The balance of this article addresses how to determine the safest possible turnaround points when divers have substantially different starting gas volumes. This is a process cave divers commonly refer to as gas matching. Some of the terminology used in this article includes:
Penetration Gas: That portion of a diver’s total breathing gas supply used to enter a cave. This should never exceed one third of the diver’s total starting gas volume (and, as we shall see, in situations in which team members have substantially different starting volumes, it should never exceed the penetration gas volume of the team member identified as the controlling diver).
Exit Gas: This is the portion of a diver’s total breathing gas supply that he or she keeps in reserve to use while exiting the cave. It is determined by subtracting the diver’s penetration gas volume from his or her total starting gas volume. Exit gas volume must always exceed penetration gas volume by a factor of at least two to one. Thus, it should (hopefully) provide sufficient reserve to deal with the unforeseen.
Turnaround Point: The dividing point between penetration gas and exit gas.
Controlling Diver: The diver with the least actual starting gas volume. Note that this is not necessarily the same as having the lowest starting gas pressure, when using cylinders of different capacities.