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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Decompression theory | 12/17 | https://en.wikipedia.org/wiki/Decompression_theory | reference | science, encyclopedia | 2026-05-05T10:06:49.112339+00:00 | kb-cron |
For example, a diver ascends from a maximum depth of 60 metres (200 ft), where the ambient pressure is 7 bars (100 psi), to a decompression stop at 20 metres (66 ft), where the pressure is 3 bars (40 psi). The first Pyle stop would take place at the halfway pressure, which is 5 bars (70 psi) corresponding to a depth of 40 metres (130 ft). The second Pyle stop would be at 30 metres (98 ft). A third would be at 25 metres (82 ft) which is less than 9 metres (30 ft) below the first required stop, and therefore is omitted. The value and safety of deep stops additional to the decompression schedule derived from a decompression algorithm is unclear. Decompression experts have pointed out that deep stops are likely to be made at depths where ingassing continues for some slow tissues, and that the addition of deep stops of any kind should be included in the hyperbaric exposure for which the decompression schedule is computed, and not added afterwards, so that such ingassing of slower tissues can be taken into account. Deep stops performed during a dive where the decompression is calculated in real-time are simply part of a multi-level dive to the computer, and add no risk beyond that which is inherent in the algorithm. There is a limit to how deep a "deep stop" can be. Some off-gassing must take place, and continued on-gassing should be minimised for acceptably effective decompression. The "deepest possible decompression stop" for a given profile can be defined as the depth where the gas loading for the leading compartment crosses the ambient pressure line. This is not a useful stop depth - some excess in tissue gas concentration is necessary to drive the outgassing diffusion, however this depth is a useful indicator of the beginning of the decompression zone, in which ascent rate is part of the planned decompression. A study by DAN in 2004 found that the incidence of high-grade bubbles could be reduced to zero providing the nitrogen concentration of the most saturated tissue was kept below 80 percent of the allowed M value and that an added deep stop was a simple and practical way of doing this, while retaining the original ascent rate.
==== Diffusion limited tissues and the "Tissue slab", and series models ====
The assumption that diffusion is the limiting mechanism of dissolved gas transport in the tissues results in a rather different tissue compartment model. In this case a series of compartments has been postulated, with perfusion transport into one compartment, and diffusion between the compartments, which for simplicity are arranged in series, so that for the generalised compartment, diffusion is to and from only the two adjacent compartments on opposite sides, and the limit cases are the first compartment where the gas is supplied and removed via perfusion, and the end of the line, where there is only one neighbouring compartment. The simplest series model is a single compartment, and this can be further reduced to a one-dimensional "tissue slab" model.
==== Bubble models ==== Bubble decompression models are a rule based approach to calculating decompression based on the idea that microscopic bubble nuclei always exist in water and tissues that contain water and that by predicting and controlling the bubble growth, one can avoid decompression sickness. Most of the bubble models assume that bubbles will form during decompression, and that mixed phase gas elimination occurs, which is slower than dissolved phase elimination. Bubble models tend to have deeper first stops to get rid of more dissolved gas at a lower supersaturation to reduce the total bubble phase volume, and potentially reduce the time required at shallower depths to eliminate bubbles. Decompression models that assume mixed phase gas elimination include:
The arterial bubble decompression model of the French Tables du Ministère du Travail 1992 The U.S. Navy Exponential-Linear (Thalmann) algorithm used for the 2008 US Navy air decompression tables (among others) Hennessy's combined perfusion/diffusion model of the BSAC'88 tables The Varying Permeability Model (VPM) developed by D.E. Yount and Hoffman (1986) at the University of Hawaii The Reduced Gradient Bubble Model (RGBM) developed by Bruce Wienke in 1990 at Los Alamos National Laboratory Michael Gernhardt proposed the Tissue Bubble Dynamics Model (1991) Wayne Gerth and Richard Vann (1997) published the Probabilistic Gas and Bubble Dynamics Model. Lewis and Crow introduced their Gas Formation Model (GFM) in 2008. The Copernicus model of Gutvik and Brubakk (2009) The most widely implemented model in dive computers is a simplified modification of the RGBM. The models of Yount and Hoffman, and Wienke, assume that bubble formation is due to supersaturation, while Gernhardt, Gerth and Vann, and Gutvik and Brubakk assume pre-existing microscopic bubble nuclei, which grow when concentration of gases in the tissues is high enough. These models are more mathematically complex, and as of 2009 were unsuitable for real-time computation by dive computer.
==== Goldman Interconnected Compartment Model ====