Manifold (scuba)

Scuba manifold

Two 300 bar scuba cylinders connected by an isolating manifold
Uses	Connection between two scuba cylinders to link gas supply

A scuba manifold is used to connect two diving cylinders containing breathing gas, providing a greater amount of gas for longer dive times or deeper dives. An isolation manifold allows the connection between the cylinders to be closed in the case of a leak from one of the cylinders or its valve or regulator, conserving the gas in the other cylinder. Diving with two or more cylinders is often associated with technical diving.

Several configurations are used, each with its own range of applications, advantages and disadvantages.

Longer and deeper dives require a greater amount of breathing gas, in turn requiring higher filling pressure, a larger cylinder or multiple cylinders. A large diameter cylinder tends to move the diver's center of mass further from the centreline, making them unbalanced in the water, and a higher pressure cylinder has a similar effect, also reducing the buoyancy of the diver, due to the thicker metal required for strength. Cylinder length is also limited by ergonomic considerations in proportion to the height of the diver. A single cylinder also presents a critical single point of failure for the breathing gas supply. Multiple-tank configurations include downstream manifolded twins, with a single regulator, independent or separate doubles which are two cylinders clamped to a backplate, but without a manifold, side mount cylinders, or upstream manifolded twins, with two complete regulator sets, which may have an isolation valve.

The function of a manifold is to connect the gas supplies of two back mounted cylinders (called doubles or twins), allowing the diver to breathe simultaneously from both. The manifold is a metal tube which is connected to the cylinder valves or directly to the cylinders, and may include an isolation valve or an outlet to connect a scuba regulator.

On an upstream manifold the left and right cylinder valves allow the corresponding first stage regulator to be shut off, leaving the entire gas supply to be used through the remaining regulator. On an isolation manifold, the central valve, called the isolating valve, separates the tanks into two independent systems, each with its own first-stage and second-stage regulators, which can prevent an upstream failure in one half of the system from losing the entire gas supply.

Manifolded twin and triple cylinder sets have been used since the days of Cousteau and Gagnan's development of the open circuit regulator, as can be seen from early photographs of the equipment. These were downstream manifolds, which connected the cylinders together by linking the outlets of the cylinder valves, and had one outlet for a regulator. This arrangement allowed larger gas storage capacity using the limited range of cylinders available. Independent valving of the manifolded cylinders also allowed the gas supply to be monitored in the absence of submersible pressure gauges, by opening and closing the valves in a specific order, as the gas was used up. The need to remember the history of valve operation and the lack of facility to connect a redundant regulator made the use of independent twins the usual alternative. This also has limitations, even when the contents can be closely monitored by using submersible pressure gauges. In 1970 a group of divers including Tom Mount, Ike Ikehara and George Benjamin came up with the concept and had the first recorded dual outlet scuba valves prototyped. These allowed upstream connection of the cylinders, with a regulator on the valved outlet of each cylinder.