Gas Matching is one of the most poorly understood and underutilized tools by Technical SCUBA Divers who dive in an overhead environment.
We all know the rule of thirds proposed by Sheck Exley in “Basic Cave Diving: A Blueprint for Survival”; a third of the gas supplies for the outward journey, a third for the return journey and a third held in reserve for any emergency.
This is a great piece of advice but does not take into account that more than likely in an emergency with some equipment failure, you will be using your Buddy’s third to exit the cave or wreck. If you and your Buddy have different size cylinders or different breathing rates, you are in a very real danger of not having enough gas to reach the surface safely.
An example of how using the rule of thirds could be dangerous is as follows:
Diver 1 is using twin 80 cu.ft cylinders (9.8L) filled to 3365 psi (232 bar) and his buddy is using twin 120 cu.ft (14.6L) cylinders also filled to 3365 psi (232 bar). Diver 1 has a SAC rate of 0.64 CFM (18 L/min) and his Buddy has a rate of 0.78 CFM (22 L/min). Using the traditional rule of thirds, Diver 1 and his Buddy’s turn pressure would be 2243 psi (155 bar). If an out-of-air emergency should occur to the Buddy at the furthest reach of the outbound leg of the dive, both divers would run out of gas about 8 minutes before reaching the exit.
The proper turnaround pressure should be 2321 psi (160 bar) for Diver 1 and 2514 psi (173 bar) for his Buddy. This will allow the buddy to have enough gas to exit using Diver 1’s reserve.
GasMatcher is an application that takes all the hard work of calculating what the reserve and your turnaround point should be. It takes into account the differences in your Cylinder sizes, whether you have twins, what your breathing rates are and what pressure your different cylinders are filled to. Using this information the application calculates what the indicated Turnaround Pressure should be for each dive team member and what amount of Gas should be held in reserve.
GasMatcher comes with many features including; how many minutes you can spend at a particular depth, how much gas supplies are in excess and a Surface Air Consumption (SAC) Calculator to assist in figuring out your individual breathing rates.
By using this tool, all Technical Divers, be they Wreck, Cave or Decompression divers can quickly and efficiently calculate their reserves and turn pressures before every dive.
As always, plan your dives, and dive your plans; safely.