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Guest
These "what if" questions present more than an interesting math problem: they can distract for focusing on prevention of the scenario presented. It is obvious that the scenario presented is completely preventable through safe and competent diving practices. For that reason, though I would be a little less sarcastic in communicating it, I fell like Greyhound does. But more importantly, herman makes a great point: you are not going to be "suddenly" out of air. So enjoy the math problem, but also always: plan and follow a safe dive profile, monitor your gauges regularly, stay close enough to you buddy so they can be a source of survival, check your gear thoroughly before every dive trip, and again before every dive. Know and recognize the signs of a low tank- you can feel it. It's fine to know the extremes of an emergency self rescue. It's better to never put yourself in a position where you have to do it.
DivemasterDennis
I do agree with your basic idea, but there are failure modes that could leave a diver suddenly and completely OOA on their primary tank. They are extremely rare but should not be completely ignored. The critical point is good planning and dive practices will render even these types of problems more a serious inconvenience rather than a hazard.