that's an interpretation that I've never heard before. being an engineer, to me thirds are always equal parts, and being a cave diver that means third in, third out, third reserve which is what we follow with any sort of penetration. What you're describing sounds just like rock bottom calculations for the part that you're reserving.
I also am an engineer and I do not really disagree with you, but this is not a cave diving or penetration thread I did not think. Okay, for planning purposes I split my gas into three not always equal amounts/partitions. As a long time user of independent doubles back when they were not acceptable until side mount came along and then it was okay, I always preserve in both tanks enough gas to complete a dive to a safe end. And whatever that end, bail out or completion is, it is part of the dive plan. Especially solo. For two buddies both diving independent doubles one would have to have quite the circus going on to end up without enough air to make a safe end to the dive. Again, this not being a technical or cave diving forum, I am talking open water, no hard or soft ceiling.
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