Quote:
| Originally Posted by JohnD what PPO2 do people use, I am having an argument with my DO as he wants to stick to 1.4 and I like 1.6 on deco stops. gets me out of the water quicker and I am usually nowhere near my 80% CNS for an individual dive. comments? pros and cons, please. |
If you're talking about shallow deco, I suspect using pure O2 (in preference to 80%) is probably more important in terms of a "clean" deco than 1.4 vs 1.6, in that you maximise the inert gas offgassing gradient. If using O2, then the extra 0.2 bar would seem primarily useful for keeping inert gas in solution. All else being equal, I'd rather deco on O2 at 4m than 80% at 6m.
Fortunately BSAC now lets you use O2 at 4m (see
http://www.bsac.org/techserv/o2use0304.htm) so if you're qualified to use it then you're presumably knowledgeable and experienced enough to make your own decisions as to what you do under the water.
If you're talking about 50% at 21m vs 18m, I'm not sure there's a great difference if diving on a nitrox backgas, though on trimix, I like to decrease the inert gas fraction as soon as possible. If you are doing extended gas switch stops from dives with a nitrox backgas, it's worth looking at the compartment loadings that result - some of them increase reasonably significantly, although you may reduce microbubble formation.
From your DO's point of view, he technically has to remain with BSAC Safe Diving Practices otherwise he is putting himself in a legal grey area in the event of an incident. Some DO's are happy to do so (especially if they themselves are qualified beyond BSAC limits and are on the same dive) others are not. If he tells you not to do violate BSAC guidelines and you choose to do so, it would make sense not to rub his nose in it - which means you and your buddy not talking about it - otherwise he would be quite entitled to restrict your diving for ignoring him. At the very least, it would be courteous not to publicise the fact that you are going against his wishes.
Iain