Redundant Air Source Diving

[Troutmaster]

Like most folks said, get a 40cf tank (sorry I don't know what size you in the UK call that) and sling it. Don't bother with a smaller tank, a 40 isn't a big tank to sling and there is no such thing as too much gas in an emergency. Make up your own mind about leaving the valve on or not, there are good arguments both ways, but make sure the reg is charged to keep water out.

As far as what gas to put in the bottle, my opinion is either 1) air or 2) nitrox as lean as it needs to be for your maximum possible depth on any of the dives you are going to do. You don't want to be putting 36 mix in and then needing to drain it and refill it with air or leaner mix for a deeper dive. If you have a too rich mix in the pony and you need it in an emergency you risk adding oxygen toxicity to your list of problems.

For the type of diving you are doing now (no deco) if you need to go to your pony, the dive is done. The pony is just there to give you enough gas to get yourself to the surface safely. So even with air in the pony, once you start breathing off it you will be heading up, so you are not going to add to your deco obligation (you should not have a deco obligation) with a leaner mix. Switch to the pony, then you and your buddy make a nice controlled ascent, including your safety stop and surface.

Then figure out what the hell went wrong to put you in the OOA situation so you can try to avoid it in the future. Then post the information in here on SB so we can all learn from it.

 

[rcs9250]

As troutmaster suggested , make sure your pony gas is safe for the dive you do.
If you stay within recreational depths you could put 28% in the pony and (technically)never have to worry about toxing as long as you mind your depth. 28% will keep you at PPO2 of 1.4. Typically regarded as safe.
If you get in a situation where your air gets low where you anticipate OOA have the pony ready to deploy in an instant. If you prescribe to the school of turning it off after pressurizing , turn it on if you think you will be using it and immediately check your pressure gauge and your depth. If you were dumb enough to run out of air you might be to deep for your gas blend too. Don't switch till your back gas is exhausted or WOB increases dramatically. If pony size permits make a normal ascent and do your stop if gas permits.
Be of the mindset that the pony is for loaning to the other moron that runs OOA.
Granted a malfunction could occur where you use the pony yourself but watch your SPG.
Keep an eye on your SPG at all times especially on deep dives.
Dive safe.

 

[R1Adam]

Thanks for all the great responses guys. You've really, given me a lot of information about something I knew nothing about. That's why I love forums like this. You can't get that sort of expertise and advice so easily anywhere else. When the water warms up a bit round here I'll invest in a pony setup and then I can hopefully do some nice dives.

I'm going away next week and I'm going to increase my training with the Wreck and Deep specialities while I'm there so will hopefully have a better insight into deep wreck diving.