decompression illness follows schedules laid out in treatment tables. Most treatments recompress to absolute, the equivalent of of water, for 4.5 to 5.5 hours with the casualty breathing pure oxygen, but taking periodic air breaks to reduce oxygen toxicity. For serious cases resulting from very deep dives, the treatment may require a chamber capable of a maximum pressure of , the equivalent of of water, and the ability to supply heliox and nitrox as a breathing gas. Surface decompression Surface decompression is a procedure in which some or all of the staged decompression obligation is done in a decompression chamber instead of in the water. This reduces the time that the diver spends in the water, exposed to environmental hazards such as cold water or currents, which will enhance diver safety. The decompression in the chamber is more controlled, in a more comfortable environment, and oxygen can be used at greater partial pressure as there is no risk of drowning and a lower risk of oxygen toxicity convulsions. A further operational advantage is that once the divers are in the chamber, new divers can be supplied from the diving panel, and the operations can continue with less delay. A typical surface decompression procedure is described in the US Navy Diving Manual. If there is no in-water 40 ft stop required the diver is surfaced directly. Otherwise, all required decompression up to and including the 40 ft (12 m) stop is completed in-water. The diver is then surfaced and pressurised in a chamber to 50 fsw (15 msw) within 5 minutes of leaving 40 ft depth in the water. If this "surface interval" from 40 ft in the water to 50 fsw in the chamber exceeds 5 minutes, a penalty is incurred, as this indicates a higher risk of DCS symptoms developing, so longer decompression is required. In the case where the diver is successfully recompressed within the nominal interval, he will be decompressed according to the schedule in the air decompression tables for surface decompression, preferably on oxygen, which is used from 50 fsw (15 msw), a partial pressure of 2.5 bar. The duration of the 50 fsw stop is 15 minutes for the Revision 6 tables. The chamber is then decompressed to 40 fsw (12 msw) for the next stage of up to 4 periods of 30 minutes each on oxygen. A stop may also be done at 30