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Reply to Terry Mc Cracken: >The book says to use the thirds rule, 1 out , 1 back , 1 in reserve, but as I >pointed out earlier there is no out and back. Is this only your bottom gas ie >the last third is reserve and not what you deco on? Is it nessacary to leave >a third in a non-physical overhead environment allowing for the glass cieling >of deco.Any problems and up I go to deco on my lift bag. Terry, I too have been instructed to use the thirds rule for open water deco dives, "bring back a third" being the responsible thing to do. As you are now wondering, I too wondered about the justification of this stance. Basically, the thirds rule is great for penetration/cave dives, but is mis-applied when adopted to open water. My conclusion was to adopt the underlying principle of cave rules, and therefore the same level of conservatism. My understanding is this: the diiver must carry sufficient gas to safely return to the surface, after suffering a breathing system failure, at any point in the dive, without relying on another diver (in the first instance). If this fails, and a diver loses all redundant breathing systems, the dive partner must be able to supply a breathing system that will allow the safe return of both divers. >My thoughts... > If diving without pony, leave enough to decompress on in one tank (or >90 BAR whichever is more) and twice that amount in the other. This should >allow me to decompress even if I lose a tank and also have enough to >decompress two divers if required (3 actually but that would be to empty). >The only senario that I can see a problem with is for my buddy to lose all >gas and for me to have lost the tank with the *2 deco in it, but this means >that we have had 3 complete and seperate system failures. When solo diving with independant twins, the goal is to have enough gas in each cylinder to complete the ascent and decompression on any one of those cylinders. This is your TAP or turn around point. You can add a buffer of 30 bar in @ cylinder, for your own comfort in an emergency situation. If diving as a team, your TAP must be modified to account for the gas consumption of your partner, but the TAP will only change if your partner has a higher SCR than yourself. This is the correct method of gas management, for OW dives. It also accounts for contingencies that the simplistic "bring back a third" rule does not, such a a partner with a SCR more than double your own (quite possible). Also, it will be increasingly difficult to carry enough gas to bail out more than one other diver, as the depth/time increases. Most divers only plan for providing gas to one other diver, their partner.
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