To prevent or minimize decompression sickness, divers must properly plan, conduct, and monitor decompression. Divers follow a decompression model to allow the release of excess inert gases dissolved in their body tissues at acceptable risk, which accumulated as a result of breathing at ambient pressures greater than surface atmospheric pressure. Decompression models take into account variables such as depth and time of dive, breathing gasses, altitude, and equipment to develop appropriate procedures for safe ascent. Decompression may be continuous or staged, where the ascent is interrupted by stops at regular depth intervals, but the entire ascent is part of the decompression, and ascent rate can be critical to harmless elimination of inert gas. What is commonly known as no-decompression diving, or more accurately no-stop decompression, relies on limiting ascent rate for avoidance of excessive bubble formation. Staged decompression may include deep stops depending on the theoretical model used for calculating the ascent schedule. Omission of decompression theoretically required for a dive profile exposes the diver to significantly higher risk of symptomatic decompression sickness, and in severe cases, serious injury or death. The risk is related to the severity of exposure and the level of supersaturation of tissues in the diver. Procedures for emergency management of omitted decompression and symptomatic decompression sickness have been published. These procedures are generally effective, but vary in effectiveness from case to case. The procedures used for decompression depend on the mode of diving, the available equipment, the site and environment, and the actual dive profile. Standardized procedures have been developed which provide an acceptable level of risk in the circumstances for which they are appropriate. Different sets of procedures are used by commercial, military, scientific and recreational divers, though there is considerable overlap where similar equipment is used, and some concepts are common to all decompression procedures. In particular, all types of surface oriented diving benefited significantly from the acceptance of personal dive computers in the 1990s, which facilitated decompression practice and allowed more complex dive profiles at acceptable levels of risk.
Decompression
Decompression in the context of diving derives from the reduction in ambient pressure experienced by the diver during the ascent at the end of a dive or hyperbaric exposure and refers to both the reduction in pressure and the process of allowing dissolved inert gases to be eliminated from the tissues during this reduction in pressure. When a diver descends in the water column the ambient pressure rises. Breathing gas is supplied at the same pressure as the surrounding water, and some of this gas dissolves into the diver's blood and other fluids. Inert gas continues to be taken up until the gas dissolved in the diver is in a state of equilibrium with the breathing gas in the diver's lungs, (see: "Saturation diving"), or the diver moves up in the water column and reduces the ambient pressure of the breathing gas until the inert gases dissolved in the tissues are at a higher concentration than the equilibrium state, and start diffusing out again. Dissolved inert gases such as nitrogen or helium can form bubbles in the blood and tissues of the diver if the partial pressures of the dissolved gases in the diver gets too high above the ambient pressure. These bubbles and products of injury caused by the bubbles can cause damage to tissues known as decompression sickness, or "the bends". The immediate goal of controlled decompression is to avoid development of symptoms of bubble formation in the tissues of the diver, and the long-term goal is to also avoid complications due to sub-clinical decompression injury. A diver who exceeds the no-decompression limit for a decompression algorithm or table has a theoretical tissue gas loading which is considered likely to cause symptomatic bubble formation unless the ascent follows a decompression schedule, and is said to have a decompression obligation.
Common procedures
Descent rate Descent rate is generally allowed for in decompression planning by assuming a maximum descent rate specified in the instructions for the use of the tables, but it is not critical. Descent slower than the nominal rate reduces useful bottom time, but has no other adverse effect. Descent faster than the specified maximum will expose the diver to greater ingassing rate earlier in the dive, and the bottom time must be reduced accordingly. In the case of real-time monitoring by dive computer, descent rate is not specified, as the consequences are automatically accounted for by the programmed algorithm.
Bottom time Bottom time is the time spent at depth before starting the ascent. Bottom time used for decompression planning may be defined differently depending on the tables or algorithm used. It may include descent time, but not in all cases. It is important to check how bottom time is defined for the tables before they are used. For example, tables using Bühlmann's algorithm define bottom time as the elapsed time between leaving the surface and the start of the final ascent at 10 metres per minute, and if the ascent rate is slower, then the excess of the ascent time to the first required decompression stop needs to be considered part of the bottom time for the tables to remain safe.
Ascent rate
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