CRDiver:
...I'm going to try slinging the bottle this season. Do you generally leave the valve on or off? I've had to use my pony in the past when my main reg freeflowed at 110', and it was nice to have it instantly ready. I'm not carrying any special deco mixes, and I know those who do often pressurize the reg at the surface and then shut it down for the dive itself. If I did that, I'm concerned about the possibility that - being inexperienced in carrying a bottle like this - I could somehow bump the second stage and depressurize the reg at depth. Is this a genuine concern?
With all due respect, my input is that slinging the tank is a superior way to dive with it. I've dived with it both ways.
1. Entanglements often occur when tank valves encounter net or fishline. A tangled tank valve is difficult to reach and clear when its an aluminum 80 valve near the back of your head. Its far
more difficult to clear when its the valve of a small bottle banded to the tank mid-way down your back.
2. You cannot reach the valve of a pony tank on your back to turn it off (or on) if its mounted valve up. Most of the time this may not be a problem. Should anything occur however, such as a freeflow in your regulator second stage, it is doubtful whether you could turn off the valve prior to your pony emptying.
3. A slung pony can be jettisoned or handed off to another diver. A pony on your back cannot.
In scuba diving the parameters that all divers face while underwater fall into two categories: those things that the diver has some control over, and those things that the diver has no control over.
All else being equal, I prefer to have control over as many things that may influence my survival as possible. The vast majority of things below I may not have control over, but whether I can reach and manipulate the valves that control my gas supply is something I can determine. When the pony bottle is slung, I can reach and manipulate the valve, avoid entanglements that I myself cannot reach, and hand the thing off or get rid of it as needed. With it on my back I can do none of those things. Its more a philosophical position than a debate over relative performance - it will still deliver gas (assuming ideal circumstances) in either position. But are you willing to bet that every dive you make will always be under ideal circumstances?
For more information, look here:
http://www.diverite.com/resource/stage/index.htm
Regards,
Doc