Untangling cubic feet, litres, PSI, bar for scuba tanks and RMV / SAC calculations

[carlcz]

I use:

Volume of tank/(divided by) working pressure of tank X(times)411
so
80cuft AL = 0.0266666666666666 X 411 = 10.96 liters (11 liter tank)
3000psi

This should be pretty close for all of your SAC/RMV calculations

 

[Bearded Diver]

Super happy I came across this post. Thank you

 

[tursiops]

Super happy I came across this post. Thank you

But there are errors in rhw OP, of course. So don't take it as the "final" answer; check out some of the more recent threads on the same subject, such as Al 80 vs HP100 useable amount of air.
The big problem is the metric folks seem intent on assuming a tank must be filled to a certain pressure, for example 200 bars, and they make their calculations of tank sizes based on that. But US tanks have a working pressure that is typically 3000 or 3300 or 3442 or 3500 psi, or 207, 228, 237, or 241 bars, respectively. The working pressure must be used if you want to know the actual tank internal volume.

 

[Kendall Raine]

The working pressure must be used if you want to know the actual tank internal volume.

Unless you can breathe a liquid.

 

[Nemrod]

I have found the best way to untangle the mess between Metric and Imperial or "Murikan" is to do just exactly that, untangle the two. There is no point in conversions, either think in one system or the other. If I need to use the Metric system, I put on that hat and do Metric. If I want to use the Imperial system then I put on that had and think and do in it. I do not convert betwixt the two. Do one, do the other but do not do both.

 

[tursiops]

I have found the best way to untangle the mess between Metric and Imperial or "Murikan" is to do just exactly that, untangle the two. There is no point in conversions, either think in one system or the other. If I need to use the Metric system, I put on that hat and do Metric. If I want to use the Imperial system then I put on that had and think and do in it. I do not convert betwixt the two. Do one, do the other but do not do both.

That's great unless you travel. On a liveaboard recently I was asked if i wanted a 12l or a 15l tank. They were both aluminum. The "12l" was in fact an AL80; the "15l" was in fact an AL100, which is rated at 3300 psi, not 3000...and both tanks would be filled to 3000. It was nice to explain to my group that the "12l" was in fact 10.6l compared to the "15l" which was in fact 12.4l. When both were filled to 3000 psi the larger tank only held 17% more gas, not 25% like the labels they were using suggested, or even 20% more which the AL80 and AL100 labels suggested.

 

[L13]

That's great unless you travel. On a liveaboard recently I was asked if i wanted a 12l or a 15l tank. They were both aluminum. The "12l" was in fact an AL80; the "15l" was in fact an AL100, which is rated at 3300 psi, not 3000...and both tanks would be filled to 3000. It was nice to explain to my group that the "12l" was in fact 10.6l compared to the "15l" which was in fact 12.4l. When both were filled to 3000 psi the larger tank only held 17% more gas, not 25% like the labels they were using suggested, or even 20% more which the AL80 and AL100 labels suggested.

Was the 12l labeled 12l, and the 15l labeled 15l? Or was that just what the dive op was calling them?

Because the volume of a tank does not change (significantly) with working or fill pressure.

A tank stamped with 12l will be 12l at any fill pressure, even 1 bar (14.7psi).

A tank stamped 15l will always have 125% of a tank stamped 12l if they are at the same fill pressure.

 

[tursiops]

Was the 12l labeled 12l, and the 15l labeled 15l? Or was that just what the dive op was calling them?

Because the volume of a tank does not change (significantly) with working or fill pressure.

A tank stamped with 12l will be 12l at any fill pressure, even 1 bar (14.7psi).

A tank stamped 15l will always have 125% of a tank stamped 12l if they are at the same fill pressure.

No stamped labels, just incorrect names. The "12l" was not a 12l tank, and the "15l" was not a 15l tank.

 

[L13]

No stamped labels, just incorrect names. The "12l" was not a 12l tank, and the "15l" was not a 15l tank.

False advertising!

 

[BlueTrin]

No stamped labels, just incorrect names. The "12l" was not a 12l tank, and the "15l" was not a 15l tank.

Seems to be common in Egypt