Saturation diving
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Saturation diving is an ambient pressure diving technique which allows a diver to remain at working depth for extended periods during which the body tissues become saturated with metabolically inert gas from the breathing gas mixture. Once saturated, the time required for decompression to surface pressure will not increase with longer exposure. The diver undergoes a single decompression to surface pressure at the end of the exposure of several days to weeks duration. The ratio of productive working time at depth to unproductive decompression time is thereby increased, and the health risk to the diver incurred by decompression is minimised. Unlike other ambient pressure diving, the saturation diver is only exposed to external ambient pressure while at diving depth. The extreme exposures common in saturation diving make the physiological effects of ambient pressure diving more pronounced, and they tend to have more significant effects on the divers' safety, health, and general well-being. Several short and long term physiological effects of ambient pressure diving must be managed, including decompression stress, high pressure nervous syndrome (HPNS), compression arthralgia, dysbaric osteonecrosis, oxygen toxicity, inert gas narcosis, high work of breathing, and disruption of thermal balance. Most saturation diving procedures are common to all surface-supplied diving, but there are some which are specific to the use of a closed bell, the restrictions of excursion limits, and the use of saturation decompression. Surface saturation systems transport the divers to the worksite in a closed bell, use surface-supplied diving equipment, and are usually installed on an offshore platform or dynamically positioned diving support vessel. Divers operating from underwater habitats may use surface-supplied equipment from the habitat or scuba equipment, and access the water through a wet porch, but will usually have to surface in a closed bell, unless the habitat includes a decompression chamber. The life support systems provide breathing gas, climate control, and sanitation for the personnel under pressure, in the accommodation and in the bell and the water. There are also communications, fire suppression and other emergency services. Bell services are provided via the bell umbilical and distributed to divers through excursion umbilicals. Life support systems for emergency evacuation are independent of the accommodation system as they must travel with the evacuation module. Saturation diving is a specialized mode 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.