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enwiki-01569712-0016-0000

Decompression equipment, Planning and monitoring decompression, Personal decompression computers The personal decompression computer, or dive computer, is a small computer designed to be worn by a diver during a dive, with a pressure sensor and an electronic timer mounted in a waterproof and pressure resistant housing and which has been programmed to model the inert gas loading of the diver's tissues in real time during a dive. Most are wrist mounted, but a few are mounted in a console with the submersible pressure gauge and possibly other instruments.

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Decompression equipment, Planning and monitoring decompression, Personal decompression computers A display allows the diver to see critical data during the dive, including the maximum and current depth, duration of the dive, and decompression data including the remaining no decompression limit calculated in real time for the diver throughout the dive. Other data such as water temperature and cylinder pressure are also sometimes displayed.

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Decompression equipment, Planning and monitoring decompression, Personal decompression computers The dive computer has the advantages of monitoring the actual dive, as opposed to the planned dive, and does not assume on a "square profile" – it dynamically calculates the real profile of pressure exposure in real time, and keeps track of residual gas loading for each tissue used in the algorithm. Dive computers also provide a measure of safety for divers who accidentally dive a different profile to that originally planned. If the diver exceeds a no-decompression limit, decompression additional to the ascent rate will be necessary. Most dive computers will provide the necessary decompression information for acceptably safe ascent in the event that the no-decompression limits are exceeded.

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Decompression equipment, Planning and monitoring decompression, Personal decompression computers The use of computers to manage recreational dive decompression is becoming the standard and their use is also common in occupational scientific diving. Their value in surface supplied commercial diving is more restricted, but they can usefully serve as a dive profile recorder.

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Decompression equipment, Planning and monitoring decompression, Personal decompression computers, Decompression using a personal decompression computer The personal decompression computer provides a real time modelling of the inert gas load on the diver according to the decompression algorithm programmed into the computer by the manufacturer, with possible personal adjustments for conservatism and altitude set by the user. In all cases the computer monitors the depth and elapsed time of the dive, and many allow user input specifying the gas mixture.

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Decompression equipment, Planning and monitoring decompression, Personal decompression computers, Decompression using a personal decompression computer Most computers require the diver to specify the mixture before the dive, but some allow the choice of mixture to be changed during the dive, which allows for the use of gas switching for accelerated decompression. A third category, mostly used by closed circuit rebreather divers, monitors the partial pressure of oxygen in the breathing mix using a remote oxygen sensor, but requires diver intervention to specify the inert gas constituents and ratio of the mix in use.

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Decompression equipment, Planning and monitoring decompression, Personal decompression computers, Decompression using a personal decompression computer The computer retains the diver's pressure exposure history, and continuously updates the calculated tissue loads on the surface, so the current tissue loading should always be correct according to the algorithm, though it is possible to provide the computer with misleading input conditions, which can nullify its reliability.

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Decompression equipment, Planning and monitoring decompression, Personal decompression computers, Decompression using a personal decompression computer This ability to provide real-time tissue loading data allows the computer to indicate the diver's current decompression obligation, and to update it for any permissible profile change, so the diver with a decompression ceiling does not have to decompress at any specific depth provided the ceiling is not violated, though the decompression rate will be affected by the depth.

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Decompression equipment, Planning and monitoring decompression, Personal decompression computers, Decompression using a personal decompression computer As a result, the diver can make a slower ascent than would be called for by a decompression schedule computed by the identical algorithm, as may suit the circumstances, and will be credited for gas elimination during the slower ascent, and penalised if necessary for additional ingassing for those tissues affected. This provides the diver with an unprecedented flexibility of dive profile while remaining within the safety envelope of the algorithm in use.

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Decompression equipment, Planning and monitoring decompression, Ratio decompression Ratio decompression (usually referred to in abbreviated form as ratio deco) is a technique for calculating decompression schedules for scuba divers engaged in deep diving without using dive tables, decompression software or a dive computer. It is generally taught as part of the "DIR" philosophy of diving promoted by organisations such Global Underwater Explorers (GUE) and Unified Team Diving (UTD) at the advanced technical diving level. It is designed for decompression diving executed deeper than standard recreational diving depth limits using trimix as a "bottom mix" breathing gas.

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Decompression equipment, Planning and monitoring decompression, Ratio decompression It is largely an empirical procedure, and has a reasonable safety record within the scope of its intended application. Advantages are reduced overall decompression time and for some versions, easy estimation of decompression by the use of a simple rule-based procedure which can be done underwater by the diver. It requires the use of specific gas mixtures for given depth ranges. The advantages claimed are flexibility in that if the depth is not known accurately, the schedule can be adjusted during the dive to allow for the actual depth, and that it allows deep dives without the use of an expensive trimix dive computer.

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Decompression equipment, Planning and monitoring decompression, Ratio decompression Limitations include that a consistent set of gases must be used which match the specific ratio model, and the specific ratio will only be relevant to a limited range of depths. As the parameters move away from the base conditions, conservatism will diverge, and the probability of symptomatic bubble formation will become more unpredictable. There is also the requirement for the diver to do mental arithmetic at depth to calculate the parameters of a safety-critical operation. This may be complicated by adverse circumstances or an emergency situation.

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Decompression equipment, Controlling depth and ascent rate A critical aspect of successful decompression is that the depth and ascent rate of the diver must be monitored and sufficiently accurately controlled. Practical in-water decompression requires a reasonable tolerance for variation in depth and rate of ascent, but unless the decompression is being monitored in real time by a decompression computer, any deviations from the nominal profile will affect the risk. Several items of equipment are used to assist in facilitating accurate adherence to the planned profile, by allowing the diver to more easily control depth and ascent rate, or to transfer this control to specialist personnel at the surface.

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Decompression equipment, Controlling depth and ascent rate, Shot lines A shot line is a rope between a float at the surface, and a sufficiently heavy weight holding the rope approximately vertical. The shot line float should be sufficiently buoyant to support the weight of all divers that are likely to be using it at the same time. As divers are seldom weighted to be very negatively buoyant, a positive buoyancy of 50 kg is considered adequate by some authorities for general commercial use. Recreational divers are free to choose lesser buoyancy at their own risk.

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Decompression equipment, Controlling depth and ascent rate, Shot lines The shot weight should be sufficient to prevent a diver from lifting it from the bottom by over-inflation of the buoyancy compensator or dry suit, but not sufficient to sink the float if the slack on the line is all taken up. Various configurations of shot line are used to control the amount of slack.

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Decompression equipment, Controlling depth and ascent rate, Shot lines The diver ascends along the shotline, and may use it purely as a visual reference, or can hold on to it to positively control depth, or can climb up it hand over hand. A Jonline may be used to fasten a diver to an anchor line or shot line during a decompression stop.

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Decompression equipment, Controlling depth and ascent rate, Shot lines, Jonlines A Jonline is a short line used by scuba divers to fasten themselves to something. The original purpose was to fasten a diver to a shot line during decompression stops in current. The line is typically around 1 m (3 feet) long and equipped with a clip at each end. One clip is fastened to the diver's harness, and the other is used to fasten the line to the shot line or anchor line. In current this relieves the diver from holding on to the line during the decompression stop, and the horizontal length of the line will absorb some or all of the vertical movement of the shot line or anchor line due to wave action.

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Decompression equipment, Controlling depth and ascent rate, Shot lines, Jonlines The jonline is named after Jon Hulbert, who is credited with its invention.

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Decompression equipment, Controlling depth and ascent rate, Shot lines, Jonlines A jonline can also be used to tether the diver's equipment to the dive boat before or after the dive. This helps the diver to put on or take off the equipment while in the water without drifting away from the boat. It is similar to a buddy line, which is used to tether two divers together during a dive.

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Decompression equipment, Controlling depth and ascent rate, Shot lines, Decompression trapezes A decompression trapeze is a device used in recreational diving and technical diving to make decompression stops more comfortable and more secure and provide the divers' surface cover with a visual reference for the divers' position.

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Decompression equipment, Controlling depth and ascent rate, Shot lines, Decompression trapezes It consists of a horizontal bar or bars suspended at the depth of intended decompression stops by buoys. The bars are of sufficient weight and the buoys of sufficient buoyancy that the trapeze will not easily change depth in turbulent water or if the divers experience buoyancy control problems.

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Decompression equipment, Controlling depth and ascent rate, Shot lines, Decompression trapezes Trapezes are often used with diving shots. When diving in tidal waters at the end of slack water, the trapeze may be released from the diving shot to drift in the current as the divers make their decompression stops.

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Decompression equipment, Controlling depth and ascent rate, Shot lines, Decompression trapezes Bottom tensioned shotline: The line passes through a ring at the weight and is tensioned by a small float, often a small lift bag which can later help lift the shot as the air expands.

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Decompression equipment, Controlling depth and ascent rate, Shot lines, Decompression trapezes Top tensioned shotline: The line passes through a ring at the float and is tensioned by a smaller weight hanging from it. This weight may be hooked to the main part of the line by a sliding clip to restrain it from swinging.

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Decompression equipment, Controlling depth and ascent rate, Shot lines, Decompression trapezes A shotline with a lazy shot – a second float with a short weighted line tethered to it at just below the depth of the deepest long decompression stop.

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Decompression equipment, Controlling depth and ascent rate, Shot lines, Decompression trapezes A shotline with a decompression trapeze – a series of crossbars suspended from a float at each end and ballasted as necessary, tethered to the main shotline.

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Decompression equipment, Controlling depth and ascent rate, Downline A downline is a rope leading from the surface down to the underwater workplace. It allows a commercial diver to travel directly to and from the job site and to control rate of descent and ascent in the same way as using a shotline. Also sometimes called a jackstay.

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Decompression equipment, Controlling depth and ascent rate, Downline A downline used for open ocean diving is much the same as a shotline, but does not reach all the way to the bottom. An open-ocean downline is weighted at the bottom, and attached to a substantial float at the surface, which may be tethered to the boat. It may be marked at intervals by knots or loops, and may be attached to decompression trapeze system. In some cases a sea anchor may be used to limit wind drift, particularly if attached to a boat with significant windage.

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Decompression equipment, Controlling depth and ascent rate, Upline Also known as a Jersey upline, an upline is a line deployed by the diver, and fixed to the bottom, usually on a wreck, to serve as a position and depth control during offshore ascents in moderate currents, where the diver wants to prevent excessive drift during decompression. The bio-degradable natural fibre line is carried on a spool and deployed connected to an inflatable decompression buoy or lift bag at the end of the dive, and the bottom end tied off to the wreck. After completing decompression and surfacing, the diver cuts the line free at the buoy, and the line sinks and naturally decomposes over a few months.

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Decompression equipment, Controlling depth and ascent rate, Surface marker buoy and delayed surface marker buoy A surface marker buoy (SMB) with a reel and line is often used by a dive leader to allow the boat to monitor progress of the dive group. This can provide the operator with a positive control of depth, by remaining slightly negative and using the buoyancy of the float to support this slight over-weighting. This allows the line to be kept under slight tension which reduces the risk of entanglement. The reel or spool used to store and roll up the line usually has slightly negative buoyancy, so that if released it will hang down and not float away.

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Decompression equipment, Controlling depth and ascent rate, Surface marker buoy and delayed surface marker buoy A delayed or deployable surface marker buoy (DSMB) is a soft inflatable tube that is attached to a reel or spool line at one end, and is inflated by the diver under water and released to float to the surface, deploying the line as it ascends. This provides information to the surface that the diver is about to ascend, and where he is. This equipment is commonly used by recreational and technical divers, and requires a certain level of skill to operate safely. Once deployed, it can be used for the same purposes as the standard surface marker and reel, and in the same way, but they are mostly used to signal the boat that the diver has started ascent or to indicate a problem in technical diving.

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Decompression equipment, Controlling depth and ascent rate, Decompression station A decompression station is a place set up to facilitate the planned decompression for a dive team.

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Decompression equipment, Controlling depth and ascent rate, Diving stages and wet bells A diving stage, sometimes known as a diving basket, is a platform on which one or two divers stand which is hoisted into the water, lowered to the workplace or the bottom, and then hoisted up again to return the diver to the surface and lift him out of the water. This equipment is almost exclusively used by surface supplied professional divers, as it requires fairly complex lifting equipment.

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Decompression equipment, Controlling depth and ascent rate, Diving stages and wet bells A diving stage allows the surface team to conveniently manage a diver's decompression as it can be hoisted at a controlled rate and stopped at the correct depth for decompression stops, and allows the divers to rest during the ascent. It also allows the divers to be relatively safely and conveniently lifted out of the water and returned to the deck or quayside.

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Decompression equipment, Controlling depth and ascent rate, Diving stages and wet bells A wet bell, or open bell, is similar to a diving stage in concept, but has an air space, open to the water at the bottom in which the divers, or at least their heads, can shelter during ascent and descent. A wet bell provides more comfort and control than a stage and allows for longer time in the water. Wet bells are used for air and mixed gas, and divers can decompress using oxygen from a mask at 12 m.

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Decompression equipment, Controlling depth and ascent rate, Diving stages and wet bells The launch and recovery system (LARS) is the equipment used to deploy and recover a stage or diving bell.

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Decompression equipment, Providing gases to accelerate decompression Reducing the partial pressure of the inert gas component of the breathing mixture will accelerate decompression as the concentration gradient will be greater for a given depth. This is achieved by increasing the fraction of oxygen in the breathing gas used, whereas substitution of a different inert gas will not produce the desired effect. Any substitution may introduce counter-diffusion complications, owing to differing rates of diffusion of the inert gases, which can lead to a net gain in total dissolved gas tension in a tissue. This can lead to bubble formation and growth, with decompression sickness as a consequence. Partial pressure of oxygen is usually limited to 1.6 bar during in water decompression for scuba divers, but can be up to 1.9 bar in-water and 2.2 bar in the chamber when using the US Navy tables for surface decompression, and up to 2.8 bar for therapeutic decompression.

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Decompression equipment, Providing gases to accelerate decompression, Stage cylinders Open circuit scuba divers by definition are independent of surface supply, and must take any gas mixture with them that is to be used on the dive. However, if they are confident of returning by a specific route, the decompression gas may be stored at appropriate places on that route. The cylinders used for this purpose are called stage cylinders, and they are usually provided with a standard regulator and a submersible pressure gauge, and are usually left at the stop with the regulator pressurised, but the cylinder valve turned off to minimise the risk of gas loss. Similar cylinders are carried by the divers when the route back is not secure. They are commonly mounted as sling cylinders, clipped to D-rings at the sides of the diver's harness.

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Decompression equipment, Providing gases to accelerate decompression, Stage cylinders Scuba divers take great care to avoid breathing oxygen enriched "deco gas" at great depths because of the high risk of oxygen toxicity. To prevent this happening, cylinders containing oxygen-rich gases must always be positively identifiable. One way of doing this is by marking them with their maximum operating depth as clearly as possible. Other safety precautions may include using different coloured regulator housing, flavoured mouthpieces, or simply placing a rubber band vertically across the mouthpiece as an alert.

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Decompression equipment, Providing gases to accelerate decompression, Surface panel gas switching Surface supplied divers may be supplied with a gas mixture suitable for accelerated decompression by connecting a supply to the surface gas panel and connecting it through the valve system to the divers. This allows accelerated decompression, usually on oxygen, which can be used to a maximum depth of 20 ft (6 m) in water for scuba and 30 ft (9 m) on surface supply. Surface supplied heliox bounce divers will be provided with mixtures suitable for their current depth, and the mixture may be changed several times during descent and ascent from great depths.

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Decompression equipment, Providing gases to accelerate decompression, Continuously variable mixture in closed circuit rebreathers Closed circuit rebreathers are usually controlled to provide a fairly constant partial pressure of oxygen during the dive (set point), and may be reset to a richer mix for decompression. The effect is to keep the partial pressure of inert gases as low as safely practicable throughout the dive. This minimises the absorption of inert gas in the first place, and accelerates the elimination of the inert gases during ascent.

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Decompression equipment, Surface decompression equipment, Deck decompression chambers A deck decompression chamber (DDC), or double-lock chamber is a two compartment pressure vessel for human occupation which has sufficient space in the main chamber for two or more occupants, and a forechamber which can allow a person to be pressurised or decompressed while the main chamber remains under constant pressure. This allows an attendant to be locked in or out during treatment of the occupant(s) of the main chamber. There is usually also a medical lock, which serves a similar function but is much smaller.

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Decompression equipment, Surface decompression equipment, Deck decompression chambers This is used to transfer medical material, food and specimens into and out of the main chamber while it is under pressure. Most deck decompression chambers are fitted with built-in breathing systems (BIBS), which supply an alternative breathing gas to the occupants (usually oxygen), and discharge the exhaled gas outside the chamber, so the chamber gas is not excessively enriched by oxygen, which would cause an unacceptable fire hazard, and require frequent flushing with chamber gas (usually air).

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Decompression equipment, Surface decompression equipment, Deck decompression chambers A deck decompression chamber is intended for surface decompression and emergency hyperbaric treatment of divers, but can be used for other hyperbaric treatment under the appropriate supervision of hyperbaric medical personnel.

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Decompression equipment, Surface decompression equipment, Deck decompression chambers Portable or mobile one and two occupant single compartment chambers are not generally intended for routine surface decompression, but may be used in an emergency.

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Decompression equipment, Surface decompression equipment, Dry bells and Saturation systems A "Saturation System" or "Saturation spread" typically includes 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 placed on a ship or ocean platform, but is more commonly capable of being moved from one vessel to another by crane. The entire system is managed from a control room (van), where depth, chamber atmosphere and other system parameters are monitored and controlled.

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Decompression equipment, Surface decompression equipment, Dry bells and Saturation systems The diving bell is the elevator or lift that transfers 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 tankage bulkhead by a trunking space, a kind of tunnel, through which the divers transfer to and from the bell. At the completion of work or a mission, the saturation diving team is decompressed gradually back to atmospheric pressure by the slow venting of system pressure, at rates of about of 15 to 30 msw (50 to 100 fsw) per day, (schedules vary).

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Decompression equipment, Surface decompression equipment, Dry bells and Saturation systems Thus the process involves only one ascent, thereby mitigating the time-consuming and comparatively risky process of multiple decompressions normally associated with non-saturation ("bounce diving") operations. The chamber gas mixture is typically controlled to maintain a nominally constant partial pressure of oxygen of between 0.3 and 0.5 bar during most of the decompression (0.44 to 0.48 bar on US Navy schedule), which is below the upper limit for long term exposure. NOAA has used rather different saturation decompression schedules for relatively shallow (less than 100 fsw) air and nitrox saturation dives, which use oxygen breathing when pressure is reduced to less than 55 fsw.

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Decompression equipment, Surface decompression equipment, Dry bells and Saturation systems The divers use surface supplied umbilical diving equipment, utilizing deep diving breathing gas, such as helium and oxygen mixtures, stored in large capacity, high pressure cylinders. The gas supplies are plumbed to the control room, where they are routed to supply the system components. 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 are supplied from the bell through umbilicals.

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Decompression equipment, Surface decompression equipment, Dry bells and Saturation systems A hyperbaric lifeboat or hyperbaric rescue unit 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 may be self-propelled and can be operated by crew while the 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. The crew would normally start decompression as soon as possible after launching.

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Decompression equipment, Surface decompression equipment, Dry bells and Saturation systems A dry bell may also be used for bounce dives to great depths, and then used as the decompression chamber during the ascent and later on board the support vessel. In this case it is not always necessary to transfer into a deck chamber, as the bell is quite capable of performing this function, though it would be relatively cramped, as a bell is usually as small as conveniently possible to minimise weight for deployment.

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Decompression illness Decompression Illness (DCI) comprises two different conditions caused by rapid decompression of the body. These conditions present similar symptoms and require the same initial first aid. Scuba divers are trained to ascent slowly from depth to avoid DCI. Although the incidence is relatively rare, the consequences can be serious and potentially fatal, especially if untreated.

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Decompression illness, Classification DCI can be caused by two different mechanisms, which result in overlapping sets of symptoms. The two mechanisms are:

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Decompression illness, Classification In any situation that could cause decompression sickness, there is also potentially a risk of arterial gas embolism, and as many of the symptoms are common to both conditions, it may be difficult to distinguish between the two in the field, and first aid treatment is the same for both mechanisms.

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Decompression illness, Signs and symptoms Below is a summary comparison of the signs and symptoms of DCI arising from its two components: Decompression Sickness and Arterial Gas Embolism. Many signs and symptoms are common to both maladies, and it may be difficult to diagnose the actual problem. The dive history can be useful to distinguish which is more probable, but it is possible for both components to manifest at the same time following some dive profiles.

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Decompression illness, Causes Decompression sickness is caused by the formation and growth of inert gas bubbles in the tissues when a diver decompresses faster than the gas can be safely disposed of through respiration and perfusion.

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Decompression illness, Causes Arterial gas embolism is caused by gas in the lungs getting into the pulmonary venous circulation through injuries to the capillaries of the alveoli caused by lung overpressure injury. These bubbles are then circulated to the tissues via the systemic arterial circulation, and may cause blockages directly or indirectly by initiating clotting.

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Decompression illness, Mechanism The mechanism of decompression sickness is different from that of arterial gas embolism, but they share the causative factor of depressurisation.

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Decompression illness, Mechanism, Decompression sickness Depressurisation causes inert gases, which were dissolved under higher pressure, to come out of physical solution and form gas bubbles within the body. These bubbles produce the symptoms of decompression sickness. Bubbles may form whenever the body experiences a reduction in pressure, but not all bubbles result in DCS. The amount of gas dissolved in a liquid is described by Henry's Law, which indicates that when the pressure of a gas in contact with a liquid is decreased, the amount of that gas dissolved in the liquid will also decrease proportionately.

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Decompression illness, Mechanism, Decompression sickness On ascent from a dive, inert gas comes out of solution in a process called "outgassing" or "offgassing". Under normal conditions, most offgassing occurs by gas exchange in the lungs. If inert gas comes out of solution too quickly to allow outgassing in the lungs then bubbles may form in the blood or within the solid tissues of the body. The formation of bubbles in the skin or joints results in milder symptoms, while large numbers of bubbles in the venous blood can cause lung damage.

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Decompression illness, Mechanism, Decompression sickness The most severe types of DCS interrupt — and ultimately damage — spinal cord function, leading to paralysis, sensory dysfunction, or death. In the presence of a right-to-left shunt of the heart, such as a patent foramen ovale, venous bubbles may enter the arterial system, resulting in an arterial gas embolism. A similar effect, known as ebullism, may occur during explosive decompression, when water vapour forms bubbles in body fluids due to a dramatic reduction in environmental pressure.

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Decompression illness, Mechanism, Arterial gas embolism When a diver holds their breath during an ascent the reduction in pressure will cause the gas to expand and the lungs will also have to expand to continue to contain the gas. If the expansion exceeds the normal capacity of the lungs, they will continue to expand elastically until the tissues reach their tensile strength limit, after which any increase in pressure difference between the gas in the lungs and the ambient pressure will cause the weaker tissues to rupture, releasing gas from the lungs into any permeable space exposed by the damaged tissue.

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Decompression illness, Mechanism, Arterial gas embolism This could be the pleural space between the lung and the chest walls, between the pleural membranes, and this condition is known as pneumothorax. The gas could also enter the interstitial spaces within the lungs, the neck and larynx, and the mediastinal space around the heart, causing interstititial or mediastinal emphysema, or it could enter the blood vessels of the venous pulmonary circulation via damaged alveolar capillaries, and from there reach the left side of the heart, from which they will be discharged into the systemic circulation.

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Decompression illness, Mechanism, Arterial gas embolism On the way out through the aorta the gas may be entrained in blood flowing into the carotid or basilar arteries. If these bubbles cause blockage in blood vessels, this is arterial gas embolism. Sufficient pressure difference and expansion to cause this injury can occur from depths as shallow as 1.2 metres (3.9 ft).

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Decompression illness, Diagnosis Definitive diagnosis is difficult, as most of the signs and symptoms are common to several conditions and there are no specific tests for DCI. The dive history is important, if reliable, and the sequence and presentation of symptoms can differentiate between possibilities. Most doctors do not have the training and experience to reliably diagnose DCI, so it is preferable to consult a diving medicine specialist, as misdiagnosis can have inconvenient, expensive and possibly life-threatening consequences.

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Decompression illness, Diagnosis Prior to 2000, there was a tendency to under-diagnose DCI, and as a result a number of cases did not get the treatment that could have produced a better result, while since 2000, there has been a swing to over-diagnosis, with consequent expensive and inconvenient treatments, and expensive inconvenient and risky evacuations that were not necessary. The presence of symptoms of pneumothorax, mediastinal or interstitial emphysema would support a diagnosis of arterial gas embolism if symptoms of that condition are also present, but AGE can occur without symptoms of other lung overpressure injuries. Most cases of arterial gas embolism will present symptoms soon after surfacing, but this also happens with cerebral decompression sickness.

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Decompression illness, Diagnosis Numbness and tingling are associated with spinal DCS, but can also be caused by pressure on nerves (compression neurapraxia). In DCS the numbness or tingling is generally confined to one or a series of dermatomes, while pressure on a nerve tends to produce characteristic areas of numbness associated with the specific nerve on only one side of the body distal to the pressure point. A loss of strength or function is likely to be a medical emergency. A loss of feeling that lasts more than a minute or two indicates a need for immediate medical attention. It is only partial sensory changes, or paraesthesias, where this distinction between trivial and more serious injuries applies.

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Decompression illness, Diagnosis Large areas of numbness with associated weakness or paralysis, especially if a whole limb is affected, are indicative of probable brain involvement and require urgent medical attention. Paraesthesias or weakness involving a dermatome indicate probable spinal cord or spinal nerve root involvement. Although it is possible that this may have other causes, such as an injured intervertebral disk, these symptoms indicate an urgent need for medical assessment. In combination with weakness, paralysis or loss of bowel or bladder control, they indicate a medical emergency.

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Decompression illness, Prevention Almost all arterial gas embolism is avoidable by not diving with lung conditions which increase the risk and not holding the breath during ascent. These conditions will usually be detected in the diving medical examination required for professional divers. Recreational divers are not all screened at this level. Complete emptying of the lungs is not recommended in emergency swimming ascents as this is thought to increase the risk by collapsing small air passages and trapping air in parts of the lung. Rate of ascent is not usually an issue for AGE.

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Decompression illness, Prevention Decompression sickness is usually avoidable by following the requirements of decompression tables or algorithms regarding ascent rates and stop times for the specific dive profile, but these do not guarantee safety, and in some cases, unpredictably, there will be decompression sickness. Decompressing for longer can reduce the risk by an unknown amount. Decompression is a calculated risk where some of the variables are not well defined, and it is not possible to define the point at which all residual risk disappears. Risk is also reduced by reducing exposure to ingassing and taking into account the various known and suspected risk factors. Most, but not all, cases are easily avoided.

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Decompression illness, Treatment Treatment for the Decompression Sickness and the Arterial Gas Embolism components of DCI may differ significantly, but that depends mostly on the symptoms, as both conditions are generally treated based on the symptoms. Refer to the separate treatments under those articles.

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Decompression illness, Treatment Urgency of treatment depends on the symptoms. Mild symptoms will usually resolve without treatment, though appropriate treatment may accelerate recovery considerably. Failure to treat severe cases can have fatal or long term effects. Some types of injuries are more likely to have long lasting effects depending on the organs involved.

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Decompression illness, Prognosis The outcome for cerebral arterial gas embolism largely depends on severity and the delay before recompression. Most cases which are recompressed within two hours do well. Recompression within six hours often produces improvement and sometimes full resolution. Delays to recompression of more than 6 to 8 hours are not often very effective, and are generally associated with delays in diagnosis and delays in transfer to a hyperbaric chamber.

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Decompression illness, Epidemiology Roughly 3 to 7 cases per 10,000 dives are diagnosed, of which about 1 in 100,000 dives are fatal.

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Decompression practice The practice of decompression by divers comprises the planning and monitoring of the profile indicated by the algorithms or tables of the chosen decompression model, to allow asymptomatic and harmless release of excess inert gases dissolved in the tissues as a result of breathing at ambient pressures greater than surface atmospheric pressure, the equipment available and appropriate to the circumstances of the dive, and the procedures authorized for the equipment and profile to be used. There is a large range of options in all of these aspects.

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Decompression practice Decompression may be continuous or staged, where the ascent is interrupted by stops at regular depth intervals, but the entire ascent is part of the decompression, and ascent rate can be critical to harmless elimination of inert gas. What is commonly known as no-decompression diving, or more accurately no-stop decompression, relies on limiting ascent rate for avoidance of excessive bubble formation. Staged decompression may include deep stops depending on the theoretical model used for calculating the ascent schedule.

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Decompression practice Omission of decompression theoretically required for a dive profile exposes the diver to significantly higher risk of symptomatic decompression sickness, and in severe cases, serious injury or death. The risk is related to the severity of exposure and the level of supersaturation of tissues in the diver. Procedures for emergency management of omitted decompression and symptomatic decompression sickness have been published. These procedures are generally effective, but vary in effectiveness from case to case.

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Decompression practice The procedures used for decompression depend on the mode of diving, the available equipment, the site and environment, and the actual dive profile. Standardized procedures have been developed which provide an acceptable level of risk in the circumstances for which they are appropriate. Different sets of procedures are used by commercial, military, scientific and recreational divers, though there is considerable overlap where similar equipment is used, and some concepts are common to all decompression procedures.

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Decompression practice, Decompression Decompression in the context of diving derives from the reduction in ambient pressure experienced by the diver during the ascent at the end of a dive or hyperbaric exposure and refers to both the reduction in pressure and the process of allowing dissolved inert gases to be eliminated from the tissues during this reduction in pressure. When a diver descends in the water column the ambient pressure rises. Breathing gas is supplied at the same pressure as the surrounding water, and some of this gas dissolves into the diver's blood and other fluids.

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Decompression practice, Decompression Inert gas continues to be taken up until the gas dissolved in the diver is in a state of equilibrium with the breathing gas in the diver's lungs, (see: "Saturation diving"), or the diver moves up in the water column and reduces the ambient pressure of the breathing gas until the inert gases dissolved in the tissues are at a higher concentration than the equilibrium state, and start diffusing out again.

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Decompression practice, Decompression Dissolved inert gases such as nitrogen or helium can form bubbles in the blood and tissues of the diver if the partial pressures of the dissolved gases in the diver gets too high above the ambient pressure. These bubbles and products of injury caused by the bubbles can cause damage to tissues known as decompression sickness, or "the bends". The immediate goal of controlled decompression is to avoid development of symptoms of bubble formation in the tissues of the diver, and the long-term goal is to also avoid complications due to sub-clinical decompression injury.

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Decompression practice, Decompression A diver who exceeds the no-decompression limit for a decompression algorithm or table has a theoretical tissue gas loading which is considered likely to cause symptomatic bubble formation unless the ascent follows a decompression schedule, and is said to have a decompression obligation.

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Decompression practice, Common procedures The descent, bottom time and ascent are sectors common to all dives and hyperbaric exposures.

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Decompression practice, Common procedures, Descent rate Descent rate is generally allowed for in decompression planning by assuming a maximum descent rate specified in the instructions for the use of the tables, but it is not critical. Descent slower than the nominal rate reduces useful bottom time, but has no other adverse effect. Descent faster than the specified maximum will expose the diver to greater ingassing rate earlier in the dive, and the bottom time must be reduced accordingly. In the case of real-time monitoring by dive computer, descent rate is not specified, as the consequences are automatically accounted for by the programmed algorithm.

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Decompression practice, Common procedures, Bottom time Bottom time is the time spent at depth before starting the ascent. Bottom time used for decompression planning may be defined differently depending on the tables or algorithm used. It may include descent time, but not in all cases. It is important to check how bottom time is defined for the tables before they are used.

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Decompression practice, Common procedures, Bottom time For example, tables using Bühlmann's algorithm define bottom time as the elapsed time between leaving the surface and the start of the final ascent at 10 metres per minute, and if the ascent rate is slower, then the excess of the ascent time to the first required decompression stop needs to be considered part of the bottom time for the tables to remain safe.

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Decompression practice, Common procedures, Ascent rate The ascent is an important part of the process of decompression, as this is the time when reduction of ambient pressure occurs, and it is of critical importance to safe decompression that the ascent rate is compatible with safe elimination of inert gas from the diver's tissues. Ascent rate must be limited to prevent supersaturation of tissues to the extent that unacceptable bubble development occurs. This is usually done by specifying a maximum ascent rate compatible with the decompression model chosen.

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Decompression practice, Common procedures, Ascent rate This will be specified in the decompression tables or the user manual for the decompression software or personal decompression computer. The instructions will usually include contingency procedures for deviation from the specified rate, both for delays and exceeding the recommended rate. Failure to comply with these specifications will generally increase the risk of decompression sickness.

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Decompression practice, Common procedures, Ascent rate Typically maximum ascent rates are in the order of 10 metres (33 ft) per minute for dives deeper than 6 metres (20 ft). Some dive computers have variable maximum ascent rates, depending on depth. Ascent rates slower than the recommended standard for the algorithm will generally be treated by a computer as part of a multilevel dive profile and the decompression requirement adjusted accordingly. Faster ascent rates will elicit a warning and additional decompression stop time to compensate.

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Decompression practice, Monitoring decompression status The decompression status of the diver must be known before starting the ascent, so that an appropriate decompression schedule can be followed to avoid an excessive risk of decompression sickness. Scuba divers are responsible for monitoring their own decompression status, as they are the only ones to have access to the necessary information. Surface supplied divers depth and elapsed time can be monitored by the surface team, and the responsibility for keeping track of the diver's decompression status is generally part of the supervisor's job.

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Decompression practice, Monitoring decompression status The supervisor will generally assess decompression status based on dive tables, maximum depth and elapsed bottom time of the dive, though multi-level calculations are possible. Depth is measured at the gas panel by pneumofathometer, which can be done at any time without distracting the diver from their activity. The instrument does not record a depth profile, and requires intermittent action by the panel operator to measure and record the current depth. Elapsed dive time and bottom time are easily monitored using a stopwatch.

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Decompression practice, Monitoring decompression status Worksheets for monitoring the dive profile are available, and include space for listing the ascent profile including decompression stop depths, time of arrival, and stop time. If repetitive dives are involved, residual nitrogen status is also calculated and recorded, and used to determine the decompression schedule. A surface supplied diver may also carry a bottom timer or decompression computer to provide an accurate record of the actual dive profile, and the computer output may be taken into account when deciding on the ascent profile. The dive profile recorded by a dive computer would be valuable evidence in the event of an accident investigation.

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Decompression practice, Monitoring decompression status Scuba divers can monitor decompression status by using maximum depth and elapsed time in the same way, and can use those to either select from a previously compiled set of surfacing schedules, or identify the recommended profile from a waterproof dive table taken along on the dive. It is possible to calculate a decompression schedule for a multilevel dive using this system, but the possibility of error is significant due to the skill and attention required, and the table format, which can be misread under task loading or in poor visibility.

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Decompression practice, Monitoring decompression status The current trend is towards the use of dive computers to calculate the decompression obligation in real time, using depth and time data automatically input into the processing unit, and continuously displayed on the output screen. Dive computers have become quite reliable, but can fail in service for a variety of reasons, and it is prudent to have a backup system available to estimate a reasonable safe ascent if the computer fails. This can be a backup computer, a written schedule with watch and depth gauge, or the dive buddy's computer if they have a reasonably similar dive profile. If only no-stop diving is done, and the diver makes sure that the no-stop limit is not exceeded, a computer failure can be managed at acceptable risk by starting an immediate direct ascent to the surface at an appropriate ascent rate.

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Decompression practice, No-decompression dives A "no-decompression", or "no-stop" dive is a dive that needs no decompression stops during the ascent according to the chosen algorithm or tables, and relies on a controlled ascent rate for the elimination of excess inert gases. In effect, the diver is doing continuous decompression during the ascent.

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Decompression practice, No-decompression dives, No-decompression limit The "no-decompression limit" (NDL) or "no-stop limit" , is the time interval that a diver may theoretically spend at a given depth without having to perform any decompression stops while surfacing. The NDL helps divers plan dives so that they can stay at a given depth for a limited time and then ascend without stopping while still avoiding an unacceptable risk of decompression sickness.

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Decompression practice, No-decompression dives, No-decompression limit The NDL is a theoretical time obtained by calculating inert gas uptake and release in the body, using a decompression model such as the Bühlmann decompression algorithm. Although the science of calculating these limits has been refined over the last century, there is still much that is unknown about how inert gases enter and leave the human body, and the NDL may vary between decompression models for identical initial conditions. In addition, every individual's body is unique and may absorb and release inert gases at different rates at different times.

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Decompression practice, No-decompression dives, No-decompression limit For this reason, dive tables typically have a degree of conservatism built into their recommendations. Divers can and do suffer decompression sickness while remaining inside NDLs, though the incidence is very low. On dive tables a set of NDLs for a range of depth intervals is printed in a grid that can be used to plan dives. There are many different tables available as well as software programs and calculators, which will calculate no decompression limits. Most personal decompression computers (dive computers) will indicate a remaining no decompression limit at the current depth during a dive. The displayed interval is continuously revised to take into account changes of depth as well as elapsed time. Dive computers also usually have a planning function which will display the NDL for a chosen depth taking the diver's recent decompression history into account.

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Decompression practice, No-decompression dives, Safety stop As a precaution against any unnoticed dive computer malfunction, diver error or physiological predisposition to decompression sickness, many divers do an extra "safety stop" in addition to those prescribed by their dive computer or tables. A safety stop is typically 1 to 5 minutes at 3 to 6 metres (10 to 20 ft). They are usually done during no-stop dives and may be added to the obligatory decompression on staged dives. Many dive computers indicate a recommended safety stop as standard procedure for dives beyond specific limits of depth and time. The Goldman decompression model predicts a significant risk reduction following a safety stop on a low-risk dive

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Decompression practice, Continuous decompression Continuous decompression is decompression without stops. Instead of a fairly rapid ascent rate to the first stop, followed by a period at static depth during the stop, the ascent is slower, but without officially stopping. In theory this may be the optimum decompression profile. In practice it is very difficult to do manually, and it may be necessary to stop the ascent occasionally to get back on schedule, but these stops are not part of the schedule, they are corrections. For example, USN treatment table 5, referring to treatment in a decompression chamber for type 1 decompression sickness, states "Descent rate - 20 ft/min. Ascent rate - Not to exceed 1 ft/min. Do not compensate for slower ascent rates. Compensate for faster rates by halting the ascent."