Function
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'sHistory
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 includingComponents
A manifold in fluid mechanics is a pipe fitting or similar device that connects multiple inputs or outputs. In this application: *Cylinder valves control gas flow into and out of the cylinders. *Manifold connector tubes are used to provide a conduit for storage pressure gas to flow between cylinders and to the outlet connectors, and usually provide a fairly rigid connection between cylinders. *Isolation valves are mounted in manifold connector tubes which may be closed to shut off flow through that tube. *Outlet valves control gas flow to the regulators. *Reserve valves (mostly obsolescent) may be used to retain part of the pressure for contingencies. In some cases a valve may perform two functions – a cylinder valve may also be an outlet valve or an isolation valve, and in some cases each function may be performed by a structurally distinct modular unit, with the modular units combined to make the manifold assembly. In other cases more than one function may be provided by a single integrated unit.Construction
The manifold structural components are usually machined from a high grade brass alloy, and chromium-plated for corrosion resistance and appearance. Brass is used because it is strong enough, acceptably corrosion resistant, easy to machine, and suitable for oxygen service. The isolation valve uses similar materials, when present. Manifold lengths are available to connect different cylinder diameters, and centreline distance may be adjustable over a small range.Upstream manifolds
Manifolds intended for use with sets where a regulator is provided for each cylinder are connected to the cylinder valves upstream of the cylinder valve seat, to a connecting port provided specifically for this purpose. Two styles of connection are commonly available for this arrangement – face seal, and barrel seal. Face seal connections are similar to the DIN regulator connection seal, and consist of an o-ring in a groove machined into the end of the manifold tube, which is clamped against the face of the valve port by a threaded component. Face seals are simple and rugged, but rely on tight connection for a reliable seal, and do not allow any adjustment for cylinder centre distance. Barrel seals use one or two O-rings in grooves around the end of the manifold tube, which seal against the bore of the valve port. They are usually screwed into the valve port with handed thread, and locked in the desired position with a lock-nut. They are generally slightly less rugged than face seal manifolds, and more vulnerable to thread damage during assembly, as they use a finer thread pitch, but allow a small amount of cylinder centre distance adjustment, and provide a reliable seal even if not completely tight. Manifolds of this type are commonly supplied in sets comprising a manifold and compatible left and right side cylinder valves with a choice of neck thread specification. The working components for all three valves in the set are usually identical. The hexagon of the left hand threadDownstream manifolds
Earlier manifolds were used to connect cylinders together downstream of the cylinder valves, using the DIN or yoke fittings on standard cylinder valves. These manifolds do not generally include an isolation valve, as the cylinder valves can be used to isolate the cylinders. However, they also do not provide for more than one regulator. Some of these earlier manifolds include a reserve valve at the connection point for the regulator, others include a reserve valve at one of the cylinder valves, or have no reserve valve.Direct manifolds
A third style of manifold, mostly of historical interest, screws directly into the cylinder neck thread of both cylinders, and provides a single valve which controls flow from both cylinders to a single connector for a regulator. These manifolds can also include a reserve valve. From a gas management point of view they are identical to a single cylinder with the same capacity.Advantages
Compared to a single cylinder of equivalent capacity: *Ergonomic – Provides a more comfortable fit of cylinders with a lower profile and centre of mass closer to the diver's centreline, for better balance in the water. Compared to independent twins: *Operational simplicity – the ability to breathe through an entire dive from a single regulator without the need to change second stages, except in an emergency or to change gases for decompression. *Only one submersible pressure gauge is necessary if the isolation valve is normally open. Isolation manifold compared to plain manifold: *Standard malfunction management – in case of a regulator or manifold malfunction a standard procedure can be used to limit the gas loss. The diver can localize the malfunction and isolate it from the functioning system by closing the necessary valves.Disadvantages
Compared to independent twins: *A manifold is aManagement of the manifold in gas supply emergencies
Regulator malfunction
If a regulator malfunctions on a set with an upstream manifold, the diver closes the relevant cylinder valve and switches to the other regulator. The entire remaining gas supply is available for the rest of the dive.Cylinder connection leak
Cylinder to manifold connection malfunction, though rare, can result in an extremely violent gas loss. On a set with an isolation manifold, the diver closes the isolating valve to preserve the gas in the cylinder which is not leaking, then uses the leaking cylinder while gas remains, and switches to the intact side cylinder when the leaky one is empty. At least half of the remaining gas volume is available for the remainder of the dive. If there is no isolation valve the entire gas supply may be lost.See also
* * * * *References
{{Underwater diving, divequ Underwater breathing apparatus Underwater diving safety equipment