Manifold (scuba)
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In scuba diving a manifold is used to connect two diving cylinders (tanks) with breathing gas, allowing longer dive times or greater safety due to redundancy. This configuration provides several advantages when more than usual gas is needed or when redundancy is needed. Diving with two or more cylinders is associated with technical diving.
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[edit] Function
Longer and deeper dives require high amounts of breathing gas. The solution is either using larger cylinder, or taking several cylinders. Large tanks (up to 18L) are very heavy and shift the divers center of mass up, making them unbalanced during swimming. Another configuration, called separate doubles, uses two tanks bound together 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 two diving cylinders (called doubles or Twins), thus allowing the diver to breath simultaneously from two tanks. The manifold is basically a metal tube (usually made of aircraft grade brass[1]) with two cylinder connectors, two first stage regulator connectors and three valves, as shown on figure above. Left and right valve allow disconnection of left and right first stage regulator, 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.
[edit] Common emergency procedures
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This section addresses out-of-air situation and free-flow malfunction, latter referring to a regulator malfunction causing uncontrollable flow of breathing gas. Common technical diving configuration is assumed, defined by DIR.
[edit] Gas sharing
The diver breathes from primary second stage regulator connected to the right-shoulder cylinder by a long (2 meters/6 feet) hose. Secondary second stage regulator is worn on his neck and is connected to the left-shoulder cylinder by a short (0.5 meter/2 feet) hose. If partner experiences out-of-air emergency, dive hands him his primary regulator, which he knows for sure is functioning properly. The diver switches then to his secondary regulator. Entire volume of gas supply is available for the two divers for remaining dive.
[edit] Primary regulator free-flow malfunction
If primary regulator malfunctions, the diver closes right-shoulder cylinder valve and switches to the secondary regulator. Entire volume of gas supply is available for remaining dive.
[edit] Secondary second stage free-flow malfunction
If secondary regulator malfunctions, the diver closes left-shoulder cylinder valve. He keeps breathing from the primary regulator. Entire volume of gas supply is available for remaining dive.
[edit] Right cylinder connection free-flow malfunction
Cylinder to manifold connection malfunction, though rare, can result in extremely violent gas loss. In case of right shoulder connection free-flow failure the diver closes isolating valve and right shoulder cylinder valve. He switches to the secondary regulator. Closing the right valve does not conserve remaining gas in the right cylinder, but rather reduces distracting and visibility degrading bubble flow. Half of the remaining gas volume is available for the rest of the dive.
[edit] Left cylinder connection free-flow malfunction
In case of left shoulder connection free-flow failure the diver closes isolating valve and left shoulder cylinder valve. He keeps breathing from the primary regulator. Half of the remaining gas volume is available for the rest of the dive.
[edit] Advantages
The manifold offers the following advantages, compared to alternatives:
- Convenience - offers better fitting of cylinders then single large tank, improving streamline properties and comfort of diver.
- Simplicity - just breath the entire dive from single regulator, no need to change second stages during the dive, unless in emergency.
- Ease of air sharing - standard procedure exists when air sharing is needed - the long hose regulator is passed to the partner. The air donor switches to the short hose.
- Standard malfunction treatment - in case of regulator or manifold malfunction standard procedure can be used to minimize gas loss. The diver should localize malfunction and isolate it from the well functioning system by closing necessary valves.
[edit] Critics
Manifold becomes single point of failure for the gas supply, according to some divers, especially in overhead environments, such as caves and wrecks.
