Diving chamber

The historically older open diving chamber, known as an open diving bell or wet bell, is in effect a compartment with an open bottom that contains a gas space above a free water surface, which allows divers to breathe underwater. The compartment may be large enough to fully accommodate the divers above the water, or may be smaller, and just accommodate head and shoulders. Internal air pressure is at the pressure of the free water surface, and varies accordingly with depth. The breathing gas supply for the open bell may be self-contained, or more usually, supplied from the surface via flexible hose, which may be combined with other hoses and cables as a bell umbilical. An open bell may also contain a breathing gas distribution panel with divers' umbilicals to supply divers with breathing gas during excursions from the bell, and an on-board emergency gas supply in high-pressure storage cylinders. This type of diving chamber can only be used underwater, as the internal gas pressure is directly proportional to the depth underwater, and raising or lowering the chamber is the only way to adjust the pressure.

A sealable diving chamber, closed bell or dry bell is a pressure vessel with hatches large enough for people to enter and exit, and a compressed breathing gas supply to raise the internal air pressure. Such chambers provide a supply of oxygen for the user, and are usually called hyperbaric chambers whether used underwater, at the water surface or on land to produce underwater pressures. However, some use the term submersible chamber to refer to those used underwater and hyperbaric chamber for those used out of water. There are two related terms that reflect particular usages rather than technically different types:

• Decompression chamber, a hyperbaric chamber used by surface-supplied divers to make their surface decompression stops
• Recompression chamber, a hyperbaric chamber used to treat or prevent decompression sickness.

When used underwater there are two ways to prevent water flooding in when the submersible hyperbaric chamber's hatch is opened. The hatch could open into a moon pool chamber, and then its internal pressure must first be equalised to that of the moon pool chamber. More commonly the hatch opens into an underwater airlock, in which case the main chamber's pressure can stay constant, while it is the airlock pressure that shifts. This common design is called a lock-out chamber, and is used in submarines, submersibles, and underwater habitats as well as diving chambers.

Another arrangement utilises a dry airlock between a sealable hyperbaric compartment and an open diving bell compartment (so that effectively the whole structure is a mixture of the two types of diving chamber).

When used underwater all types of diving chamber are attached to a diving support vessel by a strong cable for raising and lowering and an umbilical cable delivering, at a minimum, compressed breathing gas, power, and communications, and all need weights attached or built in to overcome their buoyancy. The greatest depth reached using a cable-suspended chamber is about 1,500 m (almost 1 mile); beyond this the cable becomes unmanageable.

In addition to the diving bell and hyperbaric chamber, related diving equipment includes the following:

• Underwater habitat: consists of compartments operating under the same principles as diving bells and diving chambers, but placed in a fixed position on the sea floor for long-term use.
• Submersibles and submarines differ in being able to move under their own power. The interiors are usually maintained at surface pressure, but some examples include air locks and internal hyperbaric chambers.
• There is also other deep diving equipment that has atmospheric internal pressure, including:
  • Bathysphere: name given to an experimental deep-sea diving chamber of the 1920s and 1930s.
  • Benthoscope: a successor to the bathysphere built to go to greater depths.
  • Bathyscaphe: a self-propelled submersible vessel able to adjust its own buoyancy for exploring extreme depths.
  • Atmospheric diving suit: a roughly anthropomorphic submersible for a single occupant, which maintains an internal pressure close to normal sea level atmospheric pressure

Diving bells and open diving chambers of the same principle were more common in the past owing to their simplicity, since they do not necessarily need to monitor, control and mechanically adjust the internal pressure. Secondly since internal air pressure and external water pressure on the bell wall are almost balanced, the chamber does not have to be as strong as a pressurised diving chamber (dry bell). The air inside an open bell is at the same pressure as the water at the air-water interface surface. This pressure is constant and the pressure difference on the bell shell can be higher than the external pressure to the extent of the height of the air space in the bell.

A wet diving bell or open diving chamber must be raised slowly to the surface with decompression stops appropriate to the dive profile so that the occupants can avoid decompression sickness. This may take hours, and so limits its use.

Submersible hyperbaric chambers known as closed bells or personnel transfer capsules can be brought to the surface without delay by maintaining the internal pressure and either decompressing the divers in the chamber on board the support vessel, or transferring them under pressure to a more spacious decompression chamber or to a saturation system, where they remain under pressure throughout the tour of duty, working shifts under approximately constant pressure, and are only decompressed once at the end. The ability to return to the surface without in-water decompression reduces the risk to the divers if the weather or compromised dynamic positioning forces the support vessel off station.

A diving chamber based on a pressure vessel is more expensive to construct since it has to withstand high pressure differentials. These may be bursting pressures as is the case for a dry bell used for saturation diving, where the internal pressure is matched to the water pressure at the working depth, or crushing pressures when the chamber is lowered into the sea and the internal pressure is less than ambient water pressure, such as may be used for submarine rescue.

Rescue bells are specialized diving chambers or submersibles able to retrieve divers or occupants of diving chambers or underwater habitats in an emergency and to keep them under the required pressure. They have airlocks for underwater entry or to form a watertight seal with hatches on the target structure to effect a dry transfer of personnel. Rescuing occupants of submarines or submersibles with internal air pressure of one atmosphere requires being able to withstand the huge pressure differential to effect a dry transfer, and has the advantage of not requiring decompression measures on returning to the surface, allowing a more rapid turnaround to continue the rescue effort.

A decompression chamber, or deck decompression chamber, is a pressure vessel for human occupancy used in surface supplied diving to allow the divers to complete their decompression stops at the end of a dive on the surface rather than underwater. This eliminates many of the risks of long decompressions underwater, in cold or dangerous conditions. A decompression chamber may be used with a closed bell for decompression after bounce dives, following a transfer under pressure, or the divers may surface before completing decompression and be recompressed in the chamber following stringent protocols to minimise the risk of developing symptoms of decompression sickness in the short period allowed before returning to pressure.

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  Two United States Navy sailors inside a decompression chamber about to undergo training
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  One person chamber
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  Control panel of a basic deck decompression chamber
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  The medical lock of a basic deck decompression chamber with the door closed
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  Exterior view of a basic deck decompression chamber
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  Transportable decompression chamber
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  Divers breathing oxygen during surface decompression
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  Operating a chamber from the control panel

A hyperbaric treatment chamber is a hyperbaric chamber intended for, or put into service for, medical treatment at pressures above the local atmospheric pressure.

Monoplace chamber for clinical hyperbaric oxygen treatment

Internal view of a multiplace chamber for hyperbaric oxygen therapy, showing the airtight door leading to the entry lock.

Pressure release valve and pressure gauge inside a flexible low pressure hyperbaric oxygen therapy chamber

Inside of a flexible low pressure hyperbaric oxygen therapy chamber

A hyperbaric oxygen therapy chamber is used to treat patients, including divers, whose condition might improve through hyperbaric oxygen treatment. Some illnesses and injuries occur, and may linger, at the cellular or tissue level. In cases such as circulatory problems, non-healing wounds, and strokes, adequate oxygen cannot reach the damaged area and the body's healing process is unable to function properly. Hyperbaric oxygen therapy increases oxygen transport via dissolved oxygen in serum, and is most efficacious where the haemoglobin is compromised (e.g. carbon monoxide poisoning) or where the extra oxygen in solution can diffuse through tissues past embolisms that are blocking the blood supply as in decompression illness.[2] Hyperbaric chambers capable of admitting more than one patient (multiplace) and an inside attendant have advantages for the treatment of decompression sickness (DCS) if the patient requires other treatment for serious complications or injury while in the chamber, but in most cases monoplace chambers can be successfully used for treating decompression sickness.[3] Rigid chambers are capable of greater depth of recompression than soft chambers that are unsuitable for treating DCS.

Recompression chamber

A recompression chamber is a hyperbaric treatment chamber used to treat divers suffering from certain diving disorders such as decompression sickness.[4]

Treatment is ordered by the treating physician (medical diving officer), and generally follows one of the standard hyperbaric treatment schedules such as the US Navy treatment Tables 5 or 6.[5]

When hyperbaric oxygen is used it is generally administered by built-in breathing systems (BIBS), which reduce contamination of the chamber gas by excessive oxygen.[6]

If the diagnosis of decompression illness is considered questionable, the diving officer may order a test of pressure.[7] This typically consists of a recompression to 60 feet (18 m) for up to 20 minutes. If the diver notes significant improvement in symptoms, or the attendant can detect changes in a physical examination, a treatment table is followed.

U.S. Navy Table 6 consists of compression to the depth of 60 feet (18 m) with the patient on oxygen. The diver is later decompressed to 30 feet (9.1 m) on oxygen, then slowly returned to surface pressure. This table typically takes 4 hours 45 minutes. It may be extended further. It is the most common treatment for type 2 decompression illness.

U.S. Navy Table 5 is similar to Table 6 above, but is shorter in duration. It may be used in divers with less severe complaints (type 1 decompression illness).

U.S. Navy Table 9 consists of compression to 45 feet (14 m) with the patient on oxygen, with later decompression to surface pressure. This table may be used by lower-pressure monoplace hyperbaric chambers, or as a follow-up treatment in multiplace chambers.

Schematic plan of a simple saturation system showing the main pressure vessels for human occupation
DDC - Living chamber
DTC - Transfer chamber
PTC - Personnel transfer chamber (bell)
RC - Recompression chamber
SL - Supply lock

Personnel transfer capsule

A small hyperbaric escape module

Interior of a large hyperbaric lifeboat

A hyperbaric environment on the surface comprising a set of linked pressure chambers is used in saturation diving to house divers under pressure for the duration of the project or several days to weeks, as appropriate. The occupants are decompressed to surface pressure only once, at the end of their tour of duty. This is usually done in a decompression chamber, which is part of the saturation system. The risk of decompression sickness is significantly reduced by minimizing the number of decompressions, and by decompressing at a very conservative rate.[8]

The saturation system typically comprises a complex made up of a living chamber, transfer chamber and submersible decompression chamber,[9] which is commonly referred to in commercial diving and military diving as the diving bell,[10] PTC (personnel transfer capsule) or SDC (submersible decompression chamber).[6] The system can be permanently installed on a ship or ocean platform, but is usually capable of being transferred between vessels. The system is managed from a control room, where depth, chamber atmosphere and other system parameters are monitored and controlled. The diving bell is used to transfer divers from the system to the work site. Typically, it is mated to the system utilizing a removable clamp and is separated from the system by a trunking space, through which the divers transfer to and from the bell.

The bell is fed via a large, multi-part umbilical that supplies breathing gas, electricity, communications and hot water. The bell also is fitted with exterior mounted breathing gas cylinders for emergency use. The divers operate from the bell using surface supplied umbilical diving equipment.

A hyperbaric lifeboat, hyperbaric escape module or rescue chamber may be provided for emergency evacuation of saturation divers from a saturation system.[9] This would be used if the platform is at immediate risk due to fire or sinking to get the occupants clear of the immediate danger. A hyperbaric lifeboat is self-contained and self-sufficient for several days at sea.

The process of transferring personnel from one hyperbaric system to another is called transfer under pressure (TUP). This is used to transfer personnel from portable recompression chambers to multi-person chambers for treatment, and between saturation life support systems and personnel transfer capsules (closed bells) for transport to and from the worksite, and for evacuation of saturation divers to a hyperbaric lifeboat.

Heavy duty (10 bar) hyperbaric stretcher with hatch removed, showing locking rings for hatch and for connecting to full size chambers

A hyperbaric stretcher is a lightweight pressure vessel for human occupancy (PVHO) designed to accommodate one person undergoing initial hyperbaric treatment during or while awaiting transport or transfer to a treatment chamber.[11]

Hyperbaric escape module

A saturated diver who needs to be evacuated should preferably be transported without a significant change in ambient pressure. Hyperbaric evacuation requires pressurised transportation equipment, and could be required in a range of situations:[12]

• The support vessel at risk of capsize or sinking.
• Unacceptable fire or explosion hazard.
• Failure of the hyperbaric life support system.
• A medical problem which cannot be dealt with on site.

A hyperbaric lifeboat or rescue chamber may be provided for emergency evacuation of saturation divers from a saturation system.[9] This would be used if the platform is at immediate risk due to fire or sinking, and allows the divers under saturation to get clear of the immediate danger. A hyperbaric lifeboat is self-contained and can be operated by a surface pressure crew while the chamber occupants are under pressure. It must be self-sufficient for several days at sea, in case of a delay in rescue due to sea conditions. It is possible to start decompression after launching if the occupants are medically stable, but seasickness and dehydration may delay the decompression until the module has been recovered.[13]^{: Ch. 2}

The rescue chamber or hyperbaric lifeboat will generally be recovered for completion of decompression due to the limited onboard life support and facilities. The recovery plan will include a standby vessel to perform the recovery.[14]

Bell to bell transfer may be used to rescue divers from a lost or entrapped bell. A "lost" bell is a bell which has been broken free of lifting cables and umbilical; the actual position of the bell is usually still known with considerable accuracy. This will generally occur at or near the bottom, and the divers transfer between bells at ambient pressure.[12] It is also possible in some circumstances to use a bell as a rescue chamber to transport divers from one saturation system to another. This may require temporary modifications to the bell, and is only possible if the mating flanges of the systems are compatible.[12]

The pressure vessel is the main structural component, and includes the shell of the main chamber, and if present, the shells of fore-chamber and medical or supply lock. A forechamber or entry lock may be present to provide personnel access to the main chamber while it is under pressure. A medical or stores lock may be present to provide access to the main chamber for small items while under pressure. The small volume allows quick and economical transfer of small items, as the gas lost has relatively small volume compared to the forechamber.[21]

An access door or hatch is normally hinged inward and held closed by the pressure differential, but it may also be dogged for a better seal at low pressure. There is a door or hatch at the access opening to the forechamber, the main chamber, both ends of a medical or stores lock, and at any trunking to connect multiple chambers. A closed bell has a similar hatch at the bottom for use underwater and may have a side hatch for transfer under pressure to a saturation system, or may use the bottom hatch for this purpose. The external door to the medical lock is unusual in that it opens outward and is not held closed by the internal pressure, so it needs a safety interlock system to make it impossible to open when the lock is pressurised.[21]

Viewports are generally provided to allow the operating personnel to visually monitor the occupants, and can be used for hand signalling as an auxiliary emergency communications method. Interior lighting can be provided by mounting lights outside the viewports.

Furniture is usually provided for the comfort of the occupants. Usually there are seats and/or bed facilities. Saturation systems also have tables and sanitary facilities for the occupants.

The internal pressure system includes a primary and reserve chamber gas supply, and the valves and piping to control it to pressurise and depressurise the main chamber and auxiliary compartments, and a pressure relief valve to prevent pressurisation beyond the design maximum working pressure. Valves are generally duplicated inside and outside and are labelled to avoid confusion. It is usually possible to operate a multiple occupant chamber from inside in an emergency. The monitoring equipment will vary depending on the purpose of the chamber, but will include pressure gauges for supply gas, and an accurately calibrated pressure gauge for the internal pressure of all human occupied compartments.[21]

There will also be a voice communications system between the operator and occupants. This is usually push to talk on the outside, and constantly transmitting from the inside, so that the operator can better monitor the condition of the occupants. There may also be a backup communications system.[21]

Firefighting equipment is necessary as a chamber fire is extremely dangerous to the occupants. Either fire extinguishers specially made for hyperbaric environment with non-toxic contents, or a pressurised internal water spray system can be used. Water buckets are often provided as additional equipment.[21]

Oxygen Monitor for Hyperbaric Chamber, 1969

Life support systems for saturation systems can be fairly complex, as the occupants must remain under pressure continuously for several day to weeks. Oxygen content of the chamber gas is constantly monitored and fresh oxygen added when necessary to maintain the nominal value. Chamber gas may be simply vented and flushed if it is air, but helium mixtures are expensive and over long periods very large volumes would be needed, so the chamber gas of a saturation system is recycled by passing it through a carbon dioxide scrubber and other filters to remove odours and excess moisture. Multiplace chambers that may be used for treatment usually contain a built-in breathing system (BIBS) for supply of breathing gas different from the pressurisation gas, and closed bells contain an analogous system to supply gas to the divers' umbilicals. Chambers with BIBS will generally have an oxygen monitor. BIBS are also used as an emergency breathing gas supply if the chamber gas is contaminated.[21]

Sanitation systems for washing and waste removal are required. Discharge is simple because of the pressure gradient, but must be controlled to avoid undesired chamber pressure loss or fluctuations. Catering is generally provided by preparing the food and drink outside and transferring it into the chamber through the stores lock, which is also used to transfer used utensils, laundry and other supplies.

Non-portable chambers are generally constructed from steel,[21] as it is inexpensive, strong and fire resistant. Portable chambers have been constructed from steel, aluminium alloy,[21] and fibre reinforced composites. In some cases the composite material structure is flexible when depressurised.[22][23]

A large range of working pressures are used, depending on the application of the chamber. Hyperbaric oxygen therapy is usually done at pressures not exceeding 18 msw, or an absolute internal pressure of 2.8 bar. Decompression chambers are usually rated for depths similar to the depths that the divers will encounter during planned operations. Chambers using air as the chamber atmosphere are frequently rated to depths in the range of 50 to 90 msw, and chambers, closed bells and other components of saturation systems must be rated for at least the planned operational depth. The US Navy has Heliox saturation decompression schedules for depths up to 480 msw (1600 fsw).[6] Experimental chambers may be rated for deeper depths. An experimental dive has been done to 701 msw (2300 fsw), so at least one chamber has been rated to at least this depth.[25]