Function
Longer and deeper dives require a greater amount of breathing gas, in turn requiring either a larger cylinder or multiple cylinders. A large tank (up to 18 L) is very heavy and tends to raise the diver's center of mass, making them unbalanced in water. A simple multiple-tank configuration called separate doubles consists of two mechanically attached but otherwise unconnected tanks with two complete regulator sets (with a total of two first stage regulators and two second stage regulators).
The function of a manifold is to connect the air supplies of two cylinders (called doubles or Twins), allowing the diver to breathe simultaneously from both. The manifold is a metal tube (usually made of aircraft grade brass) with two cylinder connectors, two first-stage regulator connectors and three valves, as shown in the figure above. The left and the right valves allow to disconnect the corresponding first stage regulators, leaving the entire gas supply to be used through the remaining regulator. The central valve, called the isolating valve, separates the tanks into two independent systems, each with its own first-stage and second-stage regulators.
Read more about this topic: Manifold (scuba)
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