The smallest pony bottle (extra air source)

[RAWalker]

All these numbers being pushed back and forth are really not realistic!
First off in an emergency the point is to survive first, minimizing the effects to your body is a secondary concern.
That said the pony bottle is emergency air and considering it in your dive profile as supply is foolishness. Larger bottles that are used for extending bottom times and used for decompression staging are another consideration all together.
The bottom line on a pony aka bailout bottle is to get you to the surface where you can breath non compressed air. It is for those times that your depth exceed those that would make cesa a realistic option or to just avoid cesa yet still within recreational dive limits of 130'. As for the 3 minute safety stop it is great to consider but may not be manditory and being bent at the surface is a far better situation than drowning. If you have the air supply by all means if you are clear headed and out of the worst of the emergency take as much of the safety stop time as you can. But having even a few breaths at depth puts you in a better situation than not having any reserve. If you are not solo diving a few breaths gives you time to communicate with your buddy while still independant of his/her auxilary second stage. It gives you time to get to a shallower depth where you may be able to get a breath off your primary tank due to pressure changes. Also due to pressure changes at the shallower depth you may be able to get a breath off your BCD.
So to sum it up any size bottle is better than none. If the size starts to become a consideration of transporting and you may decide to leave it home because you don't have the room for it or you don't want the extra weigh of carrying it. I'd say that's too large.
25% or more of your primary gas supply I still think that is too much to just make it to the surface but your considerations may be more conservative than mine.
I dive 80 cuft primary tanks and my choice was a 13 cuft pony.

 

[lowviz]

....................my choice was a 13 cuft pony.

Pretty much the same text, dive dual 100's, but I still like a slung 19 better, bigger but still quite tidy.

 

[Bretagus]

All these numbers being pushed back and forth are really not realistic!
First off in an emergency the point is to survive first, minimizing the effects to your body is a secondary concern.
That said the pony bottle is emergency air and considering it in your dive profile as supply is foolishness.

I disagree with this sentiment. In my mind, the pony is there to enable a safe and controlled abort in the event of a tank or reg failure. Why risk being bent on the surface when you can plan to have enough gas to abort the dive as you would any other... with a safety stop? Because of this, I feel it is vitally important that you consider how much air you would need to get top safely assuming the worst conditions. Also, the exercise of calculating the gas required to get to the surface helps to give you an idea of turnaround pressures at depth when you're relying on your buddy's air supply for redundancy. And it makes you want to buy larger tanks, haha!

Bottom line: If my back gas goes to hell, the last thing I need on my mind is hoping I was conservative enough in my estimates gas required to get to the surface. 13CuFt for up to 100ft, 19CuFt for anything deeper, IMHO.

 

[lowviz]

......Bottom line: If my back gas goes to hell, the last thing I need on my mind is hoping I was conservative enough in my estimates gas required to get to the surface. 13CuFt for up to 100ft, 19CuFt for anything deeper, IMHO.

RAWalker stated he dives an 80cuft primary. -If you go by 'not diving deeper than your tank size in cuft' then he shouldn't dive it past 80 feet, ~100 feet won't get him much bottom time. You and RAWalker say (and I agree) that a 13 cuft tank will get you back, no deco, from 100'. I still prefer a 19, oh well. BTDT, your breathing rate will skyrocket for a moment if it is a real issue.

I think that your objection revolves around 'gas planning'. Gas planning is everything (IMHO). Don't count on your buddy, you may both run out at the same time. Generally speaking, you plan your dive with your backgas and dive your plan. Your bailout is just more gear you sling before you hit the water. I seem to read RAWalker's post just this way.

 

[WD8CDH]

In fact, I'm considering (completely theoretical) situation of emergency escape from a confined space which is rapidly filling with water. When 1-2 minutes (air in my lungs) might be not enough to find a way out. So, would be nice to have some extra air source. But this thing should be light? small and easy to use. Depth is not an issue at this point, really. Maybe, there something which is already part of a standard survival gear?
Well, I remember I'd seen once something of cigar size in one of the James Bond movies.

I regularly have to carry an air pack for emergency escape from a confined space which may have an oxygen deficency (helium leak). What I carry is similar in size to a 13 cf pony and has a collapsable hood. It's bulky and really gets in the way of the work that I need to do. What would be much better for this application would be a regular size Spare Air clipped to my belt. The smaller models are the same diameter, just shorter and not enough easier to carry on the belt. It's a perfect use for a Spare Air, much better than a SCUBA emergency