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Gerrit, A couple of points. In diving independant tanks, the plan is that to breath them in such a way that at anytime if one of the tanks fails, there is enough gas to get you to the surface (including your decom). Thus, breathing one tank down to 1/2, then switching to the second tank, if the second tank dies at it's 1/2 mark, you have essentially breathed a full tank of air/gas, and now you only have 1/2 a tank to exit and decompress. If you are cave diving, and this scenario occures at max. penetration, you will not have enough gas to exit the cave, let alone decompress. Thus, most cave divers switch back and forth between the second stage regulators (and thus the tanks), and begin the exit when both tanks are down to 2/3rds of their starting pressures. At this point, 1/3 of the total gas supply has been used, and 2/3rds of the total gas supply remains for exiting and decompression. If either tank fails at this max penetration, the other tank contains enough to get the diver out of the cave - and if neither tank fails, there are hopefully ample supplies of gas to enable the diver to share with a partner, or manage other kinds of problems (ie, zero visiblity, accidently getting off the main line and having to find it before continuing out of the cave, etc.) This is less of a concern (but still a concern) with wreck diving, as penetrations are usually much shorter, and gas is consumed during max penetration digging for artifacts, or whatnot. However, the same concept of planned gas consumption would/should hold true for wreck diving. In "pure" open water diving, the turning point depends on the dive profile (depth and time) and the total planned decompression requirements. Hope this helps - John Submariner Research, Ltd. (johncrea@de*.co*)
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