Cited Passage

4-2 NOAA Diving Manual hypothetical gas loadings have “decayed” to below the limits for that depth; the diver then ascends to the next stop and the process is repeated. The J.S. Haldane decompression model goes back nearly a century, but by using it with continuously updat- ed experience, it can be used to produce reliable decom- pression tables. It is not quite correct to consider this a ‘theory” of how the human body works. Rather, it is a computational tool that allows prediction of tomorrow’s dive from yesterday’s experience. The A.A. Bühlmann model uses the same equations for on-gassing, but calcu- lates the ascent limits in a different way; it, too, is firmly based on experience. 4.1.3 Reliability of Tables Virtually any exposure to pressure imposes an obliga- tion for decompression, and even when it is done correct- ly it will create some probability of symptoms of a decom- pression disorder. For this reason, it is preferable not to use the word “safe” to describe a decompression proce- dure. In the sense that “safe” means “an acceptable level of risk” the word may be applicable, but too many people perceive that as meaning no risk at all, which is not the case. Satisfactory decompression tables can be referred to as being “reliable.” The NOAA and U.S. Navy Dive Tables in this manual are considered reliable. The limits of a decompression procedure do not rep- resent a hard line between developing or not developing decompression sickness symptoms, but rather a fuzzy boundary of “acceptable risk.” Accordingly, one should always consider DCS as a possibility. Be prepared for it psychologically and have a plan for dealing with it. There is a wide variation in the physical makeup of divers, and part of this variation is in susceptibility to decompression disorders. There are differences among individuals, and in one person at different times. There are also environmental effects. Immersion, exercise, and warmth increase gas uptake and elimination; cold and dehydration reduce them. Depending on where in the dive these conditions occur, they may be either beneficial or detrimental to the decompression. Because of the variations, a given schedule is not “safe” or “unsafe;” rather, DCS has a certain probability of occurring, therefore, decompression data is analyzed statistically. The U.S. Navy and other decompression researchers have developed means of analyzing past dives using statistics, one form of which is called “maximum likelihood.” With it, the probability of DCS can be pre- dicted from a given profile by comparing it with a collec- tion of past dives of the same general type. 4.1.4 Dive Planning Software In recent years there has been a remarkable develop- ment in the field of decompression technology; the development and marketing of commercially available computer programs for generating decompression tables. For decades it has been felt that only decompression specialists were qualified to produce decompression tables, and that all tables needed extensive testing before operational use. Several entrepreneurs have prepared and distributed computer programs that can be used to generate decom- pression tables. This has been possible because of publica- tions by Prof. Bühlmann that outline tested and accepted algorithms for computing tables. Most programs available are based, at least fundamentally, on Prof. Bühlmann’s algorithms. The different programs manage the algorithm in dif- ferent ways, especially with regard to introducing extra conservatism into the computations.

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