matt_unique:
I don't think non DIR supporting views are appreciated in this forum but I will leave that to the mods to let me know.
Your summary above (as do many of the replies above) assume the lowest common denominator in terms of awareness and preparation. It's as foolish as the anti-DIR rhetoric DIR divers complain about. Diving a pony without knowing the gas contents, diving a tank without an SPG, regulator trailing behind the diver venting gas, divers who use a pony to extend bottom time, divers who breath their primary tank dry as though they had not monitored and planned in advance, divers who cannot access the valves on their pony, etc., etc. Come on guys - all of that is nonsense in the context of the original question. None of the replies above answer the question as though it were coming from a competant diver with a properly rigged pony bottle who wants a redundant breathing system. If you ask a DIR diver why you would want doubles over a single with a Y/K valve for example you would hear that the Y/K valves offer no protection from a blown tank o-ring among many other reasons. (There are many other benefits to doubles which I do not need to list here as an acedemic exercise). When you apply this logic to divers who wish to address this shortcoming by way of a pony, who do not need doubles for relatively shallow recreational profiles, the DIR answer often changes to the rhetoric I referenced above. Does the DIR methodology have an answer for this middle ground? If the answer is "you just have to believe" then that is OK too but if not I have yet to read a direct, rational answer to the question of pony use.
The pony should serve a very specific purpose - to offer a backup breathing system in the event of primary system failure - to get to the surface (ideally with a safety stop).
--Matt
Matt,
OOA with DIR divers is almost always not about having not enough gas - its about having difficulties with problem solving an issue with gas flow. You've got plenty on your back. In fact you're carrying yours and your buddy's. The typical AL80 is what most of the recreational dive world has at its disposal. It's relatively cheap, plentiful, and with 78 cf at 3000 psi usually doesn't get you into trouble (beyond NDL). In fact, it's positively bouyant at the end too!
For a DIR diver, that's not a whole lot of gas. Having a recreational reserve of 20-40 cf means a DIR diving will turn the dive somewhere between 1000-1500 psi. And that's if we are diving all useable not 1/2 or 1/3s. This is generally not an accepted practice outside of DIR with only 78 cf of gas on your back. DIR Recreational dives with enough BG reserves usually are in the neighborhood of 100-150 cf of gas depending on depth and time. You might be able to get away with an AL80 if you're in 20-25 ft of water. Steel tanks offer the right capacity and bouyancy in a single tank for this application. In the untrained hands, there may be temptation to breathe it down to 500 psi in which case you are probably entering into NDL exposure either by depth, time, or both. In which case, many suboptimal issues. The other explanation would be poor breathing control.
If you've got AL80's, the logical choice is double them up to give up 154 cf of gas without the convolution of an independent dissimilar tanks, and management of two SPGs without access to all your gas supply. I think you can agree on the benefits of manifolded doubles:balance, ease of operation, access to gas, etc... There are other doubles options out there besides the big guns: baby 60, 72, 85. You will see that gas capacity crosses over between the singles and doubles in the 120 -130 cf range. Don't put the cart before the horse. In other words, figure out the type of diving you want to do, and how much gas you need with built in rock bottoms and either have the right singles or doubles rig. This is contrary to the prevailing logic of buying a 30-40 cf of 'emergency reserve pony.'
In either singles or doubles configuration, my emergency is right below my mouth. I don't have to turn anything on, just move it to my mouth in a split second. If multiple failures occur with my 1st stage HP Seat failure, 2nd stage freeflow, diaphram puncture, tank o-ring, etc..., my buddy has already anticipated that based on how we are reading each other during the dive and his reg is deployed and coming at me. The trained disciplined diver is more reliable than any piece of equipment because it can anticipate, think, and respond accordingly. NOW HERE'S THE KICKER... I'm not getting his reg because there's not enough gas. In a recreational environment, even with burst O-rings and free-flows, I can continue MDL ascent procedures (this is our version of the prevailing 'safety stop' in the recreational range) in a self-sufficient way completely without my buddy precisely because I have plenty of gas on my back. I've got mine and my teammates. But because we have a very conservative approach, why risk it, if we can't solve the problem in the water? We take the time to solve the problem on our own or if it's so jacked up with multiple failures, we use our buddy's reg and try to solve this complex problem without truly running out of gas.
I'm probably giving away too many tricks of the trade - someone please stop me. All of this fits together in the context of properly trained and equipped divers. In most areas around the world, there are few "trained, equipped and disciplined divers" that have this type of DIR mindset. Thus, you're back to your own self-sufficiency, solo diving or encouraging others to be like-minded to be able to dive as a unified team. Hope you're able to get the later.
Let me caveat and say this is for recreational diving. Deco diving requires dissimilar gas mixes, therefore different tanks. Oh wait, I believe there are still agencies that still teach deep air?? My bad.
Sincerely,
H2
