you show the steps on how you got 27?

If you did 11 / 0.4 ~ 27, then all you've done is calculate the conversion factor of cubic feet to liters (1 cft ~ 27 liters).

 

[jstreetley]

I was trying to conceptualize how much air is inside a standard Aluminum 80 cylinder at 3000psi.

Based on an old thread (Untangling cubic feet, litres, PSI, bar for scuba tanks and RMV / SAC calculations) it says that the actual volume inside of an Aluminum 80 is approximately 11 liters. But 11 liters is approximately 0.4 cubic feet. If that's all true then the tank holds air equal to about 27 times its actual volume, if the air was at normal atmospheric pressure. In other words, there is approximately 300 liters of air inside an 11 liter cylinder.

But that math must be incorrect, because atmospheric pressure is about 15psi, and the tanks has 3000psi, which is approximately 200 times greater. So by this math, the cylinder is holding 200 times its volume in air.

Where am I going wrong in my math and understanding?

Both come out to be around 200 for me?

80 cu ft inside a 0.4 cu ft volume = 200x atmospheric pressure
If atmospheric pressure is 15 psi then a filled pressure of 3000 psi makes sense.

 

[Kirk Bauer]

jstreetley, yes that's correct, it was indeed bad math as I suspected. All is right in the world again, thanks for your help.

 

[tursiops]

The tank holds 11 liters of air at atmospheric pressure. That is 0.4 cuft, at atmospheric pressure. think of it as the volume of water the tank would hold.
Now, put more air into the tank, by pressuring it. The tank is still the same volume, but you can put more gas into it under pressure. Now put it under 200 atmospheres pressure, so you get 200x 0.4 cuft of gas into the tank....= 80 cuft. The tank is still the same size, but the gas is under pressure.

Under 200 atmospheres pressure, the 11 liter tank can hold 200x11=2200 liters of pressurized gas.

These numbers are all approximate because (1) one atmosphere is 14.7 psi, not 15 psi, and (2) the 80 cuft tank is actually 77.4 cuft, and (3) the tank is only approximately 11 liters.

 

[JohnnyC]

Just use metric. Save yourself the annoyance.

 

[Neilwood]

Just use metric. Save yourself the annoyance.

It would be a lot simpler if everyone used metric all the time. It certainly makes calculations a hell of a lot easier.

I know we are mainly metric but still have imperial road signs for some reason and use imperial in car instruments. What is even stranger though is that fixed road signs are imperial (with miles) however if you see temporary road signs (such as for roadworks) they are usually in metric with metres. It is common to have a sign closing a lane for example saying that the lane closes in 200m with the next sign saying that the lane is closed for the next mile.

 

[Dan]

British revenge after losing their colony in 1776.

 

[Storker]

Just use metric. Save yourself the annoyance.

And in metric, an Al80 is an 11x200.