dumpsterDiver:
I have no technical training. I would not want to be associated with any agency that commonly teaches divers to use air past about 150 feet. The narcosis is too significant for too many people. I do not advocate diving as described above either.
I follow a simple (air) hockey puck computer and add deep stops where I think I need them and complete all deco shown on the computer, even though I finish up with oxygen rather than simply air (which the computer thinks that I'm using). If I had a primary tank failure, my pony is so small that I would have to abbreviate the deeper stops and move more quickly to the oxygen bottle, if I were diving solo at 190.
Double tanks might add some degree of safety, but they would slow me down underwater. There is something to be said for simplicty and this same single-tank (plus pony) rig is also used for diving 60-80 feet, although the oxygen bottle is not used for non-deco dives.
Dumpster, I do not dispute your right to dive any way you wish. Have at it. It's one of the last great underregulated frontiers. So the following comments are not directed at you or your personal choices. Instead, they are directed to other divers on this board who may be thinking about technical training.
For anyone else reading this thread who is contemplating technical training and/or who is in-between a recreational setup and a technical setup, be advised that the dive profiles dumpsterDiver is describing are extremely hazardous.
At issue is solo deco diving on air to 190' for durations (bottom times) sufficient to result in a decompression obligation requiring staged decompression stops, while using a single tank of a size that offers near-zero emergency reserves when used at 190'.
Going to 190' on air itself is not the issue. Divers in the 1970s and 1980s routinely went to 190' on air, because there were no other broadly available options in the 1970s and 1980s. Speaking from experience, you can go to 190' on air and not suffer from narcosis to the point that you suffer vertigo, dizziness, or cognitive disassociation. IOW you may be affected by narcosis to varying degrees, but you are not aware of it. You can
suddenly become aware of it, however, as a result of little things - things like sudden exertion, a vertical head-down trim, or some other rapid movement as a result of which you suddenly are overcome by dizziness and/or vertigo. Generally a solution is to ascend, but if you are mid-water and suffering from vertigo and a reduction of cognitive functioning it may be tough to initially figure out which way is "up".
While helium mixes were not known in the 1970s and 1980s, we are aware now that there are safer mixes to use to go to 190'.
Even on air, however, divers 20 and 30 years ago used double tanks when going that deep based on consumption calculations and the establishment of gas plans that offered reserves in the event of unforeseen problems. Today that planning process has been generalized as "thirds"...one third out, one third back, and the third third for reserve planning. There are other ways to calculate gas planning, and these are discussed during technical training.
The issue is that if you become task fixated at 190' and get low on gas, a CESA from 190' is not likely to be successful. Solo, of course, you have no buddy to rely on. Moreover, even if you use a bail-out bottle to swim up, at 190' there are likely to be ramifications from swimming straight to the surface after an OOA incident without stopping. Dumpster says in his quote above that in event of any failure his pony is so small that he would "
have to abbreviate the deeper stops and move more quickly to the oxygen bottle". This allows seed bubbles to initially form and then expand, increasing plausible risk of DCS, as the diver ascends from 190'.
The point is that what is being described above is a technical dive profile using more than one gas, by a diver not technically trained to use 100% Oxygen (e.g. "no technical training"), under conditions that would reasonably, foreseeably result in requiring that diver to use those gasses - which are slung in two bottles: a pony bottle and a small bottle of O2. We won't even get into the various issues that can arise during gas switches, nor protocols for safe ascents using multiple gasses - which are also covered during technical training.
Most pointedly of all, if a concern regarding not using doubles results primarily from an argument that doubles "slow you down", but you are then slinging two bottles - one a pony bottle and one an O2 bottle - logically the two bottles slung from the diver are every bit as likely to result in increased drag and "slow the diver down" as using a set of doubles. But this configuration is doubly dangerous because in addition to the increased drag of the two slung bottles, with a single tank there is insufficient gas reserve should consumption suddenly rise or some other unanticipated emergency such as entanglement in fishline suddenly occur at 190' on air while solo.
So for everyone else, be advised that there are safer ways to do precisely the same thing, and when you go through the training courses necessary to work your way to a point of diving this way you will understand what the risks are and why you configure the way you do to perform this type of diving.
Dive safe,
Doc
I better keep my mouth full.
, huge single tanks, H valves, it all to confusing for me, the amazing thing to me is that you guys use those silly little single hose regulator things and live to tell of these exploits 
. Those little regulators are going to kill you guys. After all, this stuff is life support equipment.:no 
