19 cf pony equal to what PSI in AL80?

[MichigandiverJon]

I'm sure this has been asked here before but I've been unable to find the thread to answer my question. I'm looking at getting a pony bottle and trying to get an idea of the time I'd have if I was using a 19 cf tank. If a 13 cf bottle is roughly equal to about 500 psi in an AL 80 as I've read, about what pressure in an AL80 would be roughly equal to 19cf? Thanks!

 

[olphart]

approx 750 psi X 0.26 = 20 cu ft

 

[dumpsterDiver]

500/13 = X/19


500*19/13 = X psi

 

[MichigandiverJon]

Thank you very much!

 

[Dsix36]

most aluminum tanks work like this:

actual pressure X tank size / 3000 = actual cf
so
712.5 psi X 80 / 3000 = 19 cf

712 psi is your answer

 

[JohnN]

most aluminum tanks work like this:

actual pressure X tank size / 3000 = actual cf
so
712.5 psi X 80 / 3000 = 19 cf

712 psi is your answer

Except:
AL80's are 77 cu ft (and some small change)

Using the "magic" 500 PSI: 500*77/3000 = 12.8 cu ft

HOWEVER:

The real question should be how much breathable gas is in the tank.

So let's assume your super-duper balanced 1st stage can perform wonders. . .

(500-150)*77/3000 = 8.9 cu ft

 

[MichigandiverJon]

(500-150)*77/3000 = 8.9 cu ft

So if I'm understanding right. Then at 500 psi, an AL 80 actually has 8.9 cf left. Correct?

 

[doctormike]

So if I'm understanding right. Then at 500 psi, an AL 80 actually has 8.9 cf left. Correct?

John is referring to "breathable gas", meaning that below a certain pressure the regulator will no longer deliver gas, since you aren't able to suck that last 500 PSI down to a lab-grade vacuum. So while it actually has 12.8 CUF, you can only breathe 8.9 CUF of it.

A good concept for you to learn when converting gas volumes is the baseline, which is essentially how much gas in CUF (for imperial divers) is represented by each PSI. If you know the baseline for all of your tanks, then you know how much gas you have in them, and how to compare the gas content of two different tanks.

baseline = [rated tank volume] / [service pressure]

So assuming that your aluminum 80 has an actual capacity of 77 CUF (this varies with manufacturer), and a service pressure of 3000 PSI, then that tanks baseline is 77/3000 = 0.0256.

If you have a 19 CUF pony with a service pressure of 3000 PSI, then the baseline is 19/3000 = 0.0063

You can write these two numbers on the tanks, so that you can know how much gas you have (this becomes more important in planning staged decompression dives, etc... but still good to know now).

1400 PSI in your AL80? That means that you have 1400 x 0.0256 = 36 CUF of gas

2200 PSI in your pony bottle? 2200 x 0.0063 = 14 CUF

(These numbers don't change with depth or mix, BTW)

And FINALLY, in case you ever go metric, remember that imperial volumes refer to the volume of gas at sea level that would come out of a tank filled to service pressure. So a "full" 80 has 80 (or 77) CUF of breathable gas (minus the last bit that you can't breathe). Metric tank volumes are just the liquid volume of the actual tank.

 

[LeadTurn_SD]

doctormike and I were posting almost exactly the same reply; the good doctor typed faster, and I like his reply better than mine anyway .

I'll simply second the notion that volumes trump psi when trying to compare tanks, and ultimately you will want to determine your own RMV (see the article I linked below) before deciding on appropriate sized pony / bailout bottles.

Go here for some additional info on gas management:

NWGratefulDiver.com

Best wishes.

 

[drbill]

I must be missing something... I'd just use the number of cu ft (less the margin for reg pressure) rather than converting it to PSI in an Al 80 tank. I can understand the logic and why someone might be interested in converting it to an equivalent PSI in an Al 80 but I think that's just making it more complex than the question needs to be.