Bail Out Bottle Size - Advance Open Water

[redacted]

(1) 80 to 100 ft.
(2) 13cf.
(3) 10 to 11 CF including 3+ min rest stop and normal boat reboarding.
(4) No

 

[Blackwood]

The math looks correct however the formula is WRONG. During ascend (or descend) - you need to account for the difference in SAC rate in relation to different depth. aka gas compression at depth.

His formula: C = S * (D/A) * (1/2 * D/33 + 1)

Assuming a constant ascent rate, that bolded 1/2 covers it. In other words, if you ascend at a constant rate, the average depth at which you are consuming air is halfway between the bottom and the surface, and the final parenthesis represents the average pressure at which air is being consumed (where the first represents how long it takes to get to the surface).

The forumla wouldn't work to compute gas from say 100' to 50'. For completeness, maybe he'd want to re-write it as C = S * (D2-D1)/A * (D2+D1)/66+1, and for going to the surface D1 = 0.

To the surface, his formula is correct aside from the units he suggested for C (should be CF, not CFM).

 

[Bubbletrubble]

The math looks correct however the formula is WRONG.
You formula is only good for constant depth air consumption where SAC rate is constant.
During ascend (or descend) - you need to account for the difference in SAC rate in relation to different depth. aka gas compression at depth.

@UaVaj: Perhaps you should take another look at Fish in a Barrel's post. His formula incorporates a SAC/RMV rate (independent of depth) and does, in fact, correct for the changing ambient pressure while gas is used during ascent (he uses average depth and constant ascent rate to do this). His calculation is a good-enough estimation of "ascent gas," in my opinion.

Ideally, a diver would compare his calculated number for ascent gas with actual gas used under real-world circumstances. Provided that he did his math properly, the numbers should be pretty close.

 

[Fish in a Barrel]

His formula: C = S * (D/A) * (1/2 * D/33 + 1)

Assuming a constant ascent rate, that bolded 1/2 covers it. In other words, if you ascend at a constant rate, the average depth at which you are consuming air is halfway between the bottom and the surface, and the final parenthesis represents the average pressure at which air is being consumed (where the first represents how long it takes to get to the surface).

The forumla wouldn't work to compute gas from say 100' to 50'. For completeness, maybe he'd want to re-write it as C = S * (D2-D1)/A * (D2+D1)/66+1, and for going to the surface D1 = 0.

To the surface, his formula is correct aside from the units he suggested for C (should be CF, not CFM).

You're right about the units. I edited my post to correct it. And you are also correct that my formula does assume ascent to the surface.

It also assumes a normal ascent, under no stress that would increase your SACR. And air consumed is not the same as bottle size. So you could multiply all that by a P (panic) constant for stressed SACR and then add on a U (unusable air) constant to get a rough idea of bottle size.

C = P * S * (D/A) * (1/2 * D/33 + 1) + U


...It's a really slow day at work.

 

[peterbj7]

Manufacturer data

Having owned some 600 tanks of various sizes and descriptions I can say that different manufacturers vary in the meaning of their markings. What may be true for one make may not be true for another. But I agree that in the context of recreational diving this is nit-picking. In the context of extreme technical diving I don't believe it is, though.

 

[rjack321]

Having owned some 600 tanks of various sizes and descriptions I can say that different manufacturers vary in the meaning of their markings. What may be true for one make may not be true for another. But I agree that in the context of recreational diving this is nit-picking. In the context of extreme technical diving I don't believe it is, though.

If you've owned 600 tanks you'd know that an AL80 has always held 77cf at 3000psi (and 70F). Plus or minus a cf or so in manufacturing variances.

"Extreme technical divers" don't plan their OC gas as tight as you are suggesting anyway. Its not plus minus a cubic foot, its plus a couple of extra stages.

 

[GrumpyOldGuy]

Peter's "600 tanks" includes some rather "special" ones , going back to a post in July

..... So for example, if you have an aluminium tank stamped with 3300 psi (or more likely just 3300) and there is a plus sign after the 3300 then that tank does not reach its stated rated capacity until it has been pressurised to 3300+10% = 3630 psi. If the tank has a stated capacity of 80 cu.ft. then that will only be achieved at that pressure of 3630 psi. If it is in fact pumped to 3000 psi it will only contain 80 * 3000 / 3630 cu.ft., or just over 66 cu.ft. Rather over 3/4 of what you thought it contained.....

 

[Blackwood]

Having owned some 600 tanks of various sizes and descriptions I can say that different manufacturers vary in the meaning of their markings. What may be true for one make may not be true for another. But I agree that in the context of recreational diving this is nit-picking. In the context of extreme technical diving I don't believe it is, though.

So when you plan an extreme technical dive, how do you determine cylinder capacity? Do you fill the cylinders you're going to use with water first? Was the water purified? Salt water? Tap water?

How far off were they? Were the North American system tanks further off from nominal than the European system tanks?

In my experience, working to mfr data has been sufficient, even when switching to cylinders I'd never used before and thus had no empirical data about.



Sigh. I honestly don't know how I allowed myself to drag us (you and myself) into this discussion. It's gone past my initial reason for responding (which was: the pressure to volume relationship is what matters), almost to the point of absurdity. I am rather inexperienced compared to you, and I intend no disrespect in the flippancy above.

You're right about the units. I edited my post to correct it. And you are also correct that my formula does assume ascent to the surface.

It's clear to me from your equation that you know what's going on. The unit was an obvious mistake, and the constant ascent to the surface assumption was clear. I only pointed them out so nobody could correct me in 'approving' your math

 

[fnfalman]

I am a bad AOW diver.

I don't use bailout bottles. I do carry a Spare Air now and then though.

 

[Buoyant1]

(1) 80-100 ffw

(2) 40cf slung

(3) In the quarry where I dive, it's not so much a case of "making it to the surface in an emergency" type of situation, it's more of a "get closer to the exit point at a shallower depth" thing. All of the guys I dive with sling 40's, if one person get's into trouble, dive is called, situation is stabilized, and we (the buddy team) make our way to the exit point as quickly and safely as possible. We've had a couple LONG surface swims in the past, and they aren't fun, but in the cold water environment we normally dive in, you need to do that sometimes.

(4) Never really had to use it, I've deployed it to hand off during a free flow situation, but that's about it.