Multilevel Diving Theory: Multilevel diving is a technique for extending your bottom time beyond the no decompression limit (a.k.a ‘no stop’ limit) of the deepest depth you reach. You accomplish this by ascending to shallower levels during the dive, as you ascend; your body absorbs nitrogen more slowly than if you remained at the deepest depth for the entire dive. Because you absorb nitrogen more slowly, you have more time available within the no decompression limits.
Multilevel diving-whether you use a computer, the dive wheel or both- draws directly upon decompression theory, so a rudimentary understanding of this theory helps you understand some of the limitations of multilevel diving and other dive practices. You’ll find that guidelines you use with dive computers and tables come from what we know and don’t know-about decompression.
No Decompression Limits:Reviewing what you know from your PADI Open Water Diver course, your body absorbs nitrogen from the air that you breathe during a dive. The deeper you dive, the faster you absorb nitrogen and the longer you dive, the more nitrogen you absorb. Your body tolerates a certain amount of excess nitrogen left over from a dive when you surface without developing decompression sickness; dive tables and computers track theoretical nitrogen absorption to keep your bodies nitrogen within tolerable limits. (note that in these discussions, we’re concerned with decompression sickness, rather than the more broadly defined decompression illness. See the deep diving section if you’re not familiar with the distinction)
Interestingly, the ‘tolerable limits’-the no decompression limits for recreational divers – are the starting point for decompression theory. No theory of human physiology or decompression alone can predict the no decompression limits. They’re determined only one way: through the actual results of human dives, preferably test dives.
Since no decompression limits are determined through the results of actual dives, if you limited the results of actual dives, if you limited your diving to one single depth dive in a day, you wouldn’t need a decompression theory. You would just memorize the limits established by successful dive results.
But you don’t want to make only one dive, you want to multilevel dive and you want to be ready for emergency decompression, just in case. Repetitive diving, multilevel diving and emergency decompression theory have too many variables to test all the possible combinations of dives, levels and surface intervals, so physiologists use mathematical decompression models to apply test results to this multitude of diving variables. Virtually all recreational dive computers and dive tables grew from various modifications of a decompression model published by physiologist John Scott Haldane in 1908.
Briefly, a decompression model works by mathematically predicting how much nitrogen the human body absorbs during a specific dive. As dynamic and useful as decompression models are, however, physiologists have learned that decompression theory is imperfect, and that decompression models can predict as ‘safe’ dive profiles that might not be so safe. For that reason, you can only rely on decompression theory to produce an acceptable risk of decompression sickness as far as it has been successfully tested. Even then, people vary in their physiology and susceptibility to decompression sickness. No dive table or computer can guarantee that decompression sickness will not occur, even when diving within the table or computer limits. This is one reason why it’s important to dive conservatively, well within table or computer no stop limits.
Mounting evidence suggests that repetitive deep dives produce an unacceptable high rate of decompression sickness, despite the predictions of mathematical models. For this reason, plan your repetitive dives deeper no deeper than 30 meters/100ft, regardless of what your dive table or computer might say it permits.
Tables and Multilevel Diving:Obviously, you can use dive computers for multilevel diving-that’s perhaps their prime function. Computers calculate your exact dive profile and apply a decompression model to write a custom dive table for your dive. On the other hand, you can’t use conventional dive tables for multilevel diving, even by ‘interpolating’ repetitive groups; because doing so can permit dives beyond what human dives show to work. A second concern is that attempting to calculate multilevel dives with conventional tables is at best tedious and at worst complex and error-prone. Not a good place to have an ‘oops’
The Wheel differs, though, because it was designed and tested with multilevel diving in mind. It keeps your dive plan within accepted limits, and its designed simplifies planning.
Ascent Procedures
There are two potentially hazardous conditions related to a diver’s ascent that you’re already familiar with from your Open Water diver course; lung overexpansion injuries and decompression sickness. Ascent recommendations help you avoid these. So far as decompression theory is concerned, an ascent procedure consists of three parts; 1) no decompression limit, 2) rate of ascent and 3) safety stop.
No Decompression Limit
From a theoretical point of view, the no decompression limit dictates when you’ll start to ascend. This component of ascent has no bearing on lung over expansion injuries.
Rate
The ascent rate for The Wheel has been established at a maximum of 18 meters/60 feet per minute based on human tests, though some computers specify slower rates. Ascend no faster than 18 meters/60 feet or per minute or at a rate prescribed by the table or computer you are using whichever is slower. Using The Wheel (or table) RDP, you may ascend slower than 18 meters/60 feet per minute.
Safety Stop
A safety stop is a 3 minute pause at the 5 meter/15 foot level. Safety stops have been tested to a limited degree, and show significant benefit in reducing the probability of decompression sickness. When analyzed mathematically with a decompression model, the safety stop theoretically also produces a significant reduction in absorbed nitrogen. A safety stop at 5 meters/15 feet also allows you a moment to double check your depth and time information. In addition, the stop gives you a moment to readjust your buoyancy, so it may help prevent runaway ascents through the last few meters/several feet of water, and thereby minimize the possibility of lung over expansion injuries. It’s with these reasons in mind that you want to make safety stops on virtually all dives.
PADI 5 Star National Geographic Instructor Development Center. 198/12 Rat-U-Thit Road, Patong, Phuket, Thailand. Phone: (+66) 076292052 Fax: (+66) 076293034
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