such as decompression sickness. 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. 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. 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 further information: hyperbaric treatment schedules 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 schematic plan of a simple saturation system showing the main pressure vessels for human occupationddc - living chamberdtc - transfer chamberptc - personnel transfer chamber (bell)rc - recompression chambersl - 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. 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, hyperbaric escape module 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. 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. hyperbaric transport it is sometimes necessary to transport a diver with severe symptoms of decompression illness to a more suitable facility for treatment, or to evacuate people in a hyperbaric environment which is threatened by a high risk hazard. a hyperbaric stretcher may be useful to transport a single person, a portable chamber is intended for use transporting a casualty with a chamber attendant, and hyperbaric rescue and escape systems are used to transfer groups of people. occasionally a closed bell may be used to transfer a small number (up to about 3) of divers between one hyperbaric facility and another when the necessary infrastructure is available. hyperbaric stretcher main article: hyperbaric stretcher 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. hyperbaric rescue and escape systems 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: 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. 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.: 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. closed bell rescue and escape 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. 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. history early decompression (recompression) chamber in the park at broome, western australia. the chamber is now located in the broome museum. experimental compression chambers have been used since about 1860. in 1904, submarine engineers siebe and gorman, together with physiologist leonard hill, designed a device to allow a diver to enter a closed chamber at depth, then have the chamber – still pressurised – raised and brought aboard a boat. the chamber pressure was then reduced gradually. this preventative measure allowed divers to safely work at greater depths for longer times without developing decompression sickness. in 1906, hill and another english scientist m greenwood subjected themselves to high pressure environments, in a pressure chamber built by siebe and gorman, to investigate the effects. their conclusions were that an adult could safely endure seven atmospheres, provided that decompression was sufficiently gradual. a recompression chamber intended for treatment of divers with decompression sickness was built by ce heinke and company in 1913, for delivery to broome, western australia in 1914, where it was successfully used to treat a diver in 1915. that chamber is now in the broome historical museum. structure and layout medical lock on decompression chamber. used to transfer medical supplies and food into and out of the chamber while it is under pressure. the door is locked by a 45 degree rotation. a safety interlock that prevents any rotation of the door while the lock is pressurised can be seen in the disengaged position, showing that it is safe to open the outer door. the gauge also shows that the pressure has been released. the construction and layout of a hyperbaric diving chamber depends on its intended use, but there are several features common to most chambers. there will be a pressure vessel with a chamber pressurisation and depressurisation system, access arrangements, monitoring and control systems, viewports, and often a built in breathing system for supply of alternative breathing gases. 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 stores 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. 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. 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. 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. 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. 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