TECREC TECH DEEP DIVER MANUAL, Ch4, p199
Gas Matching
In tec diving you’ll hear a fair bit about gas matching, particularly in diving that involves overhead environments (caves and wrecks). Gas matching is a technique that accounts for teams with members who have different breathing rates and cylinder sizes. It helps assure that should the diver with the highest SAC rate and gas supply have complete gas loss at the furthest penetration point, a team mate with a lower gas supply volume and lower SAC rate have sufficient gas reserve for both divers to exit. The idea is for the diver with the smaller gas volume/lower SAC rate to reserve a volume equal to one-third of the larger gas supply.
Because you can usually ascend immediately or very soon in deep open water tec diving, gas matching isn’t typically used in open water tec diving. However, if you become qualified to make wreck penetrations or cave/cavern dives, gas matching is very important.
For open water tec diving it’s sufficient to determine your actual gas volume before each dive to be sure you have enough gas, plus reserve, to make the dive as planned. However, you may find it handy in situations in which you want to return to a particular point before ascending, such as along an anchor line when there’s a moderate surface current. In this case, you might plan to come back before ascending, and gas matching provides the option of doing so if assisting a team mate with more gas and a higher SAC. (Obviously, you still should have your lift bag, deco cylinders and be prepared to ascend directly).