Optimal Pony Bottle Size for Failure at 100ft?

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What are all the potential actions that a diver needs to consider when they have no air supply?
Whether to choose cheesburger, hot dog, or tacos for lunch when we get back to the dock? If soft split fins require more effort than firmer traditional fins because you have to kick more, easier kicks to move the same distance? How should I have answered when she asked "Does the floral print bathing suit make my ass look bigger?" Did I remember to lock the garage door before leaving for the airport?
 
Aviate, Navigate, Communicate. Troubleshooting is a consistent, but subordinate task to the 3 primary ones.

Isolate-to/switch-to/get-to to a stable air source, ascend, whoops!

The SCUBA twist is the ethical burden of making a reasonable attempt to inform a buddy of the situation before disappearing to the surface. Which reorders things a bit to: Air source, communicaton (if possible), surface.
Troubleshooting is subordinate to buddy responsibility and also to forcing your survival to be in your buddys hands.
When you take the long hose from your buddy's mouth, that's a pretty effective means of communicating that you're out of gas.
 
When you take the long hose from your buddy's mouth, that's a pretty effective means of communicating that you're out of gas.
Except the query is specifically about pony use.

Some may say I've gone to the dark side, but I don't ever want to be in position of finding and asking the insta-buddy for extra air, so I take my own.

Back to the original point, within rec limits, I'm betting my 17 cu ft bottle should handle that. If an 80 get me an hour, I figure a 17 can buy me 3 minutes. Probably a pause, maybe 5 minutes, at 15.

And frankly, flashing 👍 in the general direction of where the insta-buddy, should one be assigned, serves the purpose. He should, even if missing the call, look for me for no more than one minute and begin his own climb.
 
I asked, for you to give me an answer as to what you need to think about and do when your air supply is suspended. A rote response IS the correct one- switch regulators and leave! What are all the potential actions that a diver needs to consider when they have no air supply?

Are you at the up line? Get there or ascend from where you are? Getting to the line may take few minutes. If there is current, you may need to tie into the wreck. I want the bag up as soon as possible so the crew can track me, especially if I don’t tie in. I also want to give other boats visibility that I am there.

If you consumed all of your gas, assuming you were not diving something smaller than an AL80 and this wasn’t an equipment failure, you’re likely over the NDL. Checking your computer and making a deliberate ascent is needed.

This can all be considered in planning contingent air supply. If they don’t apply, don’t do them and have a little extra gas left over.

And this isn’t “no air supply”. This is planned contingency air supply.
 
I go by the US navy dive manual. The formula is a little different we use 1.4 acfm but you are coving a lot. The only thing I wouldn't do is spend 2-3 minutes composing myself. Once there's an emergency we abort asap. The other thing is NDV Rev 6 used to say we needed either twin 80s or 100 cu foot bottle to go deeper than 60' we never go deeper than 130 unless it's an operational necessity. The Rev 7 now says to calculate all of your air consumption for a planned dive and have a contingency / more air with independent regulators for twins or more bottles staged. We don't do recreational diving so it would have been a light work job like recovery or confirming an object. I liked that you plan your dive and dive your plan. Also if it's a decompression dive, it would be on a rebreather so on air in a no D dive we come straight up, no safety stop. I'm not saying I wouldn't do it on a recreational dive but we follow the tables to the letter.

My understanding is that US Navy dive procedures are calibrated for US Navy divers, who are assumed to be at a certain level of physical conditioning, and (more importantly) are assumed to either:

1. have access to a US Navy vessel (and its facilities for treating injured divers), or else

2. to be on a mission of the type in which the operators would assume a certain risk of DCS in exchange for reducing their risk of detection by enemy forces.

There is hence, a certain danger in normal people diving for fun, following US Navy procedures.
 
Are you at the up line? Get there or ascend from where you are? Getting to the line may take few minutes. If there is current, you may need to tie into the wreck. I want the bag up as soon as possible so the crew can track me, especially if I don’t tie in. I also want to give other boats visibility that I am there.

If you consumed all of your gas, assuming you were not diving something smaller than an AL80 and this wasn’t an equipment failure, you’re likely over the NDL. Checking your computer and making a deliberate ascent is needed.

This can all be considered in planning contingent air supply. If they don’t apply, don’t do them and have a little extra gas left over.

And this isn’t “no air supply”. This is planned contingency air supply.
Did you not read what I wrote? Let me repeat it for you...

The whole idea of 2-3 minutes on the bottom after simply switching second stages seems "Wildly Pessimistic". Of course if you do have to make your way horizontally to an ascent line or mooring or a safe place to ascend, then that should be explicitly included in the calculations of emergency supply, but in most situations, I am going straight up - there is really no reason to hang around or think about options - I really have one - go up!.

Bringing in the idea of required decompression is also ridiculously off topic and outside the scope of the question.
 
@johndiver999 - Yes, I read what you wrote, possibly better than you did. You asked a question that I answered. We are in agreement that swimming to the up line is a possibility that could delay an immediate ascent and should be planned for. I’ll not insult your intelligence and say that you also see that tying into a wreck and/or shooting a bag is a possibility that could delay direct ascent and should be planned for.

Looks like we’re on the same page about two very real possibilities that would prevent an immediate, rote ascent.

Happy to disagree with you about the deco. It certainly does happen that rec divers overstay their NDL. The same kind of diver and situation where the diver would overstay their gas supply. But maybe it is irrelevant.
 
To try and bring this thread back to the OP's actual question.

I prefer a 40 for three reasons:
  1. I prefer a side-slung cylinder, so a 40, 30, and 19 all ride just about the same. I am not interested in back mounting my pony.
  2. It easily gives me PLENTY of redundant gas in the event of an emergency at any recreational depth/time.
  3. This reason is rarely discussed, but it is easier to manage logistically than a smaller cylinder. I don't need to worry about it being full for each dive. I really only need 1500-2000 psi in it for it to be a perfectly adequate emergency reserve down to around 100', which covers pretty much all my diving. That allows me to use it for months, including using it to shoot my DSMB on most dives, and I never need to fill or transfill it. I have two AL40s that I rotate through. I never have to fill then other than the annual VIP. I use one for 6 months or so and then switch to the other. I have the VIP schedules on both staggered. They rarely drop below 2200 psi in that period of time, typically closer to 2500 psi.
I am not getting embroiled in all of the minutia on the thread about procedure.
 
I prefer a 40 for three reasons:
  1. I prefer a side-slung cylinder, so a 40, 30, and 19 all ride just about the same. I am not interested in back mounting my pony.
  2. It easily gives me PLENTY of redundant gas in the event of an emergency at any recreational depth/time.
  3. This reason is rarely discussed, but it is easier to manage logistically than a smaller cylinder. I don't need to worry about it being full for each dive. I really only need 1500-2000 psi in it for it to be a perfectly adequate emergency reserve down to around 100', which covers pretty much all my diving. That allows me to use it for months, including using it to shoot my DSMB on most dives, and I never need to fill or transfill it. I have two AL40s that I rotate through. I never have to fill then other than the annual VIP. I use one for 6 months or so and then switch to the other. I have the VIP schedules on both staggered. They rarely drop below 2200 psi in that period of time, typically closer to 2500 psi.
I am not getting embroiled in all of the minutia on the thread about procedure.
That makes a lot of sense.

I do the opposite; back mount a small pony and bring a transfill whip on most every dive outing. I can top off a pony frequently and possibly transfer nitrox from one big tank to another and get another dive out of a fill and save $10-11 on a fill.

For a long time, I was very happy with using a 6 cu-ft for depths of a 100 or so, but of course it had to be topped off and I had to be committed to not exceeding ndl - something I am more inclined to do when spearfishing, rather than lobster diving. Because I hate being required to hang with sharks, lobsters aren't a problem as best I have seen.

I think it is useful to explain the logic applied and other reasons for choosing a particular configurations (even if some of them are for convenience - or even frugality).

But I have always be amazed that the assumption of being unable to switch to a pony and start an ascent in less than 30 seconds seems to be so widely accepted. Nothing wrong with that, but I don't see any good reason to apply that assumption - unless it is actually required.
 

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