Gas Matching: Why
and How Made Simple*
…or at least as simple as possible
A basic assumption cave divers make regarding gas-volume management is that each team member should always have sufficient gas, in reserve, to get both himself and a buddy out of a cave, from their maximum point of penetration. When team members’ starting gas volumes are comparable, determining appropriate gas turnaround points is fairly easy. For example:
Bill and Ted’s cylinders give them a volume of 240 cubic feet each, at a starting pressure of 3,600 psi. At this pressure, the obvious turnaround point for both divers will be 2,400 psi. Assuming both divers penetrate a cave until one hits this agreed-upon turnaround point, neither will have breathed more than 80 cubic feet (one third the starting volume for each diver). At this point, both divers have at least 160 cubic feet of gas remaining in their cylinders. So — at least, in theory — each diver has sufficient gas remaining to get both himself and his buddy out, should the need arise.
What happens, however, when two cave divers begin a dive with substantially different starting volumes? This is a situation that can arise when:
- Divers are using identical cylinders, but have substantially different starting pressures.
- Divers are using cylinders that have substantially different capacities (such as when one diver is using double 80s and his or her buddy is using double 104s).
Here is an example that better illustrates the nature of the problem:
- Big John and his buddy, Little Joe, know everything there is about cave diving (I mean, after all, you can get this information off the Internet — right?). This discovery has saved them considerable money that they might otherwise have paid some greedy cave diving instructor to give them training they don’t really need.
- Little Joe has difficulty carrying a lot of weight (and, besides, he hardly breathes anything). Thus, Joe chooses to dive with just a single 80.Big John, on the other hand, is a certified Air Sucking Dog. He is twice the size of Little Joe, and breathes air twice as fast. Therefore, so that he can have bottom times comparable to Little Joe’s, Big John decides to dive double 80s.
- Both divers "know all about that Rule of Thirds stuff," so, to determine a turnaround point, they both take 1,000 psi off their starting pressure of 3,000 psi, and agree to turn around at 2,000. True to their expectations, they both hit 2,000 psi at nearly the same time. Big John also chooses this moment to hit the ceiling and, in the process, severs one of his low-pressure hoses.
- Unfortunately, that greedy cave diving instructor Big John and Little Joe so studiously avoided was not able to teach Big John how to shut off gas flow to a damaged or free-flowing regulator. Thus, within minutes, Big John is totally and completely out of air.
- At this point, Big John has breathed one third of his starting gas volume, or 53 cubic feet. He will most likely need at least this much gas, or close to it, to exit. Little Joe has breathed 27 cubic feet of gas to reach this point, and will also need as much to exit. Thus, the two buddies together will need at least 80 cubic feet, or close to it, to make it to the exit alive. The catch is, Little Joe only has 53 cubic feet of air remaining in his single 80. (My, how inconvenient.)
This example points out why cave divers who begin dives with substantially different gas-supply volumes must take this fact into account when establishing turnaround points. If they fail to do so, they risk not having sufficient gas to exit, in the event of a catastrophic gas-system failure.
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