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Isolation manifolded doubles, or independent doubles with proper gas management procedures.
Next topic. :-)
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I’m not talking about proper gas management, this is for an emergency situation so who knows how it’d play out. Do you really want to plan your redundant air source amount on your best SAC values or give yourself some leeway?
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If responding to me, proper gas management is a qualifier to independent doubles, hence the placement of a single comma between the two alternatives to having a pony. Either solves by default the problem of sizing a pony, and the amount of reserve gas is going to be greater.
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30 cu/ft pony or better yet a set of dbles. I have dove with a pony for many dives. Excellent choice for buddy diving. But dbles is the only way I dive except for vacation diving.
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Agojo - 9/30/2015 4:59 AM 
Living in FWB where do you plan to solo dive to 99’? Only from a boat will you reach a 99’ dive in this area and then there is a whole new set of things you should consider. If no one is in the boat while you are diving you need to consider anchor slip, anchor rope break, can’t find the anchor, dive plan filed with someone topside including gps location with dive start and stop times, safety sausage with 100’ reel, etc. I’ve doubles, gave them up and settled for a buddy or a 19 pony or a depth I can CESA from.
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Returning back to Chgo. on Friday the 2nd. I don’t dive in Destin. But I do enjoy the beach and view from our place down here. The vis is just so so in the gulf. I’ll be in town to get together during the year anyways. Eating breakfast on the patio. The ocean is calm and the vis looks great today.
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lerpy - 10/06/2015 2:44 PM
If you are going to buy a pony, get a AL40. It is a great pony, and it is perfect for a deco bottle if you start getting into that. I also think it is a great idea as I note in another post about an issue I had resulting in a valve shutdown. I do dive doubles, and stand by having a redunant system even when diving a single, and not one of those Spare Air thingy’s that only have 3cu/ft, that won’t get you far.
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MDW - 10/08/2015 8:51 AM
When calculating for an actual emergency (in which you WILL be at least a little stressed) you should use a much higher SAC than your normal rate for calculation purposes. I’d say if your normal SAC is .75 you should figure on your emergency SAC being at least 1.0 or even 1.5 at least until you get to the safety stop where you may have time to calm down as you are now "almost home"
In practical terms, that means figure on needing 1.3 to 2.0 times as much emergency reserve as you calculated above. An AL30 or AL40 will give you that extra margin of safety you will want just in case of heavy breathing under stress.
So if you do choose to solo dive with "just" a pony instead of a fully redundant gas supply such as isolation manifold doubles or 2 full size side mount tanks, I would recommend slinging a larger pony such as a 30 or 40 cf tank. They are only marginally more expensive than a 19 and not that much more weight or drag to carry and have 1.5 to 2 times the gas in case you need it.
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DaveT - 2/13/2016 2:25 PM
MDW is pointing you in the correct direction. No one can tell you what amount of breathing gas you might need at99 feet without knowing how much gas you breathe. The class should educate you on gas management and calculating gas requirements. The Instructor should be able to then help you decide what redundant gas you should be using for specific dives. Along with the bottle, the method of carry comes into play.... once again ask your Instructor, that is what you are paying them for.
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