air reserve on lp96 vs n80

[FIXXERVI6]

Mech, the question I had tied in with my formula was for example only, I was trying to do the math to relate pressure vs volume in two different tanks.

maybe this did seam to spin off into a gas mangement area.... however, I appologize if I seam brash but you came accross to me as "this guy askin questions about gas management is a dumb ass"

I wanted to make sure my math was right.

 

[ERP]

The reason that gas management came up is becausethe 500psi that you based all your calculations around is a completly arbitrary amount.

People were just trying to convey what you should do, rather than what you were told to do. Even in open water with clear access to the surface the concept of rock bottom is extremely useful.

For example diving at 60ft, assuming a sac rate of 1.0cuft/minute (this might be on the high side, but better to be safe), your ascent would take about 5 minutes (including a minute at the bottom to sort out the issue), at an average depth of about 30ft, so thats 2 people * 2ATA's *5cuft = 20cuft of gas. or about 750psi (call it 800) on an AL80, and more like 600psi on the LP 95

Now this is the minimal amount of gas that you would need to start your ascent with, not the amount you should have back on the boat, so 500psi on the boat is fairly safe.

Even when diving doubles 500psi is generally considered the minimum because of gauge inaccuracies.

 

[lragsac]

MikeFerrara once bubbled...

The math in the opening post is correct except that a LP steel 95 contains about 95 cu ft at 2640 (10% overfill) and most al 80's are rated at 3000 psi.

An easy way to quicly convert presure to volume for a specific tank is to know hoe many cu ft/100 psi you have for different tank sizes. I have that memorized for the tanks I frequently use but it's a good thing to keep on a page in your wet notes.
For example an al 80...77 cu ft/3000 psi * 100 = 2.56 cu ft/100 psi. i use 2.5 in my calculations.
So...500 psi is this much volume...5 * 2.5 cu ft/100 psi = about 12.5 cu ft.
.....

of the rock bottom gas management principle
I agree, I think it should be taught in OW and AOW classes. Gas management is one of the most critical of skills, and it lays the foundation for situational awareness.

 

[lragsac]

MechDiver once bubbled...


Gas management is exactly what you are attempting to discuss. The fact that you don't know that is not my fault for trying to help you. I need a bigger ignore file

I agree with you, MechDiver, inherent in the discussion of working pressure and tank volume and "reserves" is the very notion of gas management. You can address each separately as a concept, but what is the point of leaving it fragmented and purely academic when you get in the water?

 

[FIXXERVI6]

ok ok oooops, my bad, sorry to everyone.

 

[rab]

The basic formulae needed are wrapped up in the ideal gas law:

PV=nRT

See this link for much more detail.

P = Pressure
V = Volume
n = moles of gas molecules
R = a constant
T = Temperature

So for a given gas under constant temperature, you can determine (estimate) the actual internal volume of a cylinder using:

P1 * V1 = P2 * V2 (Boyle's Law)

P1 = 1 ATA = 14.7 psi
V1 = 77 cft (equivalent air volume squeezed into tank)
P2 = 3000 psi (really 3014.7 psi since the gauge is showing 3000psi over the ambient 14.7psi)

V2 = actual internal volume = (P1 * V1) / P2
= 0.375 cft

So how much air is involved in a pressure change of 100psi in this tank?

100psi * 0.375cft = 14.7psi * V3

V3 = 2.55 cft

Quickly double-checking (sanity-checking?) the math gives a pressure change of 3000psi is 30 times our new V3 which predicts 76.6 cft. Not bad for a bit of rounding.

-Rob

P.S. Please note that this is much more from a physics/math background as I'm a new diver. The basic info in the PADI AOW text is just barely enough, but that's OK if you understand the concept from a physics perspective. In fact, I've take my psi changes for dives (so far) into account planning a deep, multi-level dive and had to back off the times due to air not NDL. I hope to actually do the dive on Saturday and see how my calculations fare.

 

[5615mike]

Thanks Mike. Good info. I wish I would have been taught this by my OW/AOW instructer.