Surface-supplied diving equipment

Surface-supplied diving is diving using equipment supplied with breathing gas using 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 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.

The copper helmeted free-flow standard diving dress is the version which made commercial diving a viable occupation, and although still used in some regions, this heavy equipment has been superseded by lighter free-flow helmets, and to a large extent, lightweight demand helmets, band masks and full-face diving masks. Breathing gases used include air, heliox, nitrox and trimix.

Saturation diving is a mode of surface supplied diving in which the divers live under pressure in a saturation system or underwater habitat and are decompressed only at the end of a tour of duty.

Airline, or hookah diving, and "compressor diving" are lower technology variants also using a breathing air supply from the surface.

Variations

There are several arrangements for supplying breathing gas to divers from the surface:
* Surface oriented diving, with or without a stage or open bell, where the diver is decompressed during the ascent or by surface decompression.
* Bell bounce diving, where the divers are transported through the water in a closed bell and locked into a surface decompression chamber for decompression, or decompressed in the bell.
* Saturation diving, where the diver is transferred under pressure from the pressurised accommodation to working depth and back in a closed bell, only decompressing once at the end of the contract.
* Standard or Heavy gear – The historical copper helmet, canvas suit and weighted boots.
* Scuba replacement – A surface-supplied arrangement where both the primary and reserve air supplies are from high-pressure cylinders. The rest of the system is identical to the standard surface supply configuration, and the full umbilical system, bailout cylinder, communications and surface air panel are used. This is more portable than most compressors and is used by commercial diving contractors as a substitute for scuba with most of the advantages and disadvantages of a regular compressor fed surface supply.
*Air-line diving uses an air line hose in place of a full diver's umbilical to supply breathing air from the surface. If any of the required components of a diver's umbilical are absent this term applies. There are subcatgories of air-line diving equipment:
** Hookah – A basic form of surface-supplied diving in which the air supply is via a single hose is often referred to as airline or Hookah (occasionally Hooka) diving. This often uses a standard scuba second stage as the delivery unit, but is also used with light full-face masks. Bailout gas may be carried, but this is not always the case. Commercial diamond divers working in the shallow zone off the west coast of South Africa under the codes of practice of the Department of Minerals and Energy use half mask and demand valve hookah, and no bailout as standard practice. Their safety record is relatively poor, as a bailout cylinder is seldom carried. It is claimed that when done using a diving compressor with suitable breathing air quality filters and an appropriate emergency gas supply, hookah diving is not more dangerous than scuba diving in the same conditions. A concern is that if the diver is supplied from a compressor in a boat, the intake must be clear of any exhaust fumes.
** Snuba and SASUBA – A system used to supply air from a cylinder mounted on a float to a recreational diver tethered by a short (approximately 6m) hose through a scuba regulator.
**Compressor diving – An even more basic system is the "Compressor diving" arrangement used in the Philippines and Caribbean for fishing. This rudimentary and highly hazardous system uses a large number of small bore plastic tubes connected to a single compressor to supply a large number of divers simultaneously. The delivery end of the hose is unencumbered by any mechanism or mouthpiece, and is simply held by the divers' teeth. Air supply is free flow and often unfiltered, and varies with depth and number of divers drawing off the system.

Alternatives

* Scuba, which is commonly used in recreational diving, is the main alternative to surface-supplied diving equipment. Scuba is available in open circuit and rebreather configurations.
* Atmospheric diving suits such as the JIM suit and the Newtsuit isolate the occupant from the ambient pressure, but are bulky and extremely expensive.
* Manned and unmanned submersibles (ROVs and AUVs) have their applications, but lack the dexterity of a diver at present (2011).
* Freediving, or breathhold diving, is extremely limited in duration and relatively high risk.

Application

Surface-supplied diving equipment and techniques are mainly used in professional diving due to the greater cost and complexity of owning and operating the equipment. This type of equipment is used in saturation diving, as the gas supply is relatively secure, and the diver can not bail out to the surface, and for diving in contaminated water, where the diver must be protected from the environment, and helmets are generally used for environmental isolation.

There has been development of low-cost airline systems for shallow recreational diving, where limited training is offset by physically limiting the depth accessible.

History

The first successful surface-supplied diving dress equipment was produced by the brothers Charles and John Deane in the 1820s. Inspired by a fire accident he witnessed in a stable in England, he designed and patented a "Smoke Helmet" to be used by firemen in smoke-filled areas in 1823. The apparatus comprised a copper helmet with an attached flexible collar and garment. A long leather hose attached to the rear of the helmet was to be used to supply air - the original concept being that it would be pumped using a double bellows. A short pipe allowed breathed air to escape. The garment was constructed from leather or airtight cloth, secured by straps.

The brothers had insufficient funds to build the equipment themselves, so they sold the patent to their employer, Edward Barnard. It was not until 1827 that the first smoke helmets were built, by German-born British engineer Augustus Siebe. In 1828 they decided to find another application for their device and converted it into a diving helmet. They marketed the helmet with a loosely attached "diving suit" so that a diver could perform salvage work but only in a full vertical position, otherwise water entered the suit.

In 1829 the Deane brothers sailed from Whitstable for trials of their new underwater apparatus, establishing the diving industry in the town. In 1834 Charles used his diving helmet and suit in a successful attempt on the wreck of at Spithead, during which he recovered 28 of the ship's cannon. In 1836, John Deane recovered timbers, guns, longbows, and other items from the rediscovered ''Mary Rose'' shipwreck.
By 1836 the Deane brothers had produced the world's first diving manual, ''Method of Using Deane's Patent Diving Apparatus'' which explained in detail the workings of the apparatus and pump, plus safety precautions.

In the 1830s the Deane brothers asked Siebe to apply his skill to improve their underwater helmet design. Expanding on improvements already made by another engineer, George Edwards, Siebe produced his own design; a helmet fitted to a full-length watertight canvas diving suit. The real success of the equipment was a valve in the helmet.

Siebe introduced various modifications on his diving dress design to accommodate the requirements of the salvage team on the wreck of HMS ''Royal George'', including making the helmet be detachable from the corselet; his improved design gave rise to the typical standard diving dress which revolutionised underwater civil engineering, underwater salvage, commercial diving and naval diving.

Equipment

The essential aspect of surface-supplied diving is that breathing gas is supplied from the surface, either from a specialized diving compressor, high-pressure 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 adds to the safety and efficiency of the working diver.

The equipment needed for surface supplied diving can be broadly grouped as diving and support equipment, but the distinction is not always clear. Diving support equipment is the equipment used to facilitate a diving operation. It is either not taken into the water during the dive, such as the gas panel and compressor, or is not integral to the actual diving, being there to make the dive easier or safer, such as a surface decompression chamber. Some equipment, like a diving stage, is not easily categorised as diving or support equipment, and may be considered as either.

Surface-supplied diving equipment is required for a large proportion of the commercial diving operations conducted in many countries, either by direct legislation, or by authorised codes of practice, as in the case of IMCA operations. Surface-supplied equipment is also required under the US Navy operational guidance for diving in harsh contaminated environments which was drawn up by the Navy Experimental Diving Unit.

Breathing apparatus

The definitive equipment for surface-supplied diving is the breathing apparatus which is supplied with primary breathing gas from the surface via a hose, which is usually part of a diver's umbilical connecting the surface supply systems with the diver, sometimes directly, otherwise via a bell umbilical and bell panel.

Lightweight demand helmets

Lightweight demand helmets are rigid structures which fully enclose the head of the diver and supply breathing gas "on demand". The flow of gas from the supply line is activated by inhalation reducing the pressure in the helmet to slightly below ambient, and a diaphragm in the demand valve senses this pressure difference and moves a lever to open the valve to allow breathing gas to flow into the helmet. This flow continues until the pressure inside the helmet again balances the ambient pressure and the lever returns to the shut position. This is exactly the same principle as used for scuba demand valves, and in some cases the same components are used. Sensitivity of the lever can often be adjusted by the diver by turning a knob on the side of the demand valve. Lightweight demand helmets are available in open circuit systems (used when breathing standard air) and closed circuit (reclaim) systems (which may be used in order to reduce costs when breathing mixed gas such as heliox and trimix: the exhaled gas is returned to the surface, scrubbed of carbon dioxide, re-oxygenated, and returned to the diver).

The helmet may be of metal or reinforced plastic composite (GRP), and is either connected to a neck dam or clamped directly to a drysuit. The neck dam is the lower part of the helmet, which seals against the neck of the diver in the same way that the neck seal of a dry suit works. Neck dams may have neoprene or latex seals, depending on diver preference. Attachment to the neck dam is critical to diver safety and a reliable locking mechanism is needed to ensure that it is not inadvertently released during a dive. When using a dry suit, the neck dam may be permanently omitted and the lower part of the helmet assembly attached directly to the suit.

The term "Lightweight" is relative; the helmets are only light in comparison with the old copper hats. They are supported only by the head and neck of the diver, and are uncomfortably heavy (Weight of KM 77 = 32.43 pounds) out of the water, as they must be ballasted for neutral buoyancy during the dive, so they don't tend to lift the diver's head with excess buoyancy. There is little difference in weight between the metal shell and GRP shell helmets because of this ballasting, and the weight is directly proportional to the total volume - smaller helmets are lighter. To avoid fatigue, divers avoid donning the helmet until just prior to entering the water. Having the helmet supported by the head has the advantage that the diver can turn the helmet to face the job without having to turn the entire upper torso. This is particularly an advantage when looking upwards. This allows the helmet to have a relatively small faceplate, which reduces overall volume and hence the weight.

Demand breathing systems reduce the amount of gas required to adequately ventilate the diver, as it needs only to be supplied when the diver inhales, but the slightly increased work of breathing caused by this system is a disadvantage at extreme levels of exertion, where free-flow systems may be better. The demand system is also quieter than free-flow, particularly during the non-inhalation phase of breathing. This can make voice communication more effective. The breathing of the diver is also audible to the surface team over the communications system, and this helps to monitor the condition of the diver and is a valuable safety feature.

=Open circuit demand helmets

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The open circuit demand system exhausts gas to the environment at ambient pressure (or a very small difference from ambient pressure required to open the exhaust valve). As a result, all exhaled gas is lost to the surroundings. For most surface orientated commercial diving where air is the breathing gas in use, this is no problem, as air is cheap and freely available. Even with nitrox it is generally more cost effective to use open circuit, as oxygen is an easily available and relatively inexpensive gas, and blending nitrox is technologically simple, both to mix and to analyse.

=Reclaim helmets

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In the case of compressed air, or Nitrox mixtures, the exhaled gas is not valuable enough to justify the expense of recycling, but helium-based mixtures are considerably more expensive, and as the depth increases, the amount of gas used (in terms of mass, or number of molecules) increases in direct proportion to the ambient pressure. As a result, gas cost is a significant factor in deep open circuit diving with helium-based mixtures for long periods. By using a return line for the exhaled gas, it can be recompressed and used again, almost indefinitely. It is necessary to remove carbon dioxide from the reclaimed gas, but this is relatively cheap and uncomplicated. It is generally removed by a scrubber, which is a filter packed with a chemical which reacts with and removes the carbon dioxide from the gas. The reclaimed gas is also filtered to remove odour and microorganisms, and oxygen is added to the required concentration. The gas is compressed for storage between uses.
There is a technical problem with recovery of the exhaled gas. Simply venting it to a return hose through a non-return valve will not work, as the hose must be maintained at exactly the ambient pressure at the depth of the helmet, otherwise the gas from the helmet will either free-flow out under pressure, or not flow out at all because of back pressure. This obstacle is overcome by using an exhaust valve working on the same principle as the demand valve, which opens the exhaust valve by using the leverage of a diaphragm sensing the pressure difference between the helmet interior pressure and the ambient pressure, This only requires the pressure in the reclaim hose to be lower than ambient at the diver to function. The same principle is used in a diving chamber's built-in breathing system (BIBS).

Free flow helmet

A free flow helmet supplies a continuous flow of air to the diver, and he breathes this as it flows past. Work of breathing is minimal, but flow rate must be high if the diver works hard, and this is noisy, affecting communications and requiring hearing protection to avoid damage to the ears. This type of helmet is popular where divers have to work hard in relatively shallow water for long periods. It is also useful when diving in contaminated environments, where the helmet is sealed onto a dry suit, and the entire system is kept at a slight positive pressure by adjusting the back-pressure of the exhaust valve, to ensure that there is no leakage into the helmet. This type of helmet is often large in volume, and as it is attached to the suit, it does not move with the head. The diver must move his body to face anything he wants to see. For this reason the faceplate is large and there is often an upper window or side windows to improve the field of vision.

=Standard diving helmet (Copper hat)

=

The helmet is usually made of two main parts: the bonnet, which covers the diver's head, and the corselet which supports the weight of the helmet on the diver's shoulders, and is clamped to the suit to create a watertight seal. The bonnet is attached and sealed to the corselet at the neck, either by bolts or an interrupted screw-thread, with some form of locking mechanism.

The helmet may be described by the number of bolts which hold it to the suit or to the corselet, and the number of vision ports, known as lights. For example, a helmet with four vision ports, and twelve studs securing the suit to the corselet, would be known as a "four light, twelve bolt helmet", and a three-bolt helmet used three bolts to secure the bonnet to the corselet, clamping the flange of the neck seal between the two parts of the helmet.

When the telephone was invented, it was applied to the standard diving dress for greatly improved communication with the diver.

The bonnet is usually a copper shell with soldered brass fittings. It covers the diver's head and provides sufficient space to turn the head to look out of the glazed faceplate and other viewports (windows). The front port can usually be opened for ventilation and communication when the diver is on deck, by being screwed out or swung to the side on a hinge. The other lights (another name for the viewports) are generally fixed. Viewports were glass on the early helmets, with some of the later helmets using acrylic, and are usually protected by brass or bronze grilles. The helmet has fittings to connect the air line and the diver's telephone.

All except the earliest helmets include a non-return valve where the airline is connected, which prevents potentially fatal helmet squeeze if the pressure in the hose is lost.
The difference in pressure between the surface and the diver can be so great that if the air line is cut at or near the surface and there is no non-return valve, the diver would be partly squeezed into the helmet by the external pressure, and injured or possibly killed.

Helmets also have a spring-loaded exhaust valve which allows excess air to leave the helmet. The spring force is adjustable by the diver to prevent the suit from deflating completely or over-inflating and the diver being floated uncontrollably to the surface. Some helmets have an extra manual valve known as a spit-cock, which can be used to vent excess air when the diver is in a position where the main exhaust can not function correctly.

The corselet, also known as a breastplate or gorget, is an oval or rectangular collar-piece resting on the shoulders, chest and back, to support the helmet and seal it to the suit, usually made from copper and brass, but occasionally steel. The helmet is usually connected to the suit by placing the holes around the rubberised collar of the suit over bolts along the rim of the corselet, and then clamping the brass straps known as brailes against the collar with wing nuts to press the rubber against the metal of the corselet rim to make a water-tight seal. An alternative method was to bolt the bonnet to the corselet over a rubber collar bonded to the top of the suit.

Most six and twelve bolt bonnets are joined to the corselet by 1/8th turn interrupted thread. The helmet neck thread is placed onto the neck of the corselet facing the divers left front, where the threads do not engage, and then rotated forward, engaging the thread and seating on a leather gasket to make a watertight seal. The helmet usually has a safety lock which prevents the bonnet from rotating back and separating underwater. Other styles of connection are also used, with the joint secured by clamps or bolts (usually three).

Band mask

A band mask is a heavy duty full-face mask with many of the characteristics of a lightweight demand helmet. In structure it is the front section of a lightweight helmet from above the faceplate to below the demand valve and exhaust ports, including the bailout block and communications connections on the sides. This rigid frame is attached to a neoprene hood by a metal clamping band, hence the name. It is provided with a padded sealing surface around the frame edge which is held firmly against the diver's face by a rubber "spider", a multiple strap arrangement with a pad behind the diver's head, and usually five straps which hook onto pins on the band. The straps have several holes so the tension can be adjusted to get a comfortable seal. A band mask is heavier than other full face masks, but lighter than a helmet, and can be donned more quickly than a helmet. They are often used by the standby diver for this reason.

Full-face mask

A full-face mask encloses both mouth and nose, which reduces the risk of the diver losing the air supply compared to a half mask and demand valve. Some models require a bailout block to provide alternative breathing gas supply from the umbilical and bailout cylinder, but are not suitable for accepting an alternative air supply from a rescue diver, while a few models accept a secondary demand valve which can be plugged into an accessory port (Draeger, Apeks and Ocean Reef). The unique Kirby Morgan 48 SuperMask has a removable DV pod which can be unclipped to allow the diver to breathe from a standard scuba demand valve with mouthpiece.

Despite the improvement in diver safety provided by the more secure attachment of the breathing apparatus to the diver's face, some models of full face mask can fail catastrophically if the faceplate is broken or detached from the skirt, as there is then no way to breathe from the mask. This can be mitigated by carrying a standard secondary second stage, and preferably also a spare half mask.

A full face mask is lighter and more comfortable for swimming than a helmet or band mask, and usually provides an improved field of vision, but it is not as secure, and does not provide the same level of protection as the heavier and more sturdily constructed equipment. The two types of equipment have different ranges of application. Most full face masks are adaptable for use with scuba or surface supply. The full face mask does not usually have a bailout block fitted, and this is usually attached to the diver's harness, with a single hose to supply the mask from main or bailout gas which is selected at the block. The strap arrangement for full face masks is usually quite secure, but not as secure as a bandmask or helmet, and it is possible for it to be dislodged in the water. However it is also quite practicable for a trained diver to replace and clear a full face mask under water without assistance, so this is more an inconvenience than a disaster unless the diver is rendered unconscious at the same time.

Breathing gas supply

Diver's umbilical

The umbilical contains a hose to supply the breathing gas and usually several other components. These usually include a communications cable (comms wire), a pneumofathometer, and a strength member, which may be the airline hose, communications cable, or a rope. When needed, a hot water supply line, helium reclaim line, video camera and lighting cables may be included. These components are neatly twisted into a multistrand cable, and are deployed as a single unit. The diver's end has underwater connectors for the electrical cables, and the air hoses are usually connected to the helmet, band mask, or bailout block by JIC fitting