1. "Rule of Thirds" does not apply to open water buddy diving. It is dangerous to do so, and will not give you enough gas to get out of an emergency in many situations.
2. "Rule of Thirds" applies only to buddy diving in an overhead environment. (EDIT: and tech/deco diving)
3. "Rule of Thirds" also has no logical basis in OW solo diving. Your gas reserve for solo diving is based upon your calculated needs, not some arbitrary "Rule of Thirds."
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If you want a more detailed explanation, here it is:
"Rule of Thirds" applies only to buddy diving in an overhead environment. The basis for "Rule of Thirds" is that your turn around when you've used 1/3 of your gas (so 2/3 of your gas remains). If you use 1/3 to penetrate then you'll need 1/3 to exit, and that leaves 1/3 in reserve.
If you buddy suffers catastrophic gas loss at the point of maximum penetration, then you will still have 2/3 of your gas - 1/3 to get you out and 1/3 to share with your buddy for the exit. For a team of two, "Rule of Thirds" in an overhead environment is cutting it dangerously close and consideration should be given to using "fourths" instead of "thirds," or carrying stage cylinders for more gas reserves.
"Rule of Thirds" is dangerous when applied to open-water buddy diving. That is because in OW diving you must also account for the gas required to ascend. So for OW diving you must use a "Modified Rule of Thirds" in which you must subtract your "rock bottom" gas from your total gas before applying "Rule of Thirds."
For example:
OW Buddy Diving on the wreck of the Spiegel Grove
Total Gas = AL80 at 3,000 PSI
"Rock Bottom" Gas = 1,200 PSI
Subtract "Rock Bottom" from total gas: 3,000 - 1,200 = 1,800 PSI
"Modified Thirds" = 1,800 divided by 3 = 600 PSI (NOT 3,000 PSI divided by 3 = 1,000 PSI - this is wrong!)
Therefore your "Turn Pressure" is 3,000 - 600 - 2,400 PSI
If you were to use "strict" Rule of Thirds (3,000/3), and you used a "Turn Pressure" of 3,000 mius one-third = 3,000-1,000 = 2,000 PSI, then you would arrive back at the mooring line at a depth of 100 feet with only 1,000 PSI remaining, which is 200 PSI below your "Rock Bottom" gas!
"Rule of Thirds" has no logical basis for OW solo diving. You must calculate your reserves based upon the dive scenario. You cannot arbitrarily use a "Rule of Thirds." For example, in the above scenario on the wreck of the Spiegel Grove: worst-case scenario is a catastrophic gas failure at your maximum distance from the mooring line. So you have to decide whether you will use your gas reserves to do an emergency drifting ascent from the wreck, or if you will make your way back to the mooring line to ascend, and you must calculate the gas requirement for your chosen contigency plan. "Rule of Thirds" is irrelevant.