Posted by mel mclaurin on October 03, 2001 at 13:10:07:
In Reply to: Re: DIR : a question re OOA...a specific answer, please. posted by AADIVER on October 01, 2001 at 17:35:13:
Imagine you have a massive blowout of your DIN (you are diving DIN arn't you? :) ) O-ring. So you roll your post off and switch to the backup reg if needed. This is a dive calling incident. You don't go on. It could be worse in that you actually blow both 1st stage o-rings, in which case you are OOA. The same goes for tank neck o-rings, 2nd stage valve seats, pressurized lines bursting, or whatever. In each case to be truely OOA you need two independent failures of equipment happening one the same dive to two independent sets of valves and regs. Very unlikely.
What is more unlikely is diver error. For example you run calculations for Deco obligations, air consumption, etc. and determine you need a certain amount of gas. Perhapse you're making a gas switch to some back gas that's hypoxic near the surface and you're using a travel mix for descent, ascent and deco stops that's narcotic or CNS toxic at depth. In this case you really don't always have access to all the gas you're carrying.
Now imagine you start a dive only to find conditions are vastly different from those anticipated. E.g. water temp is much lower, currents stronger, task loading higher for whatever reason. If the change in conditions occurs sufficiently late in the dive, at some point where aborting the dive is impractical or impossible (e.g. you've already acrued a significant deco obligation), then one of the buddies may consume gas at a rate suficiently higher than the one used for his calculations to cause an OOA.
This too is unlikely as long as divers use the rule of thirds and monitor their gas consumption as they dive. It isn't unthinkable though. Especially when diver error, conditions, and equipment failure are combined.
Imagine a buddy team that is on the return leg of their dive. If one buddy were to loose a tank neck o-ring and be forced to isolate his tanks, he'd effectively be ditching half of his air to save the other half. Instead of having a third to two thirds of his gas left for the return leg and any emergencies he now has only a sixth to a third, depending on what part of the return leg he is on. If conditions were to change for the worse suddenly, then he could easily consume all of his back gas before reaching a depth where he could safely ascend to the surface or switch to his travel/deco mix. In other words, he just went effectively OOA with only 1 equipment failure.
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