Dear Readers:
Half Times
These are exactly the same concept that you have probably encountered before with radioisotopes and their decay. The loss of molecules, from 100%, is 50% [1 half time], 25% [2 half times], 12.5% [3 half times] and so forth. Addition of dissolved nitrogen from a compartment is the same. After six halftimes, the process is generally considered more or less complete.
In his initial studies of gas uptake, John Scott Haldane considered that a person would be saturated after about 30 minutes if blood flow was equal to all portions of the body. He knew, however, that blood flow was not everywhere equal. Thus, he proposed different tissue compartments, each having its own blood flow, and all being governed by this half time concept.
To explain the longest halftimes, Haldane postulated that nitrogen was being added to tissues containing considerable amounts of lipid (=fat). The longest tissues could therefore never become appreciably shorter than their initial value, no matter what the blood flow. (This does not appear to always be true.)
Supersaturation
Haldane knew that divers could surface from depths of 33 feet directly to the surface without decompression. Clearly, he thought, the nitrogen was being held in solution. That is, it was supersaturated - - and apparently, that supersaturation was stable. Bubbles would not form.
The other possibility was that only a very few gas bubbles were forming, and that these were essentially harmless if few in number. Haldane had no way of knowing that the second possibility was closer to the truth, since Doppler bubble detectors were three-quarters of a century into the future.
The permitted supersaturation was initially thought by Haldane to be almost 2:1 - - the supersaturation from 33 fsw [2 atm] to the surface [1 atm]. Haldane believed the allowed supersaturation was the same for all halftime tissues. As tables were extended to deeper depths and longer times, this was modified to allow bigger supersaturations for small halftime comportments (fast tissues), and less supersaturation for slow tissues.
M Values
To allow the calculation of decompression tables, Dr Robert Workman (US Navy) developed a table of allowed supersaturations for each different halftime compartment and for each different depth of the decompression stops. These were the maximum pressures tolerated, and they were termed M values.
Almost all algorithms (= calculation methods) will employ M values as a concept somewhere. This concept implies that no bubbles are formed if the allowable supersaturations are never exceeded. This concept is not correct as demonstrated by Doppler bubble detect about thirty years ago. Bubbles indeed do form even in the absence of decompression sickness. These are the silent bubbles often referred to in the diving literature. Two phase decompression systems do not use the M value idea.
Halftimes and Repetitive Diving
The No-Decompression Limits are determined experimentally for each halftime. For recreational diving, they determine the maximum allowed no decompression times for each depth. The surface intervals and length of repetitive dives are governed by the gas washout halftime. There are different methods for determining the maximum dive time after a given surface interval. The method chosen by the US Navy is the Repetitive Dive Table with its letter groupings using the length of the surface interval. For maximum efficiency, it is necessary to determine what compartment is nearest to its maximum and use that halftime to determine the limit of the succeeding dive. In practice, it is too complicated and would need dive tables printed for each compartment. The US Navy simply chose to use the 120 minute tissue for off gassing since this would assure that all compartments that were shorter would be sufficiently desaturated. This would always work for all cases of decompression diving.
Recreational Repetitive Diving
In the case of recreational diving, decompression diving is not performed and the 120-minute compartment is too long. Ray Rogers thus noted that recreational divers would be better served by repetitive tables based on a 60-minute off gassing compartment. The PADI system was calculated on such a system, and the tests were performed to validate this concept.
Dive computers can calculate theoretical gas loads in all compartments and bypass the need for Repetitive Dive Tables. The calculation system (half times and maximum allowable pressures) is, however, the same. These calculation systems function only to track the gas loads and serve as a map for the partial pressures in your body. Tables do not ALLOW you to perform dives. They simply track and catalog the dissolved nitrogen. Only God and Mother Nature allow you to perform the dives. This is governed by the laws of physics.
Dr Deco :doctor:
