Re-posted from
http://www.scubaboard.com/forums/tanks-valves-bands/372271-pony-redundant-set-up-2.html .
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:
Rock Bottom and Gas Management for Recreational Divers | Spherical Chicken 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.
