PLONGEURS AU PALIER DE DECOMPRESSION
En plongée sous-marine, un palier de décompression est le temps que l'on passe à une profondeur donnée afin de réduire le taux d'azote ou d'hélium restant dans les tissus humains (sang notamment). Il entre dans le cadre des procédures de décompression en prévention des accidents de décompression (ADD). Sa profondeur et son temps sont donnés par les tables de décompression ou un ordinateur de plongée. La profondeur à laquelle il est effectué et son temps varient en fonction de la profondeur atteinte et du temps passé sous l'eau (dont dépendent la saturation du sang en gaz). Il est fréquent d'effectuer le ou les paliers sur la ligne de remontée, généralement le mouillage d'un navire, ils peuvent cependant se faire en pleine eau (à la dérive) ou en remontant collé au fond (cas des plongées depuis le bord). Il est d'usage, notamment en France, d'éviter de plonger avec des paliers dans le cadre de la plongée loisir, et en tout état de cause, en fin de plongée, d'effectuer un palier de principe de cinq minutes à cinq mètres de fond. Ce palier se justifiait anciennement pour corriger des remontées trop rapides (avant l'avènement des profondimètres électroniques, l'estimation de la vitesse était imprécise). Néanmoins, lors de plongées profondes (épaves, grottes, ou tentatives de records), des paliers profonds et longs sont nécessaires. Ils sont alors souvent effectués avec des gaz (trimix, Nitrox, Oxygène pur) dont les proportions ont été calculées au préalable afin d'être le plus efficaces possible pour une plage de profondeur.
The decompression of a diver is the reduction in ambient pressure experienced by the diver during ascent, and also the process of elimination of dissolved inert gases from the diver's body, which occurs both during the actual ascent, during pauses in the ascent known as decompression stops, and after surfacing, until either the gas concentrations reach equilibrium, or another dive is started. When a diver descends in the water the hydrostatic pressure, and therefore the ambient pressure, rises. Breathing gas is supplied at the same pressure as the surrounding water, and some of this gas dissolves into the diver's blood, and is transferred by the blood circulation to other tissues. Inert gas continues to be taken up until the gas dissolved in the diver is in a state of equilibrium with the breathing gas in the diver's lungs, (see: "Saturation diving"), or the diver moves up in the water and reduces the ambient pressure of the breathing gas until the inert gases dissolved in the tissues are at a higher concentration than the equilibrium state, and start diffusing out again. Divers breathing gas at high pressure may need to do decompression stops, but a diver who only breathes gas at atmospheric pressure when free-diving, does not usually need to do decompression stops. However, it is possible to get taravana, thought to be a form of decompression sickness, from repetitive deep free-diving with short surface intervals. Divers who only use a snorkel near the surface or use an atmospheric diving suit do not need to decompress. If the ambient pressure is reduced too quickly and to a pressure sufficiently low compared to the total concentration of dissolved gas, the dissolved inert gases such as nitrogen or helium can form bubbles in the blood and tissues of the diver in a manner similar to the fizzing of a carbonated beverage when opened.The situation is similar when a diver or hyperbaric worker is compressed and decompressed in a diving chamber or airlock. The symptoms of decompression sickness are known to be caused by damage resulting from the formation and growth of bubbles of inert gas within the tissues and by blockage of arterial blood supply to tissues by gas bubbles and other emboli consequential to bubble formation and tissue damage. The precise mechanisms of bubble formation and the damage they cause has been the subject of medical research for a considerable time and several hypotheses have been advanced and tested. Tables and algorithms for predicting the outcome of decompression schedules for specified hyperbaric exposures have been proposed, tested, and used, and usually found to be of some use but not entirely reliable. Decompression remains a procedure with some risk, but this has been reduced and is generally considered acceptable for dives within the well tested range of commercial, military and recreational diving. The immediate goal of controlled decompression is to avoid development of symptoms of bubble formation in the tissues of the diver, and the long term goal is to also avoid complications due to sub-clinical decompression injury. During effective decompression, the asymptomatic venous microbubbles present after most dives are eliminated from the diver's body in the alveolar capillary beds of the lungs. If they are not given enough time, or more bubbles are created than can be eliminated safely, the bubbles grow in size and number causing the symptoms and injuries of decompression sickness. Decompression may be continuous or staged, where the ascent is interrupted by stops at regular depth intervals, but the entire ascent is part of the decompression, and ascent rate can be critical to harmless elimination of inert gas. What is commonly known as no-decompression diving, or more accurately no-stop decompression, relies on limiting ascent rate for avoidance of excessive bubble formation. When diving with nitrogen-based breathing gases, decompression stops are typically carried out in the 3 to 21 metres (10 to 70 ft) depth range. With helium-based breathing gases the stop depths may start deeper. The period at surface pressure immediately after dives is also an important part of decompression and can be thought of as the last decompression stop of a dive. It typically takes up to 24 hours for the body to return to its normal atmospheric levels of inert gas saturation after a dive. When time is spent on the surface between dives this is known as the "surface interval" and is considered when calculating decompression requirements of the subsequent dive.