Diving chamber
A diving chamber is a vessel for human occupation, which may have an entrance that can be sealed to hold an internal pressure significantly higher than ambient pressure, a pressurised gas system to control the internal pressure, and a supply of breathing gas for the occupants.
There are two main functions for diving chambers:
- as a simple form of submersible vessel to transport divers underwater and to provide a temporary base and retrieval system in the depths;
- as a land, ship or offshore platform-based hyperbaric chamber or system, to artificially reproduce the hyperbaric conditions under the sea. Internal pressures above normal atmospheric pressure are provided for diving-related applications such as saturation diving and diver decompression, and non-diving medical applications such as hyperbaric medicine. It is also known as a pressure vessel for human occupancy, or PVHO.
Basic types of diving chambers
Open diving bell
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.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 which may be used to raise the internal pressure. Such chambers provide a supply of breathing gas for the user, and are usually called hyperbaric chambers, whether used underwater, at the water surface or on land. The term submersible chamber may be used 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 to decompress surface-supplied divers
- Recompression chamber, a hyperbaric chamber used to treat or prevent decompression sickness.
When used underwater all types of diving chambers are deployed from a diving support vessel suspended by a cable for raising and lowering and an umbilical cable delivering, at a minimum, compressed breathing gas, power, and communications. They may need ballast weights to overcome their buoyancy.
Related equipment
In addition to the diving bell and hyperbaric chamber, related Pressure Vessels for Human Occupancy 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
- * Decompression chamber: a pressure vessel used to treat a diver with controlled atmosphere and pressure to prevent related injuries. Medical hyperbaric chambers can be used to decompress divers as well as treat injuries.
- * Pressurized tunnel boring machines: just as the original term for "the bends" or "decompression sickness" came from using air pressure in mining and construction, pressurized tunnel boring machines use air pressure to force water out of the tunnel, requiring the operators to do "dry diving" where they go through the same pressurization and decompression routines as divers
- * Altitude chambers: PVHOs used to reduce pressure for training and evaluating pilots, astronauts, and related fields for high-altitude conditions
Underwater use
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. 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. 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
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 submarines, 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.
Lockout trunk
A lockout trunk or lockout chamber is a compartment on a submarine or submersible which can have the internal pressure equalised with exterior ambient pressure, allowing occupants to lock out of or into a vessel with internal pressure different to ambient pressure. If the hatch is at the bottom, it can retain gas and be pressurised with gas. it the hatch is on top, gas under the hatch will escape when it is opened, and it is likely to be pressurised by allowing water in through a flooding valve. The escape trunking on a submarine usually uses this hatch on top architecture.Out of water use
Hyperbaric chambers are also used on land and above the water:- to take surface supplied divers who have been brought up from underwater through their remaining decompression as surface decompression either after an ambient pressure ascent or after transfer under pressure from a dry bell.
- to train divers to adapt to hyperbaric conditions and decompression routines and test their performance under pressure.
- to treat divers for decompression sickness
- to treat people using raised oxygen partial pressure in hyperbaric oxygen therapy for conditions unrelated to diving.
- in saturation diving accommodation systems
- in scientific research requiring elevated gas pressures.
Lightweight portable hyperbaric chambers that can be lifted by helicopter are used by military or commercial diving operators and rescue services to carry one or two divers requiring recompression treatment to a suitable facility.