Optimal Pony Bottle Size for Failure at 100ft?

[gordonscuba]

When I do this kind of calc, I do each step in the calculation conservatively enough so that I am very sure that the pony will get me to the surface, assuming a safe ascent rate and a safety stop. However, getting to the surface is, for me, the end of the self-rescue scenario and I do not care about keeping further gas in the pony after that, nor do I require compressed gas to be available at the surface. What is the point in an emergency self-rescue of reaching the surface, breathing 5 cf from your regulator while swimming around, and still having a 20% full pony? So you can brag to the dive shop when you have it refilled? This is probably part of the reason why some of the other responders who are more experienced than I am seem quite happy with a 19 cf pony at these depths. That, and people at that level can reliably assume their SAC rate will not go up as much in an emergency because of the extreme training and experience.

Maybe you think the other calcs weren't conservative enough and so you need a final fudge factor, but then address the problem with the other calcs. (Yours look fine to me.)

 

[TooCold]

I had an interesting experience solo diving a couple of weeks ago: I had two failures within a minute of each other. It's the first dive I've had with any kind of stressful situation since I've been able to monitor and log my SAC rate / RMV. My drysuit neck seal tore and my bladder failed (not MY bladder - you know what I mean).

Long story short - my gas consumption averaged 2.5 times normal for about 90 seconds, then settled back down over the next 2 minutes. Some of that was pumping gas into the ocean before I realized the bladder wasn't holding it, but still, gas consumption is gas consumption.

When doing calculations for pony volume I had been assuming that my consumption rate would double under stress, which I thought was conservative enough. Now I realize it could probably easily triple in a real, serious emergency situation.

Theory is great, calculations are useful, reality is the teacher.

 

[Gone for diving]

Lets try a different approach. What's your turn pressure? What is your surface pressure? That was done on an AL80, right?

Remember the safety stop is optional, so the numbers already have a safety factor baked in.

Work with some real data instead of theoretical data.

I was laughing when the number kept going up and up. At one point it was up to 58 cubic feet, for a pony. I was looking at that and thinking, if he started with an 80 that leaves maybe 2 seconds of bottom time at 100' according to the bloated math. It is worse than internet advise on wiring gauge needed. Reading car forums and see people insist on running 10 gauge wire and a relay for a low wattage LED light that can safely be ran without a relay on 18 gauge wire and have the exact same output. But the armchair warriors keep stacking another safety factor on top of the last one. Quoting marine wire gauge charts and not willing to accept any voltage drop. Wiring up LED lights that have driver modules that will put out the exact same output over a huge voltage window. Oh, just go up another wire gauge to be safe, on top of being safe, on top of the baked in safety margin, which is a step above the original wiring, you know, just to be safe.

Then we have the booster packs....
With 12 g wire cranking 250 amps minimum. Surging 5 or 600 amps.

 

[CWK]

The biggest factor that determines your pony size is your own reaction to stress.

If you are a diver who stays at depth for several minutes to sort things out before beginning your ascent, then by all means get a 40cu ft pony, and you can even consider a 80cu ft pony, which will give you even more time at depth to sort things out.

However, if you are a diver who reacts by calmly ascending whilst turning off your main tank and extracting as much gas from it as possible before deploying the pony, then consider something smaller. For the depth you are discussing, skilled divers may get away with a pony that is as small as 13 cu ft or even 6cu ft.

For practical reasons I prefer a 19cu ft pony over a 6 cu ft pony.

See:

 

[Wants2divemore]

One other thing to consider is if you'll ever want to travel with the pony. A 19 cu ft pony is the largest that will fit in a standard rollerboard suitcase with the valve removed (the valve has to be removed so that TSA can verify the cylinder is empty). You can carry it in either your checked luggage or carry on.

Some will say to just rent a pony if you're traveling, however on my trip to Key West last week I couldn't find any shops close by that rented ponys. I was able to travel with my pony and get it filled so I had it for my Vandenberg dives.

 

[Gone for diving]

Some will say to just rent a pony if you're traveling, however on my trip to Key West last week I couldn't find any shops close by that rented ponys. I was able to travel with my pony and get it filled so I had it for my Vandenberg dives.

This is why in some ways sidemount or independent doubles, or even single tank and semi sidemount one, makes sense when traveling.
A second tank is easy to find,

 

[SlugLife]

Work with some real data instead of theoretical data.

Exactly. If you turn your dive at 1000psi, to have 500psi remaining at the surface....

(1000psi-500psi)/3000psi*80cu = 13.333 cu (plus buffer)

....plus whatever extra buffer (for panic, entanglements, etc) if you wish to be conservative. Within recreational limits, I usually intuitively know how much PSI I need when turning a dive, and even then I usually end up with more than the 500psi.

Even better is to actually test it with the size of pony you intend to use. Yeah, technically it should be the same amount of air, but it's nice to have proof "if I have an air-issue, and surface with the pony, there will still be 1500psi left in the pony"

 

[OMyMyOHellYes]

Why is there always an assumption that one spends 1-3 minutes at depth scratching one's ass in an emergency situmation?

In flying, we teach student pilots in their first few hours of training how to recognize a stall. And if they stall the aircraft, teach the absolute immediate action is to PUSH the nose over SMARTLY, level the wings (stop any turn/rotation) with rudder, apply power, (all of those happening at the same time, not sequentially) and as airspeed builds and stall is broken, slowly pull the nose up to arrest descent. Standard recovery should be immediate and result in no more than 100' loss in altitude. There is no "Oh, crap. We slowed down too much and our angle of attack exceeded limits and we're falling out of the sky! I wonder if I should be doing something. Yeah! I probably should do something! I wonder what I should do!" and spend a minute or three paralyzed and overwhelmed while the windscreen turns from blue to green. Or brown. Or maybe that's the underwear. The proper response is just an ingrained reaction that happens right-damned-now.

At 100 feet deep it may take about 3-5 seconds from "Oh-crap! No air!" to being on the pony and happily headed topside (that assumes no overhead environment - that would be a whole other flaming bag on the doorstep).

At the one minute mark, there's no reason someone couldn't be at 50 fsw. Practically almost CESA depth. Another minute topside. Or if they're just really conservative, maybe one minute find them at 70. And two and a half more minutes topside.

But since I know I've got a 17 or 19 cu ft bottle, I can slow that down a little and hang out at 15-20 fsw for a few minutes wondering how far I'm gonna have to swim to the boat or popping off a DSMB. And if one were to suck the pony totally dry, whooop-dee-doo. There's all you can breathe air just a few kicks upwards.

In a absolute out-of back-air crap cyclone at 130' the surface can be as close as 2:10 (two minutes, 10 seconds) away and still be within standard rec limits.

I'm not saying more air isn't better. Too much air is better than not enough air. But all the "you're gonna die - plan on panicking" aspect to training is teaching bad habits. Teaching immediate action is better. People default to their ingrained training. If they don't it wasn't ingrained training. And out of air at rec depths, if you have an immediate pony, is not a big deal to deal with. Unless a giant squid grabs you and is pulling you deeper. Then you have to assess the situation, put together a plan to retrieve the BFK strapped all handy to your calf, and you can sever a tentacle before it wraps another around you.

 

[johndiver999]

I pretty much agree with the above. Your first response (after switching over) should probably be to signal the buddy, then press the up button on the BC or if the regulator is completely non functional, then manually put a puff of air in the BC. Two kicks and you should be slowly ascending with zero effort. That is what I hope for anyway. Switching regulators should not be that stressful.

On the other hand... the reality of the situation is that catastrophic regulator failures are rare and chances are a diver who needs a pony ran the main tank down to E. For this to occur, it is also not unlikely that the diver may have been distracted or working very hard and failed to notice that he was sucking all his air down very fast (which is pretty much the reason they went OOA or low on air). So the diver may be in a very stressed situation, before the needle got pegged.

So if people want to plan for a high air consumption rate (especially for the initial part of the ascent), I don't think that is unwarranted. I do however agree that I don't plan to spend 3 minutes trying to find the inflate button on my BC.

As long as people are explicitly integrating their assumptions into the reserve calculations, I don't have a problem with being careful, especially if the person really feels like they may well freak out in an emergency.

 

[VikingDives]

Why is there always an assumption that one spends 1-3 minutes at depth scratching one's ass in an emergency situmation?

In flying, we teach student pilots in their first few hours of training how to recognize a stall. And if they stall the aircraft, teach the absolute immediate action is to PUSH the nose over SMARTLY, level the wings (stop any turn/rotation) with rudder, apply power, (all of those happening at the same time, not sequentially) and as airspeed builds and stall is broken, slowly pull the nose up to arrest descent. Standard recovery should be immediate and result in no more than 100' loss in altitude. There is no "Oh, crap. We slowed down too much and our angle of attack exceeded limits and we're falling out of the sky! I wonder if I should be doing something. Yeah! I probably should do something! I wonder what I should do!" and spend a minute or three paralyzed and overwhelmed while the windscreen turns from blue to green. Or brown. Or maybe that's the underwear. The proper response is just an ingrained reaction that happens right-damned-now.

At 100 feet deep it may take about 3-5 seconds from "Oh-crap! No air!" to being on the pony and happily headed topside (that assumes no overhead environment - that would be a whole other flaming bag on the doorstep).

At the one minute mark, there's no reason someone couldn't be at 50 fsw. Practically almost CESA depth. Another minute topside. Or if they're just really conservative, maybe one minute find them at 70. And two and a half more minutes topside.

But since I know I've got a 17 or 19 cu ft bottle, I can slow that down a little and hang out at 15-20 fsw for a few minutes wondering how far I'm gonna have to swim to the boat or popping off a DSMB. And if one were to suck the pony totally dry, whooop-dee-doo. There's all you can breathe air just a few kicks upwards.

In a absolute out-of back-air crap cyclone at 130' the surface can be as close as 2:10 (two minutes, 10 seconds) away and still be within standard rec limits.

I'm not saying more air isn't better. Too much air is better than not enough air. But all the "you're gonna die - plan on panicking" aspect to training is teaching bad habits. Teaching immediate action is better. People default to their ingrained training. If they don't it wasn't ingrained training. And out of air at rec depths, if you have an immediate pony, is not a big deal to deal with. Unless a giant squid grabs you and is pulling you deeper. Then you have to assess the situation, put together a plan to retrieve the BFK strapped all handy to your calf, and you can sever a tentacle before it wraps another around you.

It comes from technical diving. If you have a problem, taking a couple of minutes to handle the problem at depth is arguably a more reasoned solution than trying to make an ascent, under stress, while trying to do what amounts to a gas switch.

Thinking about it from a rec perspective, if you run completely OOG, you spend a second doing a switch and getting yourself under control before you start the ascent. Now you have gas, and you can take your time to manage the ascent.... that's scenario one. In your scenario, you have to switch gasses and manage your ascent simultaneously under stress. That's more likely to end up in an uncontrolled ascent.

IMO, if you are taking the time to ensure that you've got sufficient gas for the ascent, you're probably going to pay sufficient attention to your gas generally, so you will likely avoid getting task-loaded enough to run OOG. I'm not saying that's impossible, but it is less likely to occur. There are a range of other possibilities that might make you want to switch to your pony though. Being low on gas and some random dude shows up OOG, maybe a free-flowing regulator, maybe something else involving a buddy where you just can't or don't want to make like a Polaris Missle.