THE RULE OF THIRDS When do you end a dive? Most recreational divers begin an ascent with about 700 psi or 50 bar of breathing gas left in their scuba cylinders. While this reserve pressure is practically an industry standard for shallow recreational dives, it may not be sufficiently conservative for every diving scenario. In many situations, a diver may wish to use the rule of thirds for his gas management. What Is the Rule of Thirds?: The rule of thirds for gas management states: A diver can use up to one third of his breathing gas to reach his furthest point, one third of his gas for his return trip, and will keep one third of his gas as an emergency reserve. In What Scenarios Do Divers Use the Rule of Thirds?: • Dives that require navigation to reach a location and then return to a starting point. One third of the breathing gas is used for navigation to the point of interest, one third is used to return to the starting point, and one third is held in reserve. • Dives that involve penetration into an overhead environment, such as a cave or shipwreck. One third of the breathing gas is used to enter the overhead environment, one third is used to exit, and one third is held in reserve. • Dive scenarios in which extra conservatism is required or desired. What Are the Benefits of Using the Rule of Thirds in Scuba Diving?: The benefits to using the rule of thirds for scuba diving gas management are evident in emergency situations. 1. By observing the rule of thirds, any diver can rescue another diver and get him to the exit point on dives requiring navigation or overhead environment penetration. If the dive team begins navigation back to the starting point of the dive when the first diver uses one third of his breathing gas, every diver in the team has enough gas in his tank(s) to safely reach the starting point of the dive while sharing air with another diver -- one third for himself and one-third for the diver who is out of air. 2. During dives that do not allow a direct ascent to the surface at all points of the dive, or during dives where this is simply not desirable, an abundant supply of breathing gas gives the divers time to solve problems, deal with equipment malfunctions, or address navigational confusion. In these cases, having a large reserve of breathing gas also helps to lower the stress level of the dive team by removing time pressure. 3. When divers are using double tanks - either independent doubles or manifolded doubles -- a diver is able to save half his remaining gas, even if a one tank loses all its breathing gas due to an equipment failure. Even at the point of the dive furthest from the exit location, a diver will still have sufficient gas to return to the planned exit point and end the dive without sharing air.
Posted on: Wed, 31 Dec 2014 02:51:51 +0000