Rule of thirds (diving)

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In scuba diving, the rule of thirds is a rule of thumb that divers use to plan dives so they do not consume all the breathing gas from the diving cylinder before the end of the dive.

For divers following the rule, one third of the gas supply is planned for the outward journey, one third is for the return journey and one third is a safety reserve. However, when diving with a buddy with a higher breathing rate and/or different volume of gas, it may be necessary to set one third of your buddy's gas supply as your remaining 'third'. This means that that the turn point to exit is earlier, or that the diver with the lower breathing rate carries a larger volume of gas than he alone requires.

Reserves are needed at the end of dives in case the diver has gone deeper or longer than planned and must do decompression stops before being able to ascend safely to the surface. In an overhead environment, the reserve allows a diver to donate gas to an out of gas partner, with enough gas available to bring both divers to the surface.