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A diving chamber has two main functions:
- as a simpler form of submersible vessel to take divers underwater and to provide a temporary base and retrieval system in the depths;
- as a land or ship-based hyperbaric chamber to artificially reproduce the hyperbaric conditions under the sea (pressures above normal atmospheric pressure) for diving-related and non-diving medical applications such as hyperbaric medicine).
Basic types of diving chamber 
There are two basic types of submersible diving chamber differentiated by the way in which the pressure in the diving chamber is produced and controlled.
Open diving bell 
The historically older open diving chamber, open diving bell or wet bell is in effect a large diving bell, utilising an open bottom, the equivalent of a moon pool, to equalise internal air pressure and external water pressure automatically without the need, necessarily, to measure and control it. An air compressor or bottled compressed air is required to maintain the volume of the air as it becomes compressed with increasing depth, or to make up for oxygen depleted by the occupants' breathing and for carbon dioxide removed from exhaled air by a carbon dioxide scrubber system. This type of diving chamber can only be used underwater, as the internal air pressure is directly proportional to the depth underwater and raising or lowering the chamber is the only way to adjust the pressure.
Hyperbaric chamber 
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 or at the water surface or on land to produce underwater pressures. However, some use submersible chamber to refer to those used underwater and hyperbaric chamber for those used out of water. There are two related terms which 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 which 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 1500 m; beyond this the cable becomes unmanageable.
Related equipment 
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 fixed to 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 which has atmospheric internal pressure, including:
Underwater use 
As well as transporting divers, a diving chamber carries tools and equipment, breathing gas cylinders to replenish scuba tanks, and communications and emergency equipment. It provides a temporary dry air environment during extended dives for rest, eating meals, carrying out tasks which can't be done underwater, and for emergencies. Diving chambers also act as an underwater base for surface supplied diving operations, with the divers' umbilicals (air supply, etc.) attached to the diving chamber rather than to the diving support vessel.
Diving bells 
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). (Actually if h is the distance between a point on the side of the bell and the air/water interface at the bottom, the air pressure at that point is higher than the water on the other side by a water head pressure equivalent to h, but this is a small and constant amount, and is not a structural problem).
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 
Submersible hyperbaric chambers can be brought to the surface without delay to allow divers to decompress since they can maintain the same pressure at which the divers were working. The divers can stay in the chamber on the support vessel to decompress. This flexibility makes them safer to use and more useful in an accident or emergency, including problems affecting the dive support vessel, such as sudden bad weather. They are used to support saturation diving for which the decompression times are very long.
A diving chamber based on a pressure vessel is more expensive to construct since it has to withstand very high pressure differentials. These may be both crushing pressures when the chamber is lowered into the sea and the internal pressure is kept less than ambient water pressure, or it may be an outwards pressure when it is out of the water and its internal pressure is set the same as water pressure at a certain depth.
Hyperbaric chambers also require more sophisticated systems to set and control internal gas pressure. However modern manufacturing techniques and control systems have reduced the cost and this type of diving chamber is now more common than the older dive bell type.
Hyperbaric lifeboats 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 in the required decompression phase. 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.
Out of water use 
Hyperbaric chambers are also used on land and at the ocean surface
- to take surface supplied divers who have been brought up from underwater through their decompression stops, either as Surface decompression or after transfer from a wet bell. (decompression chambers)
- to train divers to adapt to hyperbaric conditions and decompression routines and test their performance under pressure.
- to treat divers for decompression sickness (recompression chambers)
- to treat people using raised oxygen pressure in hyperbaric oxygen therapy
- In saturation diving life support systems
- in scientific research requiring elevated gas pressures.
Hyperbaric chambers designed only for use out of water do not have to resist inward crushing forces, only outward expansion forces. Those for medical applications typically only operate up to two or three atmospheres, while those for diving applications may have to go to six atmospheres and above.
Decompression chamber 
A decompression chamber is a pressure vessel 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.
Two United States Navy sailors inside a decompression chamber about to undergo training
Hyperbaric treatment chamber 
Hyperbaric oxygen therapy chamber 
A hyperbaric oxygen therapy chamber is used to treat patients, including divers, whose condition might improve through hyperbaric oxygen treatment. Hyperbaric chambers capable of admitting more than one patient (multiplace) and an inside attendant have advantages for the treatment of decompression sickness (DCS). Divers with serious complications or injuries may be attended to in this manner during treatment. Multiplace chambers are capable of greater depth of recompression than soft chambers which are unsuitable for treating DCS.
Recompression chamber 
Treatment is ordered by the treating physician (medical diving officer), and is usually in accordance with the U.S. Navy Diving Tables. Other treatment tables have been developed, including the Catalina Tables, and others, including proprietary tables.
Test of pressure 
If the diagnosis of decompression illness is considered questionable, the diving officer may order a test of pressure. 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.
Representative treatment tables 
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.
Saturation diving life support systems 
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 day 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.
The "Saturation System" typically comprises a complex made up of a living chamber, transfer chamber and submersible decompression chamber, which is commonly referred to in commercial diving and military diving as the diving bell, PTC (Personnel Transfer Capsule) or SDC (Submersible Decompression Chamber). 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 or rescue chamber may be provided for emergency evacuation of saturation divers from a saturation system. 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, and can be operated from the inside by the occupants while under pressure.
Transfer under pressure 
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.
Structure and layout 
The construction and layout of a hyperbaric diving chamber depends on its intended use, but there are several features common to most chambers.
- Pressure hull
- Main chamber
- Access door or hatch
- Viewports, to allow the operating personnel to visually monitor the occupants
- Pressure control and monitoring equipment
- Lighting and communications equipment
- Firefighting equipment
- Furniture for the comfort of the occupants (usually seats and/or bed facilities)
- Pressurisation gas supply
- Built-in Breathing System (BIBS) for supply of breathing gas different from the pressurisation gas
- Forechamber (not always present) to provide personnel access to main chamber while it is under pressure
- Medical/stores lock (not always present) to provide access to the main chamber for small items while under pressure
- Some chambers are provided with arrangements which may be connected to other hyperbaric chambers to allow transfer of the occupants under pressure.
- Non-portable chambers are generally constructed from steel
- Portable chambers have been constructed from steel, aluminium alloy, and fibre reinforced composites. In some cases the composite material structure is flexible when depressurised.
Details will vary depending on the application. A generalised sequence for a stand-alone chamber is described. The operator of a commercial diving decompression chamber is generally called a Chamber operator, and the operator of a saturation system is called a Life support technician (LST).
- Pre-use checks will be conducted on the system to ensure that it is safe to operate.
- The intended occupants will be checked and authorised for compression, and will enter the chamber.
- The pressure door will be closed, communications established with the occupants, and pressurisation started.
- The operator will monitor and control the rate of pressurisation and monitor the condition of the occupants.
- Once pressurised, the operator will monitor the pressure, the run time, the chamber gas and if applicable, the independent breathing gas supply. The chamber gas quality may be controlled by carbon dioxide scrubber systems, filters and air conditioner systems and addition of oxygen as required, or by periodic ventilation by adding fresh compressed air while simultaneously releasing some of the chamber air.
- When decompression is started, the operator will notify the occupants and release chamber gas to the atmosphere or to scavenge pumps if it to be recycled. The rate of pressure reduction is controlled to follow the specified decompression schedule within tolerance.
- Compression and decompression may be interrupted if the occupants experience problems caused by the pressure change, such as ear or sinus squeezes, or symptoms of decompression illness.
- When decompression is completed, chamber pressure is equalised with ambient pressure and the doors may be opened. Occupants may exit, and will usually be checked for absence of ill-effects.
- Chamber will receive post-operation service as required to be ready for next operation or storage as applicable.
See also 
- Glossary of underwater diving terminology
- Hyperbaric medicine
- Byford Dolphin (decompression accident)
- Diving bell
- Moon pool
- Saturation diving
- Surface supplied diving
- Decompression sickness
- Hyperbaric stretcher
- Scott, David (1931). Seventy fathom deep. London: Faber & Faber.
- Scott, David (1932). The Egypt gold. London: Faber & Faber.
- "NOAA Ocean Explorer: Monitor Expedition 2002: decompression chamber". National Oceanic and Atmospheric Administration. 2002. Retrieved 3 July 2010.
- Lettnin, Heinz (1999). International textbook of Mixed Gas Diving. Flagstaff, AZ: Best Publishing. ISBN 0941332500.
- Bevan, J. (1999). "Diving bells through the centuries". South Pacific Underwater Medicine Society Journal 29 (1). ISSN 0813-1988. OCLC 16986801. Retrieved 2008-04-25.
- US Navy Diving Manual, 6th revision. United States: US Naval Sea Systems Command. 2006. Retrieved 2008-04-24.
- "Divers Go to Greater Depths With Aid of Chamber" Popular Mechanics, December 1930 first use of diving chamber by British Royal Navy divers—detail drawings on subject
- Decompression Chamber in detail
- The short film Hyperbaric Chamber (1979) is available for free download at the Internet Archive [more]
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