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Ratio decompression
Ratio decompression (usually referred to in abbreviated form as ratio deco) is a technique for calculating decompression schedules for scuba divers engaged in deep diving without using dive tables, decompression software or a dive computer. It is generally taught as part of the "DIR" philosophy of diving promoted by organisations such Global Underwater Explorers (GUE) and Unified Team Diving (UTD) at the advanced technical diving level. It is designed for decompression diving executed deeper than standard recreational diving depth limits using trimix as a "bottom mix" breathing gas.
==Theory==
The physiology behind the off-gassing of nitrogen or helium absorbed by the body from breathing gases under pressure has never been definitively established, particularly in relation to the formation of bubbles in the body's tissues,〔In the article, (Diving physics and "fizzyology" ), under "Decompression", the basis for decompression inevitably comes back to "we don't really know."〕 and a number of different algorithms have been developed over the years, based on simplified hypotheses of gas transport and absorption in body tissues, modified to fit empirical data, to predict the rate of off-gassing to reduce the risk of decompression sickness in divers to an acceptable level. However, these models do not describe the individual physiology of the diver accurately: divers have been known to suffer symptomatic decompression sickness whilst diving within the limits of dive tables or dive computers (sometimes referred to as an "undeserved hit"), and divers have exceeded No Decompression Limits but remained asymptomatic.
While Ratio Decompression is not a complete decompression model, it most resembles those of Bühlmann algorithm, and the Varying Permeability Model algorithm, with emphasis on the use of deep stops and gradient factors.
A conventional decompression profile, based on a dissolved gas model algorithm, will result in a diver ascending relatively quickly through shorter deep stops before spending a great deal of time at the shallower stops (resulting in a much sharper angle in the depth/time graph of the ascent profile), ratio deco will allow a diver to dynamically take a total decompression obligation for a given dive and create a profile which makes better use the most effective parts of the decompression profile, and spends comparatively less time at the less effective stops (resulting in a much softer curve in the depth/time graph of the ascent profile).
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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