A SIMPLE ALTERNATIVE TO WHAT TECHNICAL DIVERS DO…
Most of us are aware of the basic technical diving gas management “Rule of Thirds.” It ‘dictates’ one-third of a diver’s starting volume to begin the dive and reach the ‘destination,’ one-third to swim back to the starting point, and one-third in reserve as contingency gas. It’s a simple rule based on a team diving in an overhead environment: a cave.
It works for diving in caves, mines, inside wrecks, or any environment where direct access to the surface is either impossible (because of the overhead) or impractical (such as being a long a swim away from a safe exit point such as a mooring line). It is the go-to solution for many technical dive operations. And with minor modifications, it works for a significant number of other dives too. It’s simple, proven, and best practice.
However, it’s not the perfect solution for every recreational-level dive. Working out a better one requires a little more thought and a blanket one-size-fits-all rule will always fall short in some regard.
There are many ways to be safe and to avoid an out-of-air incident. These never end well — even when everyone involved surfaces intact but somewhat the wiser — because emergency gas sharing, in earnest, is not the best way to finish ANY dive.
Some of the methods recommended by various groups online involve more mathematical skills than most of us are comfortable with. The best and most important recommendation for most of us is to stick with a method that is simple to use and simple to apply during the final surface briefing conducted at the start of every dive.
One approach that works for most basic dives is to set aside a reserve volume, divide the remainder in half, and turn the dive to head home once half of that calculated remainder has been used. For example, if a tank is filled to 200 bar (about 3,000 psi), set aside 50 bar (about 700 psi) as a reserve, leaving 150 bar (2,300 psi) as usable gas. That means the dive is turned when the SPG reads around 125 bar (about 1850 psi). Using this method means that the swim back can be relaxed and usually gives divers an opportunity to check out the scenery. This method is simple and will get most teams back to the exit with enough gas to take the time to ascend slowly and in total control.
Three provisos: the first person whose SPG reaches the agreed reading to turn the dive is the controlling diver. When they turn, everyone turns. The second proviso is that few analogue SPGs (especially on rental gear) have been calibrated since leaving the manufacturer. Therefore, readings in the lower reaches of the dial (around the 50-bar mark and below) can be off by a surprising amount. Be conservative.
Lastly, the very best gas management method is to carry a dedicated cylinder of reserve gas… a buddy bottle.
To learn more about that, we suggest becoming a RAID Independent Diver.