Good day, Matt Beckwith,
I agree with Dive Bug Bit Me, grantwiscour and others that now is a time to practice your skills. Two of the most fundamental aspects of diving are good air management and buoyancy control. Most divers go OOA because of poor gas planning or inattentiveness, not because of catastrophic gear failure.
My advice with respect to the use of a pony bottle is re-posted from
http://www.scubaboard.com/forums/tanks-valves-bands/372271-pony-redundant-set-up-2.html#post5761909 , below:
At least PADI and NAUI teach that there is a place for a redundant air supply (i.e., a pony) in diving, especially with deep dives. The choice is really up to you.
Personally, I like the idea of a redundant air supply but at this stage in my diving I am not into doubles. I therefore opt for a pony on some dives. Dives on which I believe that a pony is reasonable are: deep, cold, solo, or any combination thereof. I may well have missed something.
There was an excellent link at:
http://www.scriptkiddie.org/diving/rockbottom.html (note: this link appears to be broken) which shows that, if you want to go deeper but within recreational limits, a 19 is a good size, and 13 is a tad too small.
The best advice that I can offer is that, if you choose to dive with a pony, practice deploying it and stowing it regularly. Since a pony has a small capacity it will run down quickly. I top mine off from my main tanks with a device such as this:
Tank to Tank Equalizer With Gauge discounts on sale IST
I find that surface-swimming face-down with one mounted on your back is a pain. You might consider a stage/deco straps such as those indicated in
Stage/Deco Straps and Hose Retainers by Dive Rite - Dive Gear Express rather than a mounting bracket.
I sling mine like this:
DIR-diver.com - Stagebottle rigging. The only objection that I have heard to slinging a bottle is that it might get in the way if you are lobster hunting. I would stress that, if you dive with a pony, deploy it regularly so that it is second nature.
I prefer to sling my pony for the following reasons:
1. I can put the pony on or take it off in seconds - no tools, no hassle. I suppose that I could "hand it off" to someone underwater, but I don't think that this will realistically be necessary;
2. I can always see my pony tank and the reg. I can see if my reg is free-flowing, or if there appears to be a leak from the O-ring. If it snags on something, I can see what it snagged and fix the situation;
3. My basic gear configuration is unchanged - I use a primary and a bungee'd octo/secondary necklace. The pony reg is bungee'd to the pony bottle, far from my other two regs, and therefore will not be confused with them; and
4. I can consult the pressure gauge on my pony - this gauge is obviously connected to the pony and therefore will not be confused with my main tank pressure.
As a final note, many tech divers will pressurize their stage bottle then turn the valve off to prevent inadvertent gas loss during a dive. Perhaps one day I will do this, but so far, my pony is pressurized and the valve stays open during the dive. This might not be great if I had to rely on that gas for a deco obligation, but I believe that I am likely to see a leak in the pony and I don't do deco dives.
Safe diving.