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Saturation diving

Saturation diving is diving for periods long enough to bring all tissues into equilibrium with the partial pressures of the inert components of the breathing gas used. It is a diving mode that reduces the number of decompressions divers working at great depths must undergo by only decompressing divers once at the end of the diving operation, which may last days to weeks, having them remain under pressure for the whole period. A diver breathing pressurized gas accumulates dissolved inert gas used in the breathing mixture to dilute the oxygen to a non-toxic level in the tissues, which can cause potentially fatal decompression sickness ("the bends") if permitted to come out of solution within the body tissues; hence, returning to the surface safely requires lengthy decompression so that the inert gases can be eliminated via the lungs. Once the dissolved gases in a diver's tissues reach the saturation point, however, decompression time does not increase with further exposure, as no more inert gas is accumulated.

Saturation diving takes advantage of this by having divers remain in that saturated state. When not in the water, the divers live in a sealed environment which maintains their pressurised state; this can be an ambient pressure underwater habitat or a saturation system at the surface, with transfer to and from the pressurised living quarters to the equivalent depth underwater via a closed, pressurised diving bell. This may be maintained for up to several weeks, and divers are decompressed to surface pressure only once, at the end of their tour of duty. By limiting the number of decompressions in this way, and using a conservative decompression schedule the risk of decompression sickness is significantly reduced, and the total time spent decompressing is minimised. Saturation divers typically breathe a helium–oxygen mixture to prevent nitrogen narcosis, and limit work of breathing, but at shallow depths saturation diving has been done on nitrox mixtures.

Most of the physiological and medical aspects of diving to the same depths are much the same in saturation and bell-bounce ambient pressure diving, or are less of a problem, but there are medical and psychological effects of living under saturation for extended periods.

Saturation diving is a specialized form of diving; of the 3,300 commercial divers employed in the United States in 2015, 336 were saturation divers. Special training and certification is required, as the activity is inherently hazardous, and a set of standard operating procedures, emergency procedures, and a range of specialised equipment is used to control the risk, that require consistently correct performance by all the members of an extended diving team. The combination of relatively large skilled personnel requirements, complex engineering, and bulky, heavy equipment required to support a saturation diving project make it an expensive diving mode, but it allows direct human intervention at places that would not otherwise be practical, and where it is applied, it is generally more economically viable than other options, if such exist.

History
On December 22, 1938, Edgar End and Max Nohl made the first intentional saturation dive by spending 27 hours breathing air at 101 feet sea water (fsw) (30.8 msw) in the County Emergency Hospital recompression facility in Milwaukee, Wisconsin. Their decompression lasted five hours leaving Nohl with a mild case of decompression sickness that resolved with recompression.

Albert R. Behnke proposed the idea of exposing humans to increased ambient pressures long enough for the blood and tissues to become saturated with inert gases in 1942. In 1957, George F. Bond began the Genesis project at the Naval Submarine Medical Research Laboratory proving that humans could in fact withstand prolonged exposure to different breathing gases and increased environmental pressures. Once saturation is achieved, the amount of time needed for decompression depends on the depth and gases breathed. This was the beginning of saturation diving and the US Navy's Man-in-the-Sea Program. The first commercial saturation dives were performed in 1965 by Westinghouse to replace faulty trash racks at 200 ft on the Smith Mountain Dam.

Peter B. Bennett is credited with the invention of trimix breathing gas as a method to eliminate high pressure nervous syndrome. In 1981, at the Duke University Medical Center, Bennett conducted an experiment called Atlantis III, which involved subjecting volunteers to a pressure of 2250 fsw (equivalent to a depth of 686 m in seawater), and slowly decompressing them to atmospheric pressure over a period of 31-plus days, setting an early world record for depth-equivalent in the process. A later experiment, Atlantis IV, encountered problems as one of the volunteers experienced euphoric hallucinations and hypomania.

The history of commercial saturation diving is closely linked to offshore oil and gas extraction. In the early 1960s exploration of the North Sea started on the premise that the Dutch gas fields might extend under the sea. This was borne out when the Gulf Tide rig hit the Ekofisk reservoir in 1969 and in 1971 Shell oil found the Brent oilfield between Norway and Shetland. From this time to the 1990s the industry developed the procedures and equipment for saturation diving from pioneering and experimental, with a somewhat dubious safety record, to a mature industry with greatly improved occupational health and safety.

When the North Sea drilling started, there was little diving support infrastructure in Europe, and the high wages attracted divers from the Gulf of Mexico oilfields, who introduced the fibre reinforced resin lightweight demand helmets from Kirby-Morgan, hot water suits from Diving Unlimited International, and the U.S. Navy Diving Manual, at the time the leading set of offshore diving procedures. Research and development money was available, and new technical developments were supported by the European Economic Community. A major challenge was developing saturation diving practices suitable to the common North Sea depth range of 100 to 180 m.

During the early drilling stages most of the diving work was for relatively short periods and was generally suitable for bell bounce diving, but the development of oilfield seabed infrastructure required much longer diver interventions, and saturation diving procedures were developed to suit. By 1982, a large amount of shallow maintenance work was becoming necessary, which brought in more air diving to service the rigs. By 2017 about 80% of North Sea diving was heliox saturation diving and the other 20% shallow air diving.

Excursion dives without decompression stops can be done both upward and downward from saturation storage pressure within limits, allowing the divers a range of working depths, and if work is required beyond excursion range, the divers can be compressed or decompressed in storage to suit the changed depth range. Further work was done by the United States Navy Experimental Diving Unit on excursion dives from February 1974 to June 1976, and the results published in the 1984 U.S. Navy Diving Manual. These tables used a partial pressure of oxygen of 0.35 to 0.4 bar during decompression, with quite slow decompression rates, which varied with the depth, getting slower as the depth decreased, with a 6-hour stop from midnight and a two-hour stop from 14:00 and a gas fraction limit of 22% for the last part of the ascent to reduce fire risk. The tables allowed decompression to start directly after return from a dive provided there had not been an upward excursion, as this was found to increase the risk of bubble development.

At the same time, the commercial diving contractor Compagnie maritime d'expertises (COMEX) had been developing slightly different decompression procedures, in which the oxygen partial pressures were higher, between 0.6 and 0.8 bar, and the ascent rates were faster to take advantage of the high PO 2. Continuous decompression without night stops was used, and excursions were allowed. Over time these were revised to use lower PO 2 and slower ascent rates, particularly at the shallower depths. Competing tables were thought to be used to gain competitive advantage, so in 1988 the Norwegian Petroleum Directorate organised a conference on saturation decompression safety under Val Hempleman, and in 1990 a conference to harmonise the saturation tables to be used in the North Sea in the Norwegian sector using input from five contractors. In 1999 the NORSOK U100 standard was published, which was a compromise using aspects of several of the tables, but which has proven in use to be sufficiently conservative and has a good safety record.

In the 1980s the Royal Navy were using an oxygen partial pressure of 0.42 bar for decompression from saturation, which is slightly higher than to 0.40 bar of the US Navy table. This reduced the time for decompression by a small percentage.

Saturation decompression in the Brazil oilfields took a slightly different route, and was originally based on company tables, until Brazil produced their own legislation in 1988, similar to that of the UK's Health and Safety Executive. In 2004 revised legislation was closer to the COMEX procedures.

By 2017 the system had settled into a chamber PO 2 of 0.5 bar while deeper than 15 msw, and limited to 22 to 23% at the end of decompression to limit fire risk.

Applications
Saturation diving has applications in scientific diving and commercial offshore diving.

Commercial offshore diving, sometimes shortened to just offshore diving, is a branch of commercial diving, with divers working in support of the exploration and production sector of the oil and gas industry in places such as the Gulf of Mexico in the United States, the North Sea in the United Kingdom and Norway, and along the coast of Brazil. The work in this area of the industry includes maintenance of oil platforms and the building of underwater structures. In this context "" implies that the diving work is done outside of national boundaries.

Saturation diving is standard practice for bottom work at many of the deeper offshore sites, and allows more effective use of the diver's time while reducing the risk of decompression sickness. Surface oriented air diving is more usual in shallower water.

Underwater habitats are underwater structures in which people can live for extended periods and carry out most of the basic human functions of a 24-hour day, such as working, resting, eating, attending to personal hygiene, and sleeping. In this context 'habitat' is generally used in a narrow sense to mean the interior and immediate exterior of the structure and its fixtures, but not its surrounding marine environment. Most early underwater habitats lacked regenerative systems for air, water, food, electricity, and other resources. However, recently some new underwater habitats allow for these resources to be delivered using pipes, or generated within the habitat, rather than manually delivered.

An underwater habitat has to meet the needs of human physiology and provide suitable environmental conditions, and the one which is most critical is breathing air of suitable quality. Others concern the physical environment (pressure, temperature, light, humidity), the chemical environment (drinking water, food, waste products, toxins) and the biological environment (hazardous sea creatures, microorganisms, marine fungi). Much of the science covering underwater habitats and their technology designed to meet human requirements is shared with diving, diving bells, submersible vehicles and submarines, and spacecraft.

Numerous underwater habitats have been designed, built and used around the world since the early 1960s, either by private individuals or by government agencies. They have been used almost exclusively for research and exploration, but in recent years at least one underwater habitat has been provided for recreation and tourism. Research has been devoted particularly to the physiological processes and limits of breathing gases under pressure, for aquanaut and astronaut training, as well as for research on marine ecosystems. Access to and from the exterior is generally vertically through a hole in the bottom of the structure called a moon pool. The habitat may include a decompression chamber, or personnel transfer to the surface may be via a closed diving bell.

Employment
Saturation diving work in support of the offshore oil and gas industries is usually contract based.

Decompression sickness
Decompression sickness (DCS) is a potentially fatal condition caused by bubbles of inert gas, which can occur in divers' bodies as a consequence of the pressure reduction as they ascend. To prevent decompression sickness, divers have to limit their rate of ascent, to reduce the concentration of dissolved gases in their body sufficiently to avoid bubble formation and growth. This protocol, known as decompression, can last for several hours for dives in excess of 50 m when divers spend more than a few minutes at these depths. The longer divers remain at depth, the more inert gas is absorbed into their body tissues, and the time required for decompression increases rapidly. This presents a problem for operations that require divers to work for extended periods at depth, as the time spent decompressing can exceed the time spent doing useful work by a large margin. However, after somewhere around 72 hours under any given pressure, depending on the ingassing model used, divers' bodies become saturated with inert gas, and no further uptake occurs. From that point onward, no increase in decompression time is necessary. The practice of saturation diving takes advantage of this by providing a means for divers to remain at depth pressure for days or weeks. At the end of that period, divers need to carry out a single saturation decompression, which is much more efficient and a lower risk than making multiple short dives, each of which requires a lengthy decompression time. By making the single decompression slower and longer, in the controlled conditions and relative comfort of the saturation habitat or decompression chamber, the risk of decompression sickness during the single exposure is further reduced.

High-pressure nervous syndrome
High-pressure nervous syndrome (HPNS) is a neurological and physiological diving disorder that results when a diver descends below about 500 ft while breathing a helium–oxygen mixture. The effects depend on the rate of descent and the depth. HPNS is a limiting factor in future deep diving. HPNS can be reduced by using a small percentage of nitrogen in the gas mixture.

Compression arthralgia
Compression arthralgia is a deep aching pain in the joints caused by exposure to high ambient pressure at a relatively high rate of compression, experienced by underwater divers. The pain may occur in the knees, shoulders, fingers, back, hips, neck or ribs, and may be sudden and intense in onset and may be accompanied by a feeling of roughness in the joints. Onset commonly occurs around 60 msw (meters of sea water), and symptoms are variable depending on depth, compression rate and personal susceptibility. Intensity increases with depth and may be aggravated by exercise. Compression arthralgia is generally a problem of deep diving, particularly deep saturation diving, where at sufficient depth even slow compression may produce symptoms. The use of trimix can reduce the symptoms. Spontaneous improvement may occur over time at depth, but this is unpredictable, and pain may persist into decompression. Compression arthralgia may be easily distinguished from decompression sickness as it starts during descent, is present before starting decompression, and resolves with decreasing pressure, the opposite of decompression sickness. The pain may be sufficiently severe to limit the diver's capacity for work, and may also limit the depth of downward excursions.

Dysbaric osteonecrosis
Saturation diving (or more precisely, long term exposure to high pressure) is associated with aseptic bone necrosis, although it is not yet known if all divers are affected or only especially sensitive ones. The joints are most vulnerable to osteonecrosis. The connection between high-pressure exposure, decompression procedure and osteonecrosis is not fully understood.

Extreme depth effects
A breathing gas mixture of oxygen, helium and hydrogen was developed for use at extreme depths to reduce the effects of high pressure on the central nervous system. Between 1978 and 1984, a team of divers from Duke University in North Carolina conducted the Atlantis series of onshore-hyperbaric-chamber-deep-scientific-test-dives. In 1981, during an extreme depth test dive to 686 metres (2251 ft) they breathed the conventional mixture of oxygen and helium with difficulty and suffered trembling and memory lapses.

A hydrogen–helium–oxygen (hydreliox) gas mixture was used during a similar on shore scientific test dive by three divers involved in an experiment for the French Comex S.A. industrial deep-sea diving company in 1992. On 18 November 1992, Comex decided to stop the experiment at an equivalent of 675 meters of sea water (msw) (2215 fsw) because the divers were suffering from insomnia and fatigue. All three divers wanted to push on but the company decided to decompress the chamber to 650 msw (2133 fsw). On 20 November 1992, Comex diver Theo Mavrostomos was given the go-ahead to continue but spent only two hours at 701 msw (2300 fsw). Comex had planned for the divers to spend four and a half days at this depth and carry out tasks.

Oxygen toxicity
Both acute and chronic oxygen toxicity are significant risks in saturation diving. The storage breathing gas exposes the divers to one continuous level of oxygen concentration for extended periods, on the order of a month at a time, which requires the gas in the habitat to be maintained at a long term tolerable partial pressure, generally around 0.4 bar, which is well tolerated, and allows for quite large accidental deviations without causing hypoxia. This may be increased during decompression, but as decompression may take over a week, the safely tolerable increase is limited, and at lower pressures oxygen partial pressure is also limited by fire hazard considerations.

Bell and excursion gas composition must suit the planned dive profile. A higher oxygen partial pressure may be tolerable over the working period, but it may be logistically preferable to use the same gas used for storage. Bailout gas may have a higher oxygen content. At one time the recommended bailout oxygen partial pressure was significantly higher than used in the main gas supply.

Thermal balance of the diver
Thermoregulation is the ability of an organism to keep its body temperature within specific bounds, even when the surrounding temperature is very different. The internal thermoregulation process is one aspect of homeostasis: a state of dynamic stability in an organism's internal conditions, maintained far from thermal equilibrium with its environment. If the body is unable to maintain a normal human body temperature and it increases significantly above normal, a condition known as hyperthermia occurs. The opposite condition, when body temperature decreases below normal levels, is known as hypothermia. It occurs when the body loses heat faster than producing it.

Body heat is lost by respiratory heat loss, by heating and humidifying (latent heat) inspired gas, and by body surface heat loss, by radiation, conduction, and convection, to the atmosphere, water, and other substances in the immediate surroundings. Surface heat loss may be reduced by insulation of the body surface. Heat is produced internally by metabolic processes and may be supplied from external sources by active heating of the body surface or the breathing gas.

Heat transfer to and via gases at higher pressure than atmospheric is increased due to the higher density of the gas at higher pressure which increases its heat capacity. This effect is also modified by changes in breathing gas composition necessary for reducing narcosis and work of breathing, to limit oxygen toxicity and to accelerate decompression. Heat loss through conduction is faster for higher fractions of helium. Divers in a helium based saturation habitat will lose or gain heat fast if the gas temperature is too low or too high, both via the skin and breathing, and therefore the tolerable temperature range is smaller than for the same gas at normal atmospheric pressure.

The heat loss situation is very different in the saturation living areas, which are temperature and humidity controlled, in the dry bell, and in the water.

The alveoli of the lungs are very effective at heat and humidity transfer. Inspired gas that reaches them is heated to core body temperature and humidified to saturation in the time needed for gas exchange, regardless of the initial temperature and humidity. This heat and humidity are lost to the environment in open circuit breathing systems. Breathing gas that only gets as far as the physiological dead space is not heated so effectively. When heat loss exceeds heat generation, body temperature will fall.

Exertion increases heat production by metabolic processes, but when breathing gas is cold and dense, heat loss due to the increased volume of gas breathed to support these metabolic processes can result in a net loss of heat, even if the heat loss through the skin is minimised.

Health effects of living under saturation conditions
There is some evidence of long term cumulative reduction in lung function in saturation divers.

Saturation divers are frequently troubled by superficial infections such as skin rashes, otitis externa and athlete's foot, which occur during and after saturation exposures. This is thought to be a consequence of raised partial pressure of oxygen, and relatively high temperatures and humidity in the accommodation.

Dysbaric osteonecrosis is considered a consequence of decompression injury rather than living under saturation conditions.

Long term cumulative exposure to high oxygen partial pressures is associated with accelerated development of cataracts.

Duration of exposure and surface intervals
The Diving Medical Advisory Council recommends that under normal circumstances the duration of a saturation dive should not exceed 28 days, and the interval between saturation exposures should generally equal the duration of the previous exposure, with a cumulative exposure of not more than 182 days in any 12 month period.

Operating procedures
Saturation diving allows professional divers to live and work at pressures greater than 50 msw (160 fsw) for days or weeks at a time, though lower pressures have been used for scientific work from underwater habitats. This type of diving allows for greater economy of work and enhanced safety for the divers. After working in the water, they rest and live in a dry pressurized habitat on, or connected to, a diving support vessel, oil platform or other floating work station, at approximately the same pressure as the work depth. The diving team is compressed to the working pressure only once, at the beginning of the work period, and decompressed to surface pressure once, after the entire work period of days or weeks. There are accepted safe upward and downward excursion limits based on the storage depth. Excursions to greater depths require decompression when returning to storage depth, and excursions to shallower depths are also limited by decompression obligations to avoid decompression sickness during the excursion. Most of the diving skills required for saturation diving are the same as for surface-oriented surface-supplied diving.

Increased use of underwater remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs) for routine or planned tasks means that saturation dives are becoming less common, though complicated underwater tasks requiring complex manual actions remain the preserve of the deep-sea saturation diver.

A person who operates a saturation diving system is called a life support technician (LST).

Personnel requirements
A saturation diving team requires at the minimum the following personnel: In some jurisdictions there will also be a diving medical practitioner on standby, but not necessarily on site, and some companies may require a diving medical technician on site. The actual personnel actively engaged in aspects of the operation are usually more than the minimum.
 * A diving supervisor (on duty during any diving operations)
 * Two life-support supervisors (working shifts while there are divers under pressure)
 * Two life-support technicians (also working shifts)
 * Two divers in the bell (working diver and bellman – they may alternate during the dive)
 * One surface stand-by diver (on duty when the bell is in the water)
 * One tender for the surface stand-by diver

Compression
Compression or blowdown to storage depth is generally at a limited rate to minimize the risk of HPNS and compression arthralgia. Norwegian standards specifies a maximum compression rate of 1 msw per minute, and a rest period at storage depth after compression and before diving.

Storage depth
Storage depth, also known as living depth, is the pressure in the accommodation sections of the saturation habitat—the ambient pressure under which the saturation divers live when not engaged in lock-out activity. Any change in storage depth involves a compression or a decompression, both of which are stressful to the occupants, and therefore dive planning should minimize the need for changes of living depth and excursion exposures, and storage depth should be as close as practicable to the working depth, taking into account all relevant safety considerations.

Atmosphere control
The hyperbaric atmosphere in the accommodation chambers and the bell are controlled to ensure that the risk of long term adverse effects on the divers is acceptably low. Most saturation diving is done on heliox mixtures, with partial pressure of oxygen in accommodation areas kept around 0.40 to 0.48 bar, which is near the upper limit for long term exposure. Carbon dioxide is removed from the chamber gas by recycling it through scrubber cartridges. The levels are generally limited to a maximum of 0.005 bar partial pressure, equivalent to 0.5% surface equivalent. Most of the balance is helium, with a small amount of nitrogen and trace residuals from the air in the system before compression.

Bell operations and lockouts may also be done at between 0.4 and 0.6 bar oxygen partial pressure, but often use a higher partial pressure of oxygen, between 0.6 and 0.9 bar, which lessens the effect of pressure variation due to excursions away from holding pressure, thereby reducing the amount and probability of bubble formation due to these pressure changes. In emergencies a partial pressure of 0.6 bar of oxygen can be tolerated for over 24 hours, but this is avoided where possible. Carbon dioxide can also be tolerated at higher levels for limited periods. US Navy limit is 0.02 bar for up to 4 hours. Nitrogen partial pressure starts at 0.79 bar from the initial air content before compression, but tends to decrease over time as the system loses gas to lock operation, and is topped up with helium.

Deployment of divers
Deployment of divers from a surface saturation complex requires the diver to be transferred under pressure from the accommodation area to the underwater workplace. This is generally done by using a closed diving bell, also known as a Personnel Transfer Capsule, which is clamped to the lock flange of the accommodation transfer chamber and the pressure equalized with the accommodation transfer chamber for transfer to the bell. The lock doors can then be opened for the divers to enter the bell. The divers will suit up before entering the bell and complete the pre-dive checks. The pressure in the bell will be adjusted to suit the depth at which the divers will lock out while the bell is being lowered, so that the pressure change can be slow without unduly delaying operations.

The bell is deployed over the side of the vessel or platform using a gantry or A-frame or through a moon pool. Deployment usually starts by lowering the clump weight, which is a large ballast weight suspended from a cable which runs down one side from the gantry, through a set of sheaves on the weight, and up the other side back to the gantry, where it is fastened. The weight hangs freely between the two parts of the cable, and due to its weight, hangs horizontally and keeps the cable under tension. The bell hangs between the parts of the cable, and has a fairlead on each side which slides along the cable as it is lowered or lifted. The bell hangs from a cable attached to the top. As the bell is lowered, the fairleads guide it down the clump weight cables to the workplace.

The bell umbilical is separate from the divers' umbilicals, which are connected on the inside of the bell. The bell umbilical is deployed from a large drum or umbilical basket and care is taken to keep the tension in the umbilical low but sufficient to remain near vertical in use and to roll up neatly during recovery.

A device called a bell cursor may be used to guide and control the motion of the bell through the air and the splash zone near the surface, where waves can move the bell significantly.

Once the bell is at the correct depth, the final adjustments to pressure are made and after final checks, the supervisor instructs the divers to lock out of the bell. The hatch is at the bottom of the bell and can only be opened if the pressure inside is balanced with the ambient water pressure. The bellman tends the working diver's umbilical through the hatch during the dive. If the diver experiences a problem and needs assistance, the bellman will exit the bell and follow the diver's umbilical to the diver and render whatever help is necessary and possible. Each diver carries back-mounted bailout gas, which should be sufficient to allow a safe return to the bell in the event of an umbilical gas supply failure.

Breathing gas is supplied to the divers from the surface through the bell umbilical. If this system fails, the bell carries an on-board gas supply which is plumbed into the bell gas panel and can be switched by operating the relevant valves. On-board gas is generally carried externally in several storage cylinders of 50 litres capacity or larger, connected through pressure regulators to the gas panel.

Helium is a very effective heat transfer material, and divers may lose heat rapidly if the surrounding water is cold. To prevent hypothermia, hot-water suits are commonly used for saturation diving, and the breathing gas supply may be heated. Heated water is produced at the surface and piped to the bell through a hot-water line in the bell umbilical, then is transferred to the divers through their excursion umbilicals. The umbilicals also have cables for electrical power to the bell and helmet lights, and for voice communications and closed circuit video cameras. In some cases the breathing gas is recovered to save the expensive helium. This is done through a reclaim hose in the umbilicals, which ducts exhaled gas exhausted through a reclaim valve on the helmet, through the umbilicals and back to the surface, where the carbon dioxide is scrubbed and the gas boosted into storage cylinders for later use.

Lock-in and lock-out
Lock-in (also lock in) is the process of passing from the outside ambient pressure to an internally pressurised space. In saturation diving the internal space of the accommodation is generally at a significantly higher pressure than the internal pressure, and an airlock is needed as an intermediate compartment. Locking into the bell from the water is done at equal pressures so an intermediate airlock is not required. The opposite process, called lock-out (or lock out), is passing out of the internally pressurised space to ambient pressure surroundings.

Lock-on and lock-off
Lock-on (or lock on) is the airtight connection of one pressurised compartment to another, and lock-off (or lock off) is the separation of two connected pressurised compartments from each other. An intermediate airlock or trunking space is needed which is equalised from ambient pressure to internal pressure after the seal has been made, and vented to ambient before disconnection.

Excursions from storage depth
It is quite common for saturation divers to need to work over a range of depths while the saturation system can only maintain one or two storage depths at any given time. A change of depth from storage depth is known as an excursion, and divers can make excursions within limits without incurring a decompression obligation, just as there are no-decompression limits for surface oriented diving. Excursions may be upward or downward from the storage depth, and the allowed depth change may be the same in both directions, or sometimes slightly less upward than downward. Excursion limits are generally based on a 6 to 8 hour time limit, as this is the standard time limit for a diving shift. These excursion limits imply a significant change in gas load in all tissues for a depth change of around 15m for 6 to 8 hours, and experimental work has shown that both venous blood and brain tissue are likely to develop small asymptomatic bubbles after a full shift at both the upward and downward excursion limits. These bubbles remain small due to the relatively small pressure ratio between storage and excursion pressure, and are generally resolved by the time the diver is back on shift, and residual bubbles do not accumulate over sequential shifts. However, any residual bubbles pose a risk of growth if decompression is started before they are eliminated. Ascent rate during excursions is limited, to minimize the risk and amount of bubble formation.

Inner ear decompression sickness is a relatively frequent symptom of DCS as a consequence of excursions in deep saturation dives, in comparison to its very low frequency in decompressions from bounce dives.

Decompression from saturation
Once all the tissue compartments have reached saturation for a given pressure and breathing mixture, continued exposure will not increase the gas loading of the tissues. From this point onward the required decompression remains the same. If divers work and live at pressure for a long period, and are decompressed only at the end of the period, the risks associated with decompression are limited to this single exposure. This principle has led to the practice of saturation diving, and as there is only one decompression, and it is done in the relative safety and comfort of a saturation habitat, the decompression is done on a very conservative profile, minimising the risk of bubble formation, growth and the consequent injury to tissues. A consequence of these procedures is that saturation divers are more likely to suffer decompression sickness symptoms in the slowest tissues, whereas bounce divers are more likely to develop bubbles in faster tissues.

Decompression from a saturation dive is a slow process. The rate of decompression typically ranges between 3 and 6 fsw (0.9 and 1.8 msw) per hour. The US Navy Heliox saturation decompression rates require a partial pressure of oxygen to be maintained at between 0.44 and 0.48 atm when possible, but not to exceed 23% by volume, to restrict the risk of fire.

For practicality the decompression is done in increments of 1 fsw at a rate not exceeding 1 fsw per minute, followed by a stop, with the average complying with the table ascent rate. Decompression is done for 16 hours in 24, with the remaining 8 hours split into two rest periods. A further adaptation generally made to the schedule is to stop at 4 fsw for the time that it would theoretically take to complete the decompression at the specified rate, i.e. 80 minutes, and then complete the decompression to surface at 1 fsw per minute. This is done to avoid the possibility of losing the door seal at a low pressure differential and losing the last hour or so of slow decompression.

Decompression following a recent excursion
Neither the excursions nor the decompression procedures currently in use have been found to cause decompression problems in isolation. However, there appears to be significantly higher risk when excursions are followed by decompression before non-symptomatic bubbles resulting from excursions have totally resolved. Starting decompression while bubbles are present appears to be the significant factor in many cases of otherwise unexpected decompression sickness during routine saturation decompression. The Norwegian standards do not allow decompression following directly on an excursion.

Emergency decompression
Very little is known with confidence about how best to decompress from saturation in an emergency. A DMAC consensus document has been issued with tentative advice on possible procedures based on a balance of perceived risk. These procedures are not supported by experience or experimental work as there is very little of either, and constitute an educated guess at best. The risk of symptomatic decompression sickness is expected to increase as the rate of decompression increases, with earlier pain only symptoms, and more serious symptoms developing later or at higher decompression rates.

Existing decompression tables for accelerated saturation decompression from the US Navy, Duke Tables, and Comex procedures were considered inadequate for the emergency scenarios envisaged, although they are faster than the schedules in general commercial use.

Recommendations include the use of high oxygen partial pressures before and during decompression, with the actual partial pressure chosen depending on the predicted total duration of the decompression. Environmental factors such as dehydration, stress, gas contamination and confinement are considered likely to affect the risk. It is considered of high importance to maintain hydration at high levels. Intravenous administration may be appropriate.

The decompression should be planned to make use of all the expected time available, at the slowest practicable rate, using the highest oxygen partial pressure appropriate for the time scale. Environmental control of the chamber should maintain temperature as closely as possible, and divers should move around enough to aid circulation, but not exercise strenuously. It is considered safer in an accelerated decompression to slow down the decompression, or stop and recompress if the situation changes than to start slow and accelerate decompression if the situation deteriorates.

Architecture of a surface saturation facility

Saturation diving systems are a variation of Pressure vessel for human occupancy, which includes rules for life support, operations, maintenance, and structural design. The "saturation system", "saturation complex" or "saturation spread" typically comprises either an underwater habitat or a surface 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 placed on a ship or ocean platform, but is more commonly capable of being moved from one vessel to another by crane. To facilitate transportation of the components, it is standard practice to construct the components as units based on the intermodal container system, some of which may be stackable to save deck space. The entire system is managed from a control room ("van"), where depth, chamber atmosphere and other system parameters are monitored and controlled. 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 an average of 15 m to 30 m per day (schedules vary). Thus the process involves only one ascent, thereby mitigating the time-consuming and comparatively risky process of in-water, staged decompression or sur-D O2 operations normally associated with non-saturation mixed gas diving. More than one living chamber can be linked to the transfer chamber through trunking so that diving teams can be stored at different depths where this is a logistical requirement. An extra chamber can be fitted to transfer personnel into and out of the system while under pressure and to treat divers for decompression sickness if this should be necessary.

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.

While in the water the divers will often use a hot water suit to protect against the cold. The hot water comes from boilers on the surface and is pumped down to the diver via the bell's umbilical and then through the diver's umbilical.

Personnel transfer capsule
A closed diving bell, also known as personnel transfer capsule or submersible decompression chamber, is used to transport divers between the workplace and the accommodations chambers. The bell is a cylindrical or spherical pressure vessel with a hatch at the bottom, and may mate with the surface transfer chamber at the bottom hatch or at a side door. Bells are usually designed to carry two or three divers, one of whom, the, stays inside the bell at the bottom and is to the working divers. Each diver is supplied by an umbilical from inside the bell. The bell has a set of high pressure gas storage cylinders mounted on the outside containing on-board reserve breathing gas. The on-board gas and main gas supply are distributed from the bell gas panel, which is controlled by the bellman. The bell may have viewports and external lights. The divers' umbilicals are stored on racks inside the bell during transfer, and are tended by the bellman during the dive.

Bell handling system
The bell is deployed from a gantry or A-frame, also known as a bell launch and recovery system (LARS), on the vessel or platform, using a winch. Deployment may be over the side or through a moon pool.
 * The handling system must be able to support the dynamic loads imposed by operating in a range of weather conditions.
 * It must be able to move the bell through the air/water interface (splash zone) in a controlled way, fast enough to avoid excessive movement caused by wave action.
 * A bell cursor may be used to limit lateral motion through and above the splash zone.
 * It must keep the bell clear of the vessel or platform to prevent impact damage or injury.
 * It must have sufficient power for fast retrieval of the bell in an emergency, and fine control to facilitate mating of the bell and transfer flange, and to accurately place the bell at the bottom.
 * It must include a system to move the bell between the mating flange of the transfer chamber and the launch/retrieval position.

Transfer chamber
The transfer chamber is where the bell is mated to the surface saturation system for transfer under pressure (TUP). It is a wet surface chamber where divers prepare for a dive and strip off and clean their gear after return. Connection to the bell may be overhead, through the bottom hatch of the bell, or lateral, through a side door.

Accommodation chambers
The accommodation chambers may be as small as 100 square feet. This part is generally made of multiple compartments, including living, sanitation, and rest facilities, each a separate unit, joined by short lengths of cylindrical trunking. It is usually possible to isolate each compartment from the others using internal pressure doors. Catering and laundry are provided from outside the system and locked in and out as required.

Recompression chamber
A recompression chamber may be included in the system so that divers can be given treatment for decompression sickness without inconveniencing the rest of the occupants. The recompression chamber may also be used as an entry lock, and to decompress occupants who may need to leave before scheduled.

Mating flange for transportable chamber
One or more of the external doors may be provided with a mating flange or collar to suit a portable or transportable chamber, which can be used to evacuate a diver under pressure. The closed bell can be used for this purpose, but lighter and more easily portable chambers are also available. There will usually also be a mating flange for the hyperbaric rescue and escape system.

Supply lock
A small lock, also known as an equipment lock or medical lock, is used for transfer of supplies into and out of the pressurized system. This would normally include food, medical supplies, clothing, bedding etc.

Trunking
The pressurised compartments of the system are connected through access trunking: relatively short and small diameter bolted between the external flanges of the larger compartments, with pressure seals, forming passageways between the chambers, which can be isolated by pressure doors.

Life support systems
The life support system provides breathing gas and other services to support life for the personnel under pressure. It includes the following components:
 * Breathing gas supply, distribution and recycling equipment: scrubbers, filters, boosters, compressors, mixing, monitoring, and storage facilities
 * Chamber climate control system – control of temperature and humidity, and filtration of gas
 * Instrumentation, control, monitoring and communications equipment
 * Fire suppression systems
 * Sanitation systems

The life support system for the bell provides and monitors the main supply of breathing gas, and the control station monitors the deployment and communications with the divers. Primary gas supply, power and communications to the bell are through a bell umbilical, made up from a number of hoses and electrical cables twisted together and deployed as a unit. This is extended to the divers through the diver umbilicals (excursion umbilical]]s). The accommodation life support system maintains the chamber environment within the acceptable range for health and comfort of the occupants. Temperature, humidity, breathing gas quality sanitation systems and equipment function are monitored and controlled.

Life support system operation
The life support system is operated by the life-support technicians under the life-support supervisor, who are part of the saturation diving team. There will be at least two life support technicians working shifts, as one must be on duty at all times while there are divers under pressure.

Hot water system
Divers working in cold water, particularly when breathing helium-based gases (which increase the rate of heat transfer), may rapidly lose body heat and suffer from hypothermia. Hypothermia is unhealthy, can be life-threatening, and reduces diver effectiveness. This can be ameliorated with a hot water system. A diver hot water system heats filtered seawater and pumps it to the divers through the bell and diver umbilicals. This water can be used to heat the breathing gas before it is inhaled. The divers breathing gas is mainly heated on dives below 150 metres, and the region will dictate what temperature the water is heated to so that it will then flow through the diver's hot water suit to keep the diver warm.

Emergency heating of the bell
There is a need for emergency heating of divers trapped in a closed diving bell. The breathing gas may be helium based, at a high pressure, and the ambient water temperature may be quite low, down to 2°C, with a typical temperature in the North Sea of about 5°C. The bell itself is usually made of steel, a good thermal conductor, and the quality of bell insulation is variable, so the internal atmosphere tends to match the water temperature fairly soon after the primary heating fails. Divers trapped in bells for long periods have been subjected to various degrees of hypothermia when the primary heating systems failed. There have been deaths attributed to this cause.

Passive systems were the first to be developed to a stage where they were considered functionally sufficient, and are relatively simple, economical and immediately available, and are used as standard equipment when applicable. Personal insulation for the diver, in the form of an insulated bag, combined with a breathing gas heat exchanger to conserve the heat of exhaled gas, and heat liberated by a personal carbon dioxide scrubber kept within the insulation layer round the diver is usually sufficient to keep the divers in thermal balance while waiting for rescue. The scrubber has an orinasal mask, and the bag is secured to the inside of the bell by a harness, to prevent the diver from collapsing if rendered unconscious, and potentially blocking access to the bell by rescuers.

Communication systems
Helium and high pressure both cause hyperbaric distortion of speech. The process of talking underwater is influenced by the internal geometry of the life support equipment and constraints on the communications systems as well as the physical and physiological influences of the environment on the processes of speaking and vocal sound production. The use of breathing gases under pressure or containing helium causes problems in intelligibility of diver speech due to distortion caused by the different speed of sound in the gas and the different density of the gas compared to air at surface pressure. These parameters induce changes in the vocal tract formants, which affect the timbre, and a slight change of pitch. Several studies indicate that the loss in intelligibility is mainly due to the change in the formants.

The difference in density of the breathing gas causes a non-linear shift of low-pitch vocal resonance, due to resonance shifts in the vocal cavities, giving a nasal effect, and a linear shift of vocal resonances which is a function of the velocity of sound in the gas, known as the Donald Duck effect. Another effect of higher density is the relative increase in intensity of voiced sounds relative to unvoiced sounds. The contrast between closed and open voiced sounds and the contrast between voiced consonants and adjacent vowels decrease with increased pressure. Change of the speed of sound is relatively large in relation to depth increase at shallower depths, but this effect reduces as the pressure increases, and at greater depths a change in depth makes a smaller difference. Helium speech unscramblers are a partial technical solution. They improve intelligibility of transmitted speech to surface personnel.

The communications system may have four component systems.
 * The hardwired intercom system, an amplified voice system with speech unscrambler to reduce the pitch of the speech of the occupants of the pressurized system. This system will provide communications between the main control console and the bell and accommodation chambers. This two-way system is the primary communications mode.
 * Wireless through-water communications between bell and main control console is a backup system in case of failure of the hardwired system with the bell.
 * Closed circuit video from cameras on the bell and diver helmets allow visual monitoring of the dive and the divers by the supervisor.
 * A sound powered phone system may be provided as a backup voice communication system between bell and control console

Bulk gas supplies
Gas storage and blending equipment are provided to pressurize and flush the system, and treatment gases should be available appropriate to the planned storage depths. Bulk stock of premixed gas is usually provided to suit the planned depth of the operation, and separate bulk stock of helium and oxygen to make up additional requirements, adjust chamber gas composition as the oxygen is used up, and mix decompression gas.

Bulk gas is usually stored in manifolded groups of storage cylinders known as "quads", which usually carry about 16 high pressure cylinders, each of about 50 litres internal volume mounted on a frame for ease of transport, or larger frames carrying larger capacity high pressure "tubes". These tube frames are usually designed to be handled by intermodal container handling equipment, so are usually made in one of the standard sizes for intermodal containers.

Gas reclaim systems
[[File:Reclaim schematic.png|thumb|upright=2|Schematic diagram of a heliox breathing gas reclaim system BGP: bell gas panel

S1: first water separator

BP1: bell back-pressure regulator

U: bell umbilical

F1: first gas filter

BP2: topside back-pressure regulator

R1, R2: serial gas receivers

F2: second gas filter

B: booster pump

Sc1, Sc2: parallel scrubbers

C: gas cooler

S2: last water separator

VT: volume tank

PR: pressure regulator

MGP: main gas panel]]

A helium reclaim system (or push-pull system) may be used to recover helium based breathing gas after use by the divers as this is more economical than losing it to the environment in open circuit systems. The recovered gas is passed through a scrubber system to remove carbon dioxide, filtered to remove odours and other impurities, and pressurised into storage containers, where it may be mixed with oxygen to the required composition. Alternatively the recycled gas can be more directly recirculated to the divers.

During extended diving operations very large amounts of breathing gas are used. Helium is an expensive gas and can be difficult to source and supply to offshore vessels in some parts of the world. A closed circuit gas reclaim system can save around 80% of gas costs by recovering about 90% of the helium based breathing mixture. Reclaim also reduces the amount of gas storage required on board, which can be important where storage capacity is limited. Reclaim systems are also used to recover gas discharged from the saturation system during decompression.

A reclaim system will typically consist of the following components:

Topside components: Underwater components:
 * A reclaim control console, which controls and monitors the booster pump, oxygen addition, diver supply pressure, exhaust hose pressure and make-up gas addition.
 * A gas reprocessing unit, with low-pressure carbon dioxide scrubber towers, filters' receivers and back-pressure regulator which will remove carbon dioxide and excess moisture in a condensation water trap. Other gases and odours can be removed by activated carbon filters.
 * A gas booster, to boost the pressure of the reclaimed gas to the storage pressure.
 * A gas volume tank
 * A storage system of pressure vessels to hold the boosted and reconstituted gas mixture until it is used. This functions as a buffer to allow for the variations of gas volume in the rest of the system due to pressure changes.
 * Dive control panel
 * A bell gas supply panel, to control the supply of gas to the bell.
 * The bell umbilical, with the supply and exhaust hoses between the topside system and the bell.
 * Internal bell gas panel to supply the gas to the divers, and bell reclaim equipment, which controls the exhaust hose back-pressure, and can shut off the reclaim hose if the diver's gas supply is interrupted. A scrubber for the bell atmosphere and water trap would be included.
 * Diver excursion umbilicals, with supply and exhaust hoses between the bell and the divers
 * Reclaim helmets which supply gas to the divers on demand, with reclaim back-pressure regulators which exhaust the exhaled gas to the return line.
 * Bell back-pressure regulator with water trap

In operation the gas supply from the reclaim system is connected to the topside gas panel, with a backup supply at a slightly lower pressure from mixed gas storage which will automatically cut in if the reclaim supply pressure drops. The bellman will set onboard gas supply to a slightly lower pressure than surface supply pressure to the bell gas panel, so that it will automatically cut in if surface supply is lost. After locking out of the bell the diver will close the diverter valve and open the return valve on the helmet, to start the gas reclaim process. Once this is running, the reclaim control panel will be adjusted to make up the metabolic oxygen usage of the diver into the returned gas. This system will automatically shut down oxygen addition if the flow of exhaled gas from the diver fails, to avoid an excessive oxygen fraction in the recycled gas. There is an indicator light to show whether the return gas is flowing.

The gas supplied to the diver's helmet passes through the same hoses and demand valve as for the open circuit system, but the exhaled gas passes out into the reclaim valve at slightly above ambient pressure, which is considerably above atmospheric pressure, so the flow must be controlled to prevent dropping the helmet internal pressure and causing the demand valve to free-flow. This is achieved by using back-pressure regulators to control the pressure drop in stages. The reclaim valve itself is a demand triggered back-pressure regulator, and there is another back pressure regulator at the bell gas panel, and one at the surface before the receiver tanks. Each of these back-pressure regulators is set to allow about a 1 bar pressure drop.

Exhaust gas returns to the bell through the diver's umbilical exhaust hose, where it passes through a water separator and trap then through a back-pressure regulator which controls the pressure in the exhaust hose and which can be monitored on a pressure gauge in the bell and adjusted by the bellman to suit the excursion depth of the diver. The gas then passes through the bell umbilical exhaust hose to the surface via a non-return valve and another water trap. When the gas enters the surface unit it goes through a coalescing water separator and micron particle filter, and a float valve, which protects the reclaim system from large volumes of water in the event of a leak at depth. Another back-pressure regulator at the surface controls the pressure in the bell umbilical. The gas then passes into the receiver tanks, where oxygen is added at a flow rate calculated to compensate for metabolic use by the diver.

Before entering the boosters, the gas passes through a 0.1 micron filter. The gas is then boosted to storage pressure. Redundant boosters are provided to keep the system running while a booster is serviced. The boosters are automatically controlled to match the diver's gas consumption, and the boosted gas passes through a scrubber where the carbon dioxide is removed by a material like sodalime. Like the boosters, there are at least two scrubbers in parallel, so that they can be isolated, vented and repacked alternately while the system remains in operation. The gas then passes through a cooling heat exchanger to condense out any remaining moisture, which is removed by another 1 micon coalescing filter before it reaches the volume storage tank, where it remains until returned to the gas panel to be used by the divers. While in the volume tank, the gas can be analysed to ensure that it is suitable for re-use, and that the oxygen fraction is correct and carbon dioxide has been removed to specification before it is delivered to the divers. If necessary any lost gas can be compensated by topping up the volume tank from the high pressure storage. Gas from the volume tank is fed to the topside gas panel to be routed back to the bell and diver.

Sanitation system
The sanitation system includes hot and cold water supply for washbasins and showers, drainage, and marine toilets with holding tank and discharge system.

Control consoles
It is common for the control room to be installed in an ISO intermode container for convenience of transport. There are three main control panels, for life support, dive control and gas management.

Gas management panel
The gas management panel includes pressure regulation of gases from high pressure storage, and distribution to the consumers. Gases will include air, oxygen and heliox mixes

Saturation control panel
The chamber control panel will typically include depth gauges for each compartment, including trunking, blowdown and exhaust valves, oxygen monitoring and other gas analysis equipment, make-up system for oxygen replenishment, valves for supplying therapeutic breathing mixture, closed circuit television monitoring displays, and monitoring systems with alarms for temperature and pressure in the system chambers.

Dive control panel
The dive control panel will include depth gauges for bell internal and external pressure, diver and bellman depth, and trunking pressure for transfer to the accommodation chambers. There will also be breathing gas pressure gauges and control valves for each diver, and blowdown and exhaust valves for the bell interior, diver communications systems with speech unscramblers, a through-water emergency communications system to the bell, controls, monitors and recording equipment for helmet and bell mounted video cameras, oxygen analysers for diver breathing gas, oxygen and carbon dioxide analysers for bell and reclaim gas, alarms for reclaim gas flow, dynamic positioning and hot water supply.

Fire suppression system
Firefighting systems include hand held fire extinguishers to automatic deluge systems. Special fire extinguishers which do not use toxic materials must be used. In the event of a fire, toxic gases may be released by burning materials, and the occupants will have to use the built-in breathing systems (BIBS) until the chamber gas has been flushed sufficiently. When a system with oxygen partial pressure 0.48 bar is pressurized below about 70 msw (231fsw), the oxygen fraction is too low to support combustion (less than 6%), and the fire risk is low. During the early stages of compression and towards the end of decompression the oxygen levels will support combustion, and greater care must be taken.

Built in breathing systems
Built in breathing systems are installed for emergency use and for treatment of decompression sickness. They supply breathing gas appropriate to the current function, which is supplied from outside the pressurized system and also vented to the exterior, so the exhaled gases do not contaminate the chamber atmosphere.

Hyperbaric rescue and escape systems
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 "lost" bell (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).

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.

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.

The International Maritime Organisation (IMO) and IMCA recognise that though the number of hyperbaric evacuations which have been successfully carried out is small, and the likelihood of an incident needing hyperbaric evacuation is extremely low, the risk is sufficient to justify requiring the equipment to be available. The original meaning for the term hyperbaric evacuation system covered the system that actually transported the divers away from the working hyperbaric system such as a hyperbaric rescue chamber, a self-propelled hyperbaric lifeboat, or hyperbaric rescue vessel, all of which float and carry short term life-support systems of varied endurance, but it has more recently come to include all of the equipment that would support a hyperbaric evacuation, such as a life support package that can be connected to a recovered hyperbaric rescue unit, to provide interim life support until decompression facilities are available, and the hyperbaric reception facility where divers can be decompressed and treated in relative comfort. The four main classes of problem that must be managed during a hyperbaric evacuation are thermal balance, motion sickness, dealing with metabolic waste products, and severely cramped and confined conditions.

Bell to bell transfer may be used to rescue divers from a lost or entrapped bell. This will generally occur at or near the bottom, and the divers transfer between bells at ambient pressure. It is 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.

Evacuation of a single diver who is medically stable, or a single diver with an attendant, may be possible using a hyperbaric stretcher or a small portable chamber if the duration of the trip is short, the pressure is suitable and the locking flanges are compatible.

Platforms
Most saturation diving is done offshore, in the vicinity of drilling and production platforms, or for salvage work, and requires precise positioning of the bell during the dive. In deep water this is usually done from a specialised diving support vessel, or a suitable vessel of opportunity on which a saturation system has been temporarily installed. Positioning can either be by a substantial anchor pattern, which may interfere with other anchoring spreads already established, and which presents its own set of hazards, or by dynamic positioning, which must be sufficiently reliable and fail-safe for the expected conditions.

Underwater habitats
Scientific saturation diving is usually conducted by researchers and technicians known as aquanauts living in an underwater habitat, a structure designed for people to live in for extended periods, where they can carry out almost all basic human functions: working, resting, eating, attending to personal hygiene, and sleeping, all while remaining under pressure beneath the surface.

Depth records
The diving depth record for offshore diving was achieved in 1988 by a team of professional divers (Th. Arnold, S. Icart, J.G. Marcel Auda, R. Peilho, P. Raude, L. Schneider) of the Comex S.A. industrial deep-sea diving company performing pipe line connection exercises at a depth of 534 meters of sea water (msw) (1752 fsw) in the Mediterranean Sea during a record scientific dive.

In the real working conditions of the offshore oil industry, in Campos Basin, Brazil, Brazilian saturation divers from the DSV Stena Marianos (later Mermaid Commander (2006)) performed a manifold installation for Petrobras at 316 m depth in February 1990. When a lift bag attachment failed, the equipment was carried by the bottom currents to 328 m depth, and the Brazilian diver Adelson D'Araujo Santos Jr. made the recovery and installation.

In 1992, Greek diver Theodoros Mavrostomos of the Comex S.A. achieved a simulated 701 m of seawater depth in an on shore hyperbaric chamber. He took 43 days to complete the record experimental dive, where a hydrogen–helium–oxygen gas mixture was used as breathing gas.

The complexity, medical problems and accompanying high costs of professional diving to such extreme depths and the development of deep water atmospheric diving suits and ROVs in offshore oilfield drilling and production have effectively eliminated the need for ambient pressure manned intervention at extreme depths.

Training and registration
Training of saturation divers generally takes place at commercial diving schools registered to train saturation divers, and having the required infrastructure and equipment. Diver training standards for saturation divers are published by a small number of organisations, and there is some international recognition of equivalence. The prerequisites for starting training are generally that the diver is already qualified as a bell diver and has a specified number of dives and hours of experience since qualifying.

Training of saturation divers generally starts with a competent and at least moderately experienced surface oriented bell diver and concentrates on the additional knowledge and skills required for saturation diving. There is a large additional technical component related to the specialised equipment. For the South African Department of Labour Class I Diver, the additional knowledge and skills include:
 * A basic knowledge of the history of mixed gas and saturation diving,
 * An understanding of modular and diving support vessel based saturation diving systems, saturation life-support systems including environmental control, diver heating systems, sump drains and hyperbaric toilet discharges
 * An understanding and practical operating skills for closed diving bells, their standard and emergency equipment, handling systems, bell and excursion umbilicals and personal diving equipment, and their testing and maintenance requirements,
 * An understanding and practical operating skills for transfer under pressure and closed bell diving from 4-point moored and dynamically positioned vessels
 * An understanding of gas supplies and saturation consumables, including minimum gas requirements, gas transfer pumps, gas blending, and gas reclaim systems,
 * An understanding and practical experience in committing divers to saturation, and pressurisation
 * An understanding of split level saturation diving
 * Knowledge of the minimum personnel requirements for saturation diving operations and the responsibilities of the diving team members, including the superintendent, supervisor, life support supervisor, life support technician, support and systems technicians, gas man, and the bellman and diver, and experience and skills as diver and bellman
 * Knowledge of saturation decompression procedures, emergency saturation decompression and hyperbaric evacuation and practical experience of standard procedures and simulated emergency procedures.
 * Certification as a level 2 first aider, with additional knowledge of saturation hygiene, saturation first aid requirements and the deep diving compression disorders, high pressure nervous syndrome and compression arthralgia.

Safety and risk
The purpose of saturation diving is to extend the useful working time for dives without increasing the exposure to risk of decompression sickness. There is a trade-off against other risks associated with living under high-pressure saturation conditions, and the financial cost is high due to the complex infrastructure and expensive equipment and consumables required. The risk of decompression sickness is reduced at the cost of increased risk due to being committed to the saturation environment for the duration of the decompression schedule associated with the storage depth. Hyperbaric evacuation from saturation is possible, but not universally available, and is logistically complicated. Having an evacuation system on standby is expensive.

Some notable saturation diving incidents include:

 * Bibby Topaz diving accident, documented by the 2019 film Last Breath
 * Bibby Topaz diving accident, documented by the 2019 film Last Breath
 * Bibby Topaz diving accident, documented by the 2019 film Last Breath
 * Bibby Topaz diving accident, documented by the 2019 film Last Breath
 * Bibby Topaz diving accident, documented by the 2019 film Last Breath
 * Bibby Topaz diving accident, documented by the 2019 film Last Breath
 * Bibby Topaz diving accident, documented by the 2019 film Last Breath
 * Bibby Topaz diving accident, documented by the 2019 film Last Breath

Working conditions
The living and working conditions of the saturation diver are unusual. There are shifting demands and large contrasts. The diver is required to be adaptable to the variations in work and coworkers, and to do so for several weeks at a time. The future of the occupation is uncertain and linked to the oil and gas industry. The work environment and living conditions on contract tend to be monotonous, but they are interspersed with periods of leisure.There is a conflict between family commitments and extended work periods in relative isolation, but in the continuous close proximity with a small group of co-workers. The job is prestigious, the pay is good and there are fairly long periods of leisure between jobs. The occupation needs mental endurance, flexibility and willingness to adapt and learn. Maintaining personal routines may help in maintaining mental health. The quality of the team can have a large effect on the members. The ability to get along well with each other and mutual trust are important to cooperation and efficiency in a group who depend on each other for safety and sometimes for survival. A sense of humour is an advantage though there is a tendency for this to be relatively dark, possibly due to the high risk environment.

In arts and media
For saturation diving in fiction, see Pressure (2015), The Abyss (1989), Sphere (1987), Goliath Awaits (1981), Dykket (The Dive) (1989), Pioneer (Pionér) (2013) and The Neptune Factor (1973).

In 2019, Netflix released Last Breath, a documentary which tells the story of Chris Lemons, a saturation diver who survived 38 minutes without a surface-supplied breathing gas supply after the vessel's dynamic positioning system failed during a storm, setting off a red alert. The two working divers started returning to the bell, but the ship drifted from the work site, dragging the bell with it, and his umbilical was snagged and severed under the load. He was able to return to the workplace using his bailout set, so was easily found by an ROV from the ship, but his bailout gas was insufficient for the time it took to get the ship back on position for a rescue attempt from the bell. Although presumed dead by support crew aboard the vessel, he was recovered by the second diver and successfully resuscitated in the bell. It has been hypothesised that his survival may have been a result of hypothermia, high partial pressure of oxygen in the bailout gas, or a combination. The ROV video footage shows him twitching while unconscious, which is consistent with an oxygen toxicity blackout.

title: Saturation diving


text:
Talk:Plestiodon anthracinus

This article uses text from the public domain website of the U.S. Army Corps of Engineers at, which is also given as a reference in the article. Lupo 14:41, 26 Feb 2004 (UTC)

odd name for a scale
The sentence "One postmental scale is present" looks extremely odd. I might suggest that a postmedial scale is intended, but I'm not well versed in biology. Any volunteers? Carrionluggage 03:59, 2 January 2006 (UTC) Sorry - I guess it is OK - I found it also at Carrionluggage 04:12, 2 January 2006 (UTC)

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Talk:Plestiodon anthracinus


text:
Trump (magazine)

Trump was a glossy magazine of satire and humor, mostly in the forms of comics features and short stories. It was edited by Harvey Kurtzman and published by Hugh Hefner, with only two issues produced in 1957. The first issue appeared in January 1957. The magazine's mascot was a trumpeter herald in the style of John Tenniel's Alice in Wonderland illustrations. Kurtzman began work on Trump shortly after leaving Mad following a break with its publisher William M. Gaines. Mad also lost two of its top cartoonists in the dispute's aftermath, when Will Elder and Jack Davis chose to follow Kurtzman. Wally Wood was also recruited for the Trump team in the form of an either-or option, but he chose to stay at Mad. Other notable artists, including Al Jaffee and Arnold Roth, appeared in Trump.

Production
Sales were reportedly good for Trump, especially for a new title with a 50-cent cover price, then considered high. But the project was ill-fated. The magazine featured glossy (and costly) production standards and had the misfortune of debuting at the same time that a financial crunch forced publisher Hefner to scale back his non-Playboy publishing interests. This put an end to Trump before the magazine ever got the chance to develop a steady readership.

Speaking about the magazine's short run later, Hefner said, "I gave Harvey an unlimited budget, and he exceeded it."

Aftermath
Kurtzman spearheaded additional humorous publications such as Humbug and Help!. Kurtzman and Elder continued to work at Playboy for another three decades, chiefly producing Little Annie Fanny, which made use of copious sight gags. Jack Davis became a top freelancer for advertising agencies and various magazines, including a return to Mad in 1965.

title: Trump (magazine)


text:
Jazz (disambiguation)

Jazz is a style of music and its subgenres.

Jazz may also refer to:

Computing

 * Jazz (computer), a development platform used by Microsoft
 * Jazz (mobile network operator), a mobile network operator in Pakistan
 * IBM Jazz, a team collaboration platform
 * Lotus Jazz, a spreadsheet program for Macintosh

Literature

 * Jazz (magazine) or Jazz & Pop, a 1962–1972 magazine
 * Jazz (manga), a Japanese comic book
 * Jazz (novel), a 1992 novel by Toni Morrison
 * Jazz (Henri Matisse), a book collecting pochoir prints based on cut-outs by Henri Matisse
 * Jazz (picture book), a 2006 children's book by Walter Dean Myers, illustrated by Christopher Myers
 * Jazz Magazine, a French magazine about jazz, founded 1954

Music

 * Jazz, Ltd., a Dixieland jazz band, nightclub, and record label in Chicago
 * Jazz (singer), member of Black Buddafly
 * Jazz (Ry Cooder album) (1978)
 * Jazz (John Handy album) (1962)
 * Jazz (Queen album) (1978)
 * Jazz (Wallace Roney album) (2007)
 * Jazz (Tenacious D EP) (2012)
 * Jazz (Casey Abrams album), (2019)
 * "Jazz (We've Got)", a song by A Tribe Called Quest from The Low End Theory

Sports

 * Jazz (wrestler) or Carlene Begnaud (born 1973), professional wrestler
 * Utah Jazz, NBA basketball team in Salt Lake City, Utah
 * FC Jazz, a Finnish association football team

Other uses

 * Jazz Aviation, an airline in Canada
 * Jazz (apple)
 * Jazz (design), a design introduced on disposable cups in 1992
 * Jazz (Kanso series), a 1979 series of paintings by Nabil Kanso
 * Jazz (Italian EMU), Italian Electric multiple units (EMU) used for commuter regional trains
 * Jazz (perfume), a fragrance by Yves Saint Laurent
 * Jazz (soft drink), a soda product from Pepsi-Cola
 * Jazz (TV series), a 2001 documentary produced by Ken Burns for PBS
 * Jazz (Transformers), a robot superhero character
 * Jazz (word), etymology and usage
 * Honda Jazz or Honda Fit, a hatchback
 * Pro-Design Jazz, an Austrian paraglider design

People with the name

 * Jazz Carlin (born 1990), Welsh competitive swimmer
 * Jazz Cartier (born 1993), Canadian rapper
 * Jazz Chisholm Jr. (born 1998), Bahamian baseball player
 * Jazz Gillum (1904–1966), American blues harmonica player
 * Jazz Guignard (active 1930s), Haitian jazz musician
 * Jazz Hamilton (born 1965), Puerto Rican saxophonist
 * Jazz Janewattananond (born 1995), Thai professional golfer
 * Jazz Jennings (born 2000), American LGBTQ rights activist
 * Jazz Ocampo (born 1997), Filipina actress and model
 * Jazz Raycole (born 1988), American actress and dancer
 * Jazz Richards (born 1991), Welsh footballer
 * Jazz Summers (1944–2015), British music manager
 * Jazz Tevaga (born 1995), New Zealand rugby league player

Fictional

 * Jazz Curtis, a character from Home and Away
 * Jazz Fenton, an animated character from Nickelodeon's Danny Phantom

title: Jazz (disambiguation)


text:
11 Downing Street

11 Downing Street in London, also known colloquially in the United Kingdom as Number 11, is the official residence of the Chancellor of the Exchequer (who traditionally also has the title of Second Lord of the Treasury). The residence, in Downing Street in London, was built alongside the official residence of the Prime Minister at Number 10 in 1682.

The first Chancellor to live there was Henry Petty-Fitzmaurice in 1806, but Number 11 did not become the Chancellor's official residence until 1828.

From 1997 to 2022, prime ministers Tony Blair, Gordon Brown, David Cameron, Theresa May and Boris Johnson chose to reside for all or part of their term of office in the flat above Number 11, as its residential apartment is larger than at Number 10. Rishi Sunak broke with this by resuming residence in the smaller flat above Number 10. Current Prime Minister Keir Starmer, his wife Victoria, and their two children are expected to live above Number 11.

Background
Number 11 is part of a (blackened) yellow-brick Georgian-era converted mansion. The building overlooks St. James's Park and Horse Guards Parade and consists—from left to right—of Numbers 12, 11 and 10.

Number 11 is located on the left side of Number 10, the official residence of the Prime Minister (or First Lord of the Treasury) since the early 19th century. Number 12, to the left of Number 11, is the official residence of the Chief Whip, but it is now used as the Prime Minister's press office.

As a result of many internal alterations over the years, the three terraced houses are internally a single complex; one can walk from number 11 to number 10, via an internal connecting door, without using the street doors. The Cabinet Office on Whitehall is also directly connected to these at its rear making up an executive office of the prime Minister and senior Privy Councillors.

The terraced house was one of several built by Sir George Downing between 1682 and 1684 to designs by Christopher Wren. It was altered c. 1723–35; refaced c. 1766–75 by Kenton Couse and with early C.19 alterations. Along with Number 10, it underwent a major reconstruction by Raymond Erith, 1960–64. Despite reconstruction, the interior retains a fine staircase with carved bracket tread ends and three slender turned balusters per tread. The fine Dining Room of 1825–26 is by Sir John Soane.

Recent occupancy
When Tony Blair became Prime Minister in 1997 he chose to reside in Number 11, rather than Number 10, as it has a larger living area; Blair was living with his wife and their several young children, while Gordon Brown, the Chancellor of the Exchequer, was still a bachelor. In 2007, when Brown became Prime Minister, he at first chose to live in Number 11, but soon moved back to Number 10.

Following the 2010 general election, the incoming prime minister, David Cameron, moved into 11, and George Osborne chose to remain in his Notting Hill home. In early August 2011, Osborne moved into Number 10.

Boris Johnson lived in Number 11, instead of his first and second Chancellor (Sajid Javid and Rishi Sunak, respectively). In March 2020, Johnson refurbished the residential apartment at Number 11. An Electoral Commission inquiry investigated the financing of this refurbishment. This was known in the press as the Cash-for-Curtains scandal.

Rishi Sunak broke with this by resuming residence in the smaller flat above Number 10.

title: 11 Downing Street


text:
Yevgeny Kafelnikov

Yevgeny Aleksandrovich Kafelnikov (Евгений Александрович Кафельников; born 18 February 1974) is a Russian former world No. 1 tennis player. He won two Grand Slam singles titles; the 1996 French Open and the 1999 Australian Open, and a gold medal at the 2000 Sydney Olympics. He also won four Grand Slam doubles titles, and is the most recent man to have won both the men's singles and doubles titles at the same Grand Slam tournament (which he accomplished at the 1996 French Open). In 2019, Kafelnikov was inducted into the International Tennis Hall of Fame.

Career
In his breakthrough year in 1994, Kafelnikov won three titles, reached the Hamburg Masters final and beat world top-5 players on six occasions. His ranking rose from 102 at the beginning of the year, to a year-end ranking of 11.

In 1995, he reached his first Grand Slam semifinals, beating world No. 1, Andre Agassi, in straight sets in the quarterfinals. He also defeated three top-10 players (Michael Stich, Goran Ivanisevic and Boris Becker) on his way to the title in Milan.

At the 1996 French Open, Kafelnikov became the first Russian to ever win a Grand Slam title, defeating Michael Stich in the final in straight sets, having beaten world No. 1, Pete Sampras, in the semifinals.

Kafelnikov was finalist at the 1997 ATP Tour World Championships, and won three titles during that season. In doubles, he won both the French Open and US Open partnering Daniel Vacek.

At the 1999 Australian Open, 10th seed Kafelnikov won his second singles Grand Slam title, defeating Thomas Enqvist in the final in four sets. He also won in Rotterdam and Moscow, was runner-up at the Canadian Open and reached the semifinals of the US Open.

Seeded fifth, Kafelnikov won the gold medal in the men's singles tournament at the 2000 Olympic Games, beating second seed Gustavo Kuerten in the quarterfinals and Tommy Haas in the final in five sets. He also reached the final of the Australian Open and the quarterfinals of the French Open.

In 2001, he defeated world No. 1 Gustavo Kuerten in the quarterfinals of the US Open for the loss of just seven games, before losing to Lleyton Hewitt in the semifinals. Kafelnikov was also a finalist at the Paris Masters, quarterfinalist at the Australian Open and French Open, and won a record fifth consecutive title in Moscow.

Kafelnikov won his fourth and final doubles Grand Slam at the French Open in 2002, partnering Paul Haarhuis, and his final career singles title, in Tashkent. He was also a member of Russia's Davis Cup-winning team in 2002.

Kafelnikov played his last ATP Tour match in October 2003 (in St Petersburg). In total, he won 53 titles across singles and doubles during his career, and he remains the last male player to win both singles and doubles titles at the same grand slam.

Post-retirement
Since retiring from tennis, Kafelnikov cashed three times at the 2005 World Series of Poker. He also played golf on the European Tour at the 2005, 2008, 2013, 2014 and 2015 Russian Open, 2012, 2013 and 2014 Austrian Open, and the 2014 Czech Masters, plus several Challenge Tour events, without making any cuts. During the 2008 Miami Masters, Kafelnikov coached Marat Safin (in the absence of Safin's usual coach, Hernán Gumy). In 2009 and 2010, he participated in the ATP Champions Tour (for retired ATP-professional tennis players), finishing in third place in tournaments in Chengdu, Bogotá and São Paulo.

Kafelnikov was inducted into the International Tennis Hall of Fame in 2019, not counting enough votes in his previous nominations in 2012 (compared to Gustavo Kuerten and Jennifer Capriati), in 2015 (compared to David Hall and Amélie Mauresmo), and in 2018 (compared to Michael Stich and Helena Suková). In May 2017, Kafelnikov was extensively interviewed by the most popular Russian website Sports.ru considering his current political preferences. In August 2020, Kafelnikov announced his plans to settle in [Western / old] Europe.

In March 2021, Kafelnikov faced another wave of rumors about his retirement in 2003 as really being caused by ATP's desire to avoid a betting scandal considering his match in Lyon against Fernando Vicente. Russian volleyball player Aleksey Spiridonov said in an interview: "Kafelnikov is corrupt. He made bets against himself during his career. And then he finished abruptly when being grabbed by the ass. I know. My friends work in the offices. And someone in an interview also said that Kafelnikov offered him to bet against himself and lose the match. Who pinned him down? There was no [powerful] tennis federation back then. I think, the gangsters". One week after his match in 2003, Kafelnikov said about the accusations: "This is a complete bullshit, but now in the locker room, they [players] look at me like at an enemy of the people. Even in my country where I have always been a role model they [people] have begun to look at me that way. I talked to Fernando and he said his mother was crying on the phone because of this. Those who made this mess should be punished. The article says Kafelnikov has been involved in match-fixing and it rips me to pieces. I do not want to be associated with betting in any way".

To a lesser extent, the player has been commemorated for his outspoken jealousy towards the much more impressive incomes of golf players in general, if compared to his colleagues in tennis. Lindsay Davenport correspondingly voiced some support for Kafelnikov's claim who was also worried "it would be a shame to see the public lose sympathy in me just because I am making such a statement". In January 2001, during the 2001 Australian Open, he concluded: "If you look at the golfers, we are taking an extreme example now, of course, the golfers make $540,000 a week to the winner. And this is the lowest tournament that they have on the U.S. Tour. If you look at the tennis players, to win a tournament, win five matches [at the] absolutely lowest level tournament you make only $42,000. I think it is quite bizarre to see that kind of money in a tennis game."

Singles
1Held in Stockholm till 1994, held as Stuttgart Masters from 1995 until 2001. Held as Madrid Masters from 2002 onwards.

Doubles
1Held in Stockholm till 1994, held as Stuttgart Masters from 1995 until 2001. Held as Madrid Masters from 2002 onwards.

Team titles
2002 – Davis Cup winner with Russia

2000, 2001, 2002 - World Team Cup finalist with Russia

Tennis records

 * He played exclusively with and endorsed racquets from Austrian company Fischer throughout his career.
 * In the episode of Sports Night "Shane", Dan and Jeremy spend over an hour recording and rerecording a ten-second commercial voiceover because Dan cannot say Yevgeny Kafelnikov.
 * Kafelnikov is the only male player in the open era to have won two or more Grand Slam singles titles without also winning a Masters Series title, despite having reached five Masters Series finals.
 * He won the Kremlin Cup in Moscow for a record five consecutive times from 1997 to 2001.

Other interests

 * Kafelnikov is an avid supporter of Spartak Moscow FC.
 * Kafelnikov is a professional golfer, he has won the Russian Amateur Open Championship of Golf in 2011. Note that this national tournament should not be confused with the Russian Open tournament that wasn't held that year.
 * Kafelnikov starred in Virtua Tennis, an arcade tennis game.

Awards

 * 1994–1999, 2001: The Russian Cup in the nomination Male Player of the Year
 * 2000: The Russian Cup in the nomination Male Player of the Century
 * 2002: The Russian Cup in the nomination Team of the Year (with M. Safin, M. Youzhny, S. Leonyuk, B. Sobkin, A. Cherkasov, V. Okhapkin, S. Yasnitsky, A. Glebov)

title: Yevgeny Kafelnikov


text:
Black ice

Black ice, sometimes called clear ice, is a coating of glaze ice on a surface, for example on streets or on lakes. The ice itself is not black, but visually transparent, allowing the often black road below to be seen through it and light to be transmitted. The typically low levels of noticeable ice pellets, snow, or sleet surrounding black ice means that areas of the ice are often next to invisible to drivers or people walking on it. Thus, there is a risk of slippage and subsequent accident due to the unexpected loss of traction.

Definitions
The term black ice in the United States is defined by the National Weather Service as "patchy ice on roadways or other transportation surfaces that cannot easily be seen.  It is often clear (not white) with the black road surface visible underneath. It is most prevalent during the early morning hours, especially after snowmelt on the roadways has a chance to refreeze overnight when the temperature drops below freezing. Black ice can also form when roadways are slick from rain and temperatures drop below freezing overnight."

The World Meteorological Organization definitions of black ice are:

 * A popular alternative for the term glaze. A thin sheet of ice, relatively dark in appearance, may form when light rain or drizzle falls on a road surface that is at a temperature below freezing point or, alternatively, when water already on the road surface subsequently freezes when its temperature falls below freezing point. It may also be formed when supercooled fog droplets are intercepted by buildings, fences, and vegetation.
 * Thin, new ice on freshwater or saltwater, appearing dark in color because of its transparency, a result of its columnar grain structure.

On roads and pavements
The American Meteorological Society Glossary of Meteorology includes the definition of black ice as "a thin sheet of ice, relatively dark in appearance, [that] may form when light rain or drizzle falls on a road surface that is at a temperature below 0 C." Because it represents only a thin accumulation, black ice is highly transparent and thus difficult to see compared with snow, frozen slush, or thicker ice layers. In addition, it is often interleaved with wet pavement, which is nearly identical in appearance.

This condition makes driving, cycling or walking on affected surfaces extremely dangerous. Deicing with salt (sodium chloride) is effective down to temperatures of about -18 C. Other compounds such as magnesium chloride or calcium chloride have been used for extremely cold temperatures since the freezing-point depression of their solutions is lower.

At lower temperatures (below 0 F), black ice can form on roadways when the moisture from automobile exhaust, such as motorcycles and cars, condenses on the road surface.

Such conditions caused multiple accidents in Minnesota when the temperatures dipped below that point for a prolonged period of time in mid-December 2008. Salt's ineffectiveness at melting ice at these temperatures compounds the problem. Black ice may form even when the ambient temperature is several degrees above the freezing point of water 0 C, if the air warms suddenly after a prolonged cold spell that has left the surface of the roadway well below the freezing point of water.

On December 1, 2013, heavy post-Thanksgiving weekend traffic encountered black ice on the westbound I-290 expressway in Worcester, Massachusetts. A chain reaction series of crashes resulted, involving three tractor-trailers and over 60 other vehicles. The ice formed suddenly on a long downward slope, surprising drivers coming over the crest of a hill, who could not see crashed vehicles ahead until it was too late to stop on the slick pavement.

On February 11, 2021, icing in Dallas and Fort Worth, Texas, contributed to a crash on I-35W which authorities called a "mass casualty event". Over 100 cars were involved in the pileup and multiple fatalities were reported.

On bridges
Bridges and overpasses can be especially dangerous. Black ice forms first on bridges and overpasses because air can circulate both above and below the surface of the elevated roadway when the ambient temperature drops, causing the pavement temperature on the bridge to fall rapidly.

Case study in Minnesota
The I-35W Mississippi River bridge in Minneapolis, Minnesota, was well known for its black ice before it collapsed in 2007 into the Mississippi River. It had caused several pileups during its 40-year life. On December 19, 1985, the temperature reached -34 °C. Cars crossing the bridge experienced black ice and there was a massive pile up of crashed vehicles on the bridge on the northbound side. In February and in December 1996, the bridge was identified as the single most treacherous cold-weather spot in the local freeway system, because of the almost frictionless thin layer of black ice that regularly formed when temperatures dropped below freezing. The bridge's proximity to Saint Anthony Falls contributed significantly to the icing problem and the site was noted for frequent spinouts and collisions.

By January 1999, Mn/DOT began testing magnesium chloride solutions and a mixture of magnesium chloride and a corn-processing byproduct to see whether either would reduce the black ice that appeared on the bridge during the winter months. In October 1999, the state embedded temperature-activated nozzles in the bridge deck to spray the bridge with potassium acetate solution to keep the area free of winter black ice. The system came into operation in 2000.

On water
When the temperature is below freezing and the wind is calm, such as under a high atmospheric pressure at night in the fall, a thin layer of ice will form over open water of a lake. If the depth of the body of water is large enough, its color is black and can be seen through the ice, thus the name black ice. On lakes, black ice is also commonly overlain by white ice formed from refrozen snow or slush towards spring. Black ice transmits more light than white ice which can be important for primary producers in lakes, such as phytoplankton. Black ice has a higher bearing capacity than white ice which is important to consider for ice safety reasons when doing recreational activities on lakes, such as ice skating and ice fishing.

Ice can also be formed by seawater spray and water vapour freezing upon contact with a vessel's superstructure when temperature is low enough. Ice formed in this manner is known as rime. As the formation progress, the aboveboard weight of the vessel increases and may ultimately cause capsizing. Furthermore, rime ice may impede the correct functioning of important navigational instruments on board, such as radar or radio installations. Different strategies for the removal of such ice are employed: chipping away the ice or even using fire hoses in an attempt to remove the ice.

In mountains
Black ice is a significant hazard for climbers and scramblers. Cold weather is common at high altitudes, and black ice quickly forms on rock surfaces. Loss of traction is as sudden and unexpected as on a pavement or road, but can be dangerous if the rock is in an exposed position with a drop below. An ice-axe and crampons are essential use in such circumstances as they will help to prevent a fall, and a belay rope will help to arrest a fall.

title: Black ice