Cave divers use two separate regulators, attached to double cylinders, so that, should one fail, they have a backup instantly available. When a regulator fails, however, it does not shut off a diver’s gas supply, but rather begins to free flow. Cave divers must be able to shut off the gas supply to the free-flowing regulator so that they do not lose all their remaining breathing gas.
To allow this, cave divers use a special tank manifold with separate on/off valves for each regulator. When looking at these manifolds, it is not instantly apparent how they work. That is what these drawings help explain.
When all the valves are on, gas flows simultaneously from both cylinders to both regulators.
When one of the regulator valves is turned off (such as would be necessary in the case of a free flow), gas continues to flow from both tanks to the remaining regulator.
Should one side of the manifold become damaged, creating a leak that cannot be shut down, the center isolator valve can be closed. This creates what is, in essence, two separate tanks and regulators. This way, at least half the diver’s remaining gas supply can be prevented from leaking out through the damaged side of the manifold.
In a situation such as this, the affected diver would breathe from the damaged side until the gas is depleted. He would then switch to the remaining side. At this point, he should be so close to the cave entrance that what remains will be more than sufficient.