19 cf pony equal to what PSI in AL80?

[LeadTurn_SD]

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.

Yep.

I think the direction several folks are trying to gently move the discussion in is to volumes. Once a diver grasps the relationship between the SPG reading, the volume remaining in the tank, and the effect of depth on rate of consumption of that remaining volume, suddenly a light goes on and the SPG is now seen as a timepiece rather than a pressure gauge:

"At this depth, with this volume tank, working this hard, my spg is really telling me that I can only stay 'X' minutes before I need to start heading up towards sunlight and seagulls".

It took me a bit of diving to make that connection.

Most folks are smarter than I am and grasp the connection more quickly .

Best wishes.

 

[nimoh]

simple solution is just to notice that a 19 is about a quarter of an 80, so 3000/4 = 750psi.

 

[MichigandiverJon]

Thank you all for the great advice. Yes, I see now that volume and computing my RMV is really the right way to go here. I was obviously going at this wrong, because the time i'd have on a 19 cf bottle was really what I was trying to guess at. Thanks again all for your patience.

 

[Basking Ridge Diver]

because the time i'd have on a 19 cf bottle was really what I was trying to guess at.

There are formulas that you can use that you do not need to guess at - if you know your SAC/RMV rate.

Volume = (SAC/RMV) * Pressure * Time
So using some basic numbers
Volume = .6 *2.82(60ft ATA) * 10(mins) = 16.92 cu ft
or
take the formula and solve for time
Time = Volume / (SAC/RMV) * Pressure
So using some basic numbers
Time = 19 (pony) / .6 * 2.82 = 11.22 mins

I use excel - to do a lot of preplans - you can figure in your descent - bottom time and ascent with safety stop if you want to go crazy....
But this may or may not help you if you know your SAC/RMV - that is the most critical value.

 

[JohnN]

Thank you all for the great advice. Yes, I see now that volume and computing my RMV is really the right way to go here. I was obviously going at this wrong, because the time i'd have on a 19 cf bottle was really what I was trying to guess at. Thanks again all for your patience.

For the sake of simplicity, lets say your SAC = 1.0 cu ft/min and poo happens at 100'

1 minute to sort things out, a 30 ft/min ascent to 15 ft, a 3 minute stop and then you surface:

1 min @ 100' = 1.0 * 4(atm) = 4 cu ft

85 ft @ 30 ft/min = 2.8 min, 2.8 min @ 43 ft (average depth) =2.8 * (43/33 + 1) = 6.5 cu ft

3 minutes @ 15 ft, 3*(15/33 + 1) = 4.4 cu ft

go to surface (@ 10 ft min) = 1.5 min @ 7.5 ft = 1.7 cu ft


so:

4 + 6.5 + 4.4 + 1.7 = 16.6 cu ft (and you're very close to getting all the available air out of your bottle -- that number is about 18 cu ft)

 

[doctormike]

simple solution is just to notice that a 19 is about a quarter of an 80, so 3000/4 = 750psi.

A good rule of thumb. The only problem with that (in terms of teaching a new diver how to think about tank volumes) is that you will be off if you are comparing tanks with different service pressures (i.e. aluminum vs. steel).

 

[MichigandiverJon]

A good rule of thumb. The only problem with that (in terms of teaching a new diver how to think about tank volumes) is that you will be off if you are comparing tanks with different service pressures (i.e. aluminum vs. steel).

Thanks doctormike, I actually do switch frequently between hp120 steel and al80 depending on my dive.

 

[Kharon]

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.

I'm sure I have breathed tanks down to zero - at least that's what the guage showed on the surface and purging produced no air flow. Yeah, I know that there is error in what (at least some) guages show. However, if you couldn't breathe any of the last 500 psi the recommendation (requirement in some places) to board/exit wouldn't be 500 psi - it would probably be 1000psi. You certainly can breathe more than 8.9 cf of a 13cf pony and it doesn't take a $700 reg to do it. I've breathed one dry just to see what it felt like with a Cressi AC2/XS2 reg set (about $150).

 

[doctormike]

I'm sure I have breathed tanks down to zero - at least that's what the guage showed on the surface and purging produced no air flow. Yeah, I know that there is error in what (at least some) guages show. However, if you couldn't breathe any of the last 500 psi the recommendation (requirement in some places) to board/exit wouldn't be 500 psi - it would probably be 1000psi. You certainly can breathe more than 8.9 cf of a 13cf pony and it doesn't take a $700 reg to do it. I've breathed one dry just to see what it felt like with a Cressi AC2/XS2 reg set (about $150).

Oh, yeah... you can definitely suck it down to below 500 PSI, I think that he was just referring to the fact that at depth (or even on the surface) there is always SOME gas that the reg won't deliver (depending on the reg and the ambient pressure). So in case of an "empty" tank, you wouldn't be able to breathe it below ambient pressure no matter what. Kind of a minor difference in most calculations, unless you are talking about significant depths.

For his sample calculation, he chose 150 PSI. I guess that would be ambient pressure at 10 ATA, right (15 PSI at sea level), or about 300 FSW? If you breathe a tank "empty" underwater, it will start to deliver more gas as you ascend as ambient pressure drops.

 

[MichigandiverJon]

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

Originally Posted by Kharon
I'm sure I have breathed tanks down to zero - at least that's what the guage showed on the surface and purging produced no air flow. Yeah, I know that there is error in what (at least some) guages show. However, if you couldn't breathe any of the last 500 psi the recommendation (requirement in some places) to board/exit wouldn't be 500 psi - it would probably be 1000psi. You certainly can breathe more than 8.9 cf of a 13cf pony and it doesn't take a $700 reg to do it. I've breathed one dry just to see what it felt like with a Cressi AC2/XS2 reg set (about $150).



Before this, I hadn't thought of tank volumes in near as much detail before and this is really interesting to me! I think what John and DoctorMike are referring to is the fact that there is measurable volume in the tank that simply can't be used. i.e. The 4.1 or so cubic feet of air over the 8.9 usable is simply the base volume of the tank at atmospheric pressure. Hence DoctorMike's inclusion of the phrase "Lab grade vacuum"