Scuba manifold

Scuba manifold
	Two 300 bar scuba cylinders connected by a downstream isolation manifold
Uses	Connection between scuba cylinders to link gas supply

Last updated February 01, 2024

A scuba manifold is a device incorporating one or more valves and one or more gas outlets with scuba regulator connections, used to connect two or more 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. Almost all manifold assemblies include one cylinder valve for each cylinder, and the overwhelming majority are for two cylinders.

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'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.^[1] 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.^[1]^[2]^[3]

The manifold functionally combines usually two,^[1] but occasionally three or more cylinders in a way that allows the combined contents to be delivered to the diver through usually one or two regulators. Any arrangement that will perform this function is theoretically possible, but there are only a few arrangements that are commonly seen in practice, and these are a rigid assembly comprising a combination of cylinder valves, manifold connector tubes, isolation valves and reserve valves, with a connection to each cylinder at the neck thread and an outlet connector for each regulator.^[1] A fairly rigid support system to carry the cylinders is also needed, but is not normally part of the manifold system. In practice, scuba manifold systems connect the cylinders at storage pressure, the pressure can be balanced between cylinders, and the cylinders can be simultaneously filled through the manifold from one filling connection.^[1] It is usually possible to isolate cylinders from the manifold or from the outlet connectors, and the gas mixture is, as a general rule, the same in all of the cylinders.^[1] Manifolds combining more than three cylinders are occasionally used for open circuit scuba depth record attempts.

The function of the most commonly used scuba manifolds is to connect the gas supplies of two back mounted cylinders (called doubles or twins), allowing the diver to breathe simultaneously from both.^[1]

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.^[1]

History

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.^[2]

Components

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.^[1]^[3]
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.^[1]^[3]
Isolation valves are mounted in manifold connector tubes which may be closed to shut off flow through that tube.^[1]
Outlet valves control gas flow to the regulators.^[1]^[3]
Reserve valves (mostly obsolescent) may be used to retain part of the pressure for contingencies.^[3]

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.^[3]

Construction

The manifold structural components are usually machined from a high grade brass alloy,^[4] 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.^[2] 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 thread lock nut generally has a groove machined into it to alert the technician to the presence of left hand thread.^[5]

Upstream manifolds may be plain or isolation manifolds.

Plain manifolds simply connect the interiors of the two cylinders together, allowing gas flow between them at all times. If one leaks, the gas from the other will also be lost.

Isolation manifolds connect the gas spaces of the two cylinders when the isolation valve is open, and isolate them when it is closed. If one cylinder leaks, the gas from the other may be protected by closing the isolation valve.

Downstream 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.^[3]

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.^[3]

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.^[6]
Only one submersible pressure gauge is necessary if the isolation valve is normally open.^[6]

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.^[1]^[6]

Disadvantages

Compared to independent twins:

A manifold is a single point of failure for the gas supply, especially dangerous in overhead environments such as caves or wrecks. An isolating manifold localises the single point of failure to the isolating valve itself.
An upstream manifolded set must be emptied to split if needed as singles.

Compared to a single large cylinder:

The manifold, valves and second cylinder are an additional cost, both for capital outlay and maintenance.
The twin set is usually heavier than the equivalent single.

Compared to side-mount:

The manifold and valves are vulnerable to damage by impact with the overhead, and to snagging.
The valves are difficult to reach for many divers, reducing the effectiveness of isolation procedures.

Management 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.^[1]^[6]

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.^[1]^[6]

Related Research Articles

A scuba set, originally just scuba, is any breathing apparatus that is entirely carried by an underwater diver and provides the diver with breathing gas at the ambient pressure. Scuba is an anacronym for self-contained underwater breathing apparatus. Although strictly speaking the scuba set is only the diving equipment that is required for providing breathing gas to the diver, general usage includes the harness or rigging by which it is carried and those accessories which are integral parts of the harness and breathing apparatus assembly, such as a jacket or wing style buoyancy compensator and instruments mounted in a combined housing with the pressure gauge. In the looser sense, scuba set has been used to refer to all the diving equipment used by the scuba diver, though this would more commonly and accurately be termed scuba equipment or scuba gear. Scuba is overwhelmingly the most common underwater breathing system used by recreational divers and is also used in professional diving when it provides advantages, usually of mobility and range, over surface-supplied diving systems and is allowed by the relevant legislation and code of practice.

Aqua-Lung was the first open-circuit, self-contained underwater breathing apparatus to achieve worldwide popularity and commercial success. This class of equipment is now commonly referred to as a twin-hose diving regulator, or demand valve. The Aqua-Lung was invented in France during the winter of 1942–1943 by two Frenchmen: engineer Émile Gagnan and Jacques Cousteau, who was a Naval Lieutenant. It allowed Cousteau and Gagnan to film and explore underwater more easily.

Ice diving is a type of penetration diving where the dive takes place under ice. Because diving under ice places the diver in an overhead environment typically with only a single entry/exit point, it requires special procedures and equipment. Ice diving is done for purposes of recreation, scientific research, public safety and other professional or commercial reasons.

A diving cylinder or diving gas cylinder is a gas cylinder used to store and transport high pressure gas used in diving operations. This may be breathing gas used with a scuba set, in which case the cylinder may also be referred to as a scuba cylinder, scuba tank or diving tank. When used for an emergency gas supply for surface supplied diving or scuba, it may be referred to as a bailout cylinder or bailout bottle. It may also be used for surface-supplied diving or as decompression gas. A diving cylinder may also be used to supply inflation gas for a dry suit or buoyancy compensator. Cylinders provide gas to the diver through the demand valve of a diving regulator or the breathing loop of a diving rebreather.

A diving regulator is a pressure regulator that controls the pressure of breathing gas for diving. The most commonly recognised application is to reduce pressurized breathing gas to ambient pressure and deliver it to the diver, but there are also other types of gas pressure regulator used for diving applications. The gas may be air or one of a variety of specially blended breathing gases. The gas may be supplied from a scuba cylinder carried by the diver, in which case it is called a scuba regulator, or via a hose from a compressor or high-pressure storage cylinders at the surface in surface-supplied diving. A gas pressure regulator has one or more valves in series which reduce pressure from the source, and use the downstream pressure as feedback to control the delivered pressure, or the upstream pressure as feedback to prevent excessive flow rates, lowering the pressure at each stage.

<span class="mw-page-title-main">Surface-supplied diving</span> Underwater diving breathing gas supplied from the surface

Surface-supplied diving is a mode of underwater diving using equipment supplied with breathing gas through a diver's umbilical from the surface, either from the shore or from a diving support vessel, sometimes indirectly via a diving bell. This is different from scuba diving, where the diver's breathing equipment is completely self-contained and there is no essential link to the surface. The primary advantages of conventional surface supplied diving are lower risk of drowning and considerably larger breathing gas supply than scuba, allowing longer working periods and safer decompression. Disadvantages are the absolute limitation on diver mobility imposed by the length of the umbilical, encumbrance by the umbilical, and high logistical and equipment costs compared with scuba. The disadvantages restrict use of this mode of diving to applications where the diver operates within a small area, which is common in commercial diving work.

A full-face diving mask is a type of diving mask that seals the whole of the diver's face from the water and contains a mouthpiece, demand valve or constant flow gas supply that provides the diver with breathing gas. The full face mask has several functions: it lets the diver see clearly underwater, it provides the diver's face with some protection from cold and polluted water and from stings, such as from jellyfish or coral. It increases breathing security and provides a space for equipment that lets the diver communicate with the surface support team.

A pressure regulator is a valve that controls the pressure of a fluid to a desired value, using negative feedback from the controlled pressure. Regulators are used for gases and liquids, and can be an integral device with a pressure setting, a restrictor and a sensor all in the one body, or consist of a separate pressure sensor, controller and flow valve.

In underwater diving, an alternative air source, or more generally alternative breathing gas source, is a secondary supply of air or other breathing gas for use by the diver in an emergency. Examples include an auxiliary demand valve, a pony bottle and bailout bottle.

Vintage scuba is scuba equipment dating from 1975 and earlier, and the practice of diving using such equipment.

Freeflow in underwater diving apparatus is a continuous flow of gas from a storage or supply unit. In scuba diving it is usually undesirable and considered a malfunction, while in surface supplied diving it may be a malfunction or a user selected option in demand systems, or the standard mode of operation in freeflow systems.

Scuba gas management is the aspect of scuba diving which includes the gas planning, blending, filling, analysing, marking, storage, and transportation of gas cylinders for a dive, the monitoring and switching of breathing gases during a dive, efficient and correct use of the gas, and the provision of emergency gas to another member of the dive team. The primary aim is to ensure that everyone has enough to breathe of a gas suitable for the current depth at all times, and is aware of the gas mixture in use and its effect on decompression obligations, nitrogen narcosis, and oxygen toxicity risk. Some of these functions may be delegated to others, such as the filling of cylinders, or transportation to the dive site, but others are the direct responsibility of the diver using the gas.

Scuba skills are skills required to dive safely using SCUBA, an acronym for self-contained underwater breathing apparatus, known as a scuba set. Most of these skills are relevant to both open-circuit scuba and rebreather scuba, and also to surface-supplied diving. Certain scuba skills, which are critical to divers' safety, may require more practice than standard recreational training provides.

Surface-supplied diving equipment (SSDE) is the equipment required for surface-supplied diving. The essential aspect of surface-supplied diving is that breathing gas is supplied from the surface, either from a specialised diving compressor, high-pressure gas storage cylinders, or both. In commercial and military surface-supplied diving, a backup source of surface-supplied breathing gas should always be present in case the primary supply fails. The diver may also wear a bailout cylinder which can provide self-contained breathing gas in an emergency. Thus, the surface-supplied diver is less likely to have an "out-of-air" emergency than a scuba diver using a single gas supply, as there are normally two alternative breathing gas sources available. Surface-supplied diving equipment usually includes communication capability with the surface, which improves the safety and efficiency of the working diver.

Diving procedures are standardised methods of doing things that are commonly useful while diving that are known to work effectively and acceptably safely. Due to the inherent risks of the environment and the necessity to operate the equipment correctly, both under normal conditions and during incidents where failure to respond appropriately and quickly can have fatal consequences, a set of standard procedures are used in preparation of the equipment, preparation to dive, during the dive if all goes according to plan, after the dive, and in the event of a reasonably foreseeable contingency. Standard procedures are not necessarily the only courses of action that produce a satisfactory outcome, but they are generally those procedures that experiment and experience show to work well and reliably in response to given circumstances. All formal diver training is based on the learning of standard skills and procedures, and in many cases the over-learning of the skills until the procedures can be performed without hesitation even when distracting circumstances exist. Where reasonably practicable, checklists may be used to ensure that preparatory and maintenance procedures are carried out in the correct sequence and that no steps are inadvertently omitted.

A scuba cylinder valve or pillar valve is a high pressure manually operated screw-down shut off valve fitted to the neck of a scuba cylinder to control breathing gas flow to and from the pressure vessel and to provide a connection with the scuba regulator or filling whip. Cylinder valves are usually machined from brass and finished with a protective and decorative layer of chrome plating. A metal or plastic dip tube or valve snorkel screwed into the bottom of the valve extends into the cylinder to reduce the risk of liquid or particulate contaminants in the cylinder getting into the gas passages when the cylinder is inverted, and blocking or jamming the regulator.

The mechanism of diving regulators is the arrangement of components and function of gas pressure regulators used in the systems which supply breathing gases for underwater diving. Both free-flow and demand regulators use mechanical feedback of the downstream pressure to control the opening of a valve which controls gas flow from the upstream, high-pressure side, to the downstream, low-pressure side of each stage. Flow capacity must be sufficient to allow the downstream pressure to be maintained at maximum demand, and sensitivity must be appropriate to deliver maximum required flow rate with a small variation in downstream pressure, and for a large variation in supply pressure, without instability of flow. Open circuit scuba regulators must also deliver against a variable ambient pressure. They must be robust and reliable, as they are life-support equipment which must function in the relatively hostile seawater environment, and the human interface must be comfortable over periods of several hours.

References

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Beresford, Michael (2006). CMAS-ISA Advanced Nitrox Diver Manual (3rd ed.). CMAS-ISA.
1 2 3 Mount, Tom (August 2008). "9: Equipment Configuration". In Mount, Tom; Dituri, Joseph (eds.). Exploration and Mixed Gas Diving Encyclopedia (1st ed.). Miami Shores, Florida: International Association of Nitrox Divers. p. 95. ISBN 978-0-915539-10-9.
1 2 3 4 5 6 7 8 Roberts, Fred M. (1963). Basic Scuba: Self contained underwater breathing apparatus: Its operation, maintenance and use (2nd ed.). New York: Van Nostrand Reinholdt.
↑ "OMS SCUBA Valves & Manifolds". OMS. 2013. Retrieved 6 May 2013.
↑ "DGX Premium Modular Valve, Right (Typical Side)". www.divegearexpress.com. Dive Gear Express. Retrieved 27 January 2021.
1 2 3 4 5 Jablonski, Jarrod (2006). Doing It Right: The Fundamentals of Better Diving. High Springs, Florida: Global Underwater Explorers. ISBN 0-9713267-0-3.

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[Adv._Nitrox-1] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Beresford, Michael (2006). CMAS-ISA Advanced Nitrox Diver Manual (3rd ed.). CMAS-ISA.

[Mount_2008b-2] 1 2 3 Mount, Tom (August 2008). "9: Equipment Configuration". In Mount, Tom; Dituri, Joseph (eds.). Exploration and Mixed Gas Diving Encyclopedia (1st ed.). Miami Shores, Florida: International Association of Nitrox Divers. p. 95. ISBN 978-0-915539-10-9.

[Roberts_1963-3] 1 2 3 4 5 6 7 8 Roberts, Fred M. (1963). Basic Scuba: Self contained underwater breathing apparatus: Its operation, maintenance and use (2nd ed.). New York: Van Nostrand Reinholdt.

[oms-4] "OMS SCUBA Valves & Manifolds". OMS. 2013. Retrieved 6 May 2013.

[DGE_Modular-5] "DGX Premium Modular Valve, Right (Typical Side)". www.divegearexpress.com. Dive Gear Express. Retrieved 27 January 2021.

[Jablonski_2006-6] 1 2 3 4 5 Jablonski, Jarrod (2006). Doing It Right: The Fundamentals of Better Diving. High Springs, Florida: Global Underwater Explorers. ISBN 0-9713267-0-3.

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