What you described is correct but again it is not changing anything... We are not discussing breathing from the failed valve fully open. or from a blown burst disk.
When you breath from the tank valve directly you crank it just enough to make the smallest stream of bubbles and catch those bubbles from a distance when their velocity is tolerable. At that moment the pressure inside the bubbles will be equal to ambient or at most close...
We can discuss ad nauseum how much it will be at the tip of the dip tube or when the tank blow up but it's not relevant to the original post, do not you find ?
Topic creates a context of discussion and we discuss it in the context.
Unless we want to discuss the obviously idiotic case when the operator in question decides to put the LP hose or the tank valve into their mouth directly...
Darwin Award mode:
1) An OOA situation is not an usual situation i.e. there is already something wrong. e.g. someone forgot to plan the dive or to dive the plan, maybe distraction maybe a contingent and unexpected event.
2) How many degrees turning the tank open make the difference between a quasi laminar flow and a turbulent flow?
maybe in under stress my motoric will emphasize all movements and drive the opening more toward too-much than exactly-what-I-need.
3) Why not adding some more stress juggling with a tank without regulator. Maybe I can improve my panic threshold and/or try to kill myself while holding the tank in place with one hand, opening the valve with the other hand, making a funnel to direct the flow to my mouth while trying to avoid drowning with my third hand, control my buoyancy while ascending, releasing air from the BCD with my fourth hand while regulating the flow from the tank to compensate for the change in ambient pressure and maybe I can also throw a nice esmb at 15ft to make deco or a safety stop which I will probably need whereas I sucked all the air?
4) How fast will my emergency tank deplete when free flowing compared to using an on-demand regulator?
If you have doubts go to the bottom of your favorite pool check your gauge, free flow your regulator for a couple of minutes and then check again. Think that the pressure differential between intermediate pressure and ambient pressure is less than 150psi and the differential between tank pressure and ambient in more than 1500psi and Remember that your estimate of flow at something about 10ft should be project to the flow at e.g. 100ft.
5) Given the first fault, the best effort to make things difficult I am quite sure someone wants to add some more complication, maybe can shot some nice movie to immortalize the operation for the posterity and certify the feat for the jury of the Darwin Award.
Now I am not telling that it is impossible, my personal opinion is that in an unusual situation I prefer to have the easiest set of operations to do to bail my a** back home than juggling with a valve on a bare tank especially considering that an investment of some 100$ can make the difference.
Do It Right mode:
1) Plan your dive conservatively and stay away from an OOA situation
2) Shall an OOA happens be sure you have the easiest possible management of the situation.
e.g. a buddy next to you with plenty of air and a spare reg or a tank with a nice regulator hanging somewhere in a suitable place so that you do not even need to turn it and can just stick the regulator in your mouth and comfortably surface, almost as doing a normal ascent. It dramatically increase the chance you'll dive another day.
