Multilevel 1.9 PDA

4. How to plan Multi-Level Dives

But how can complex dives like extreme exposure cave dives be conducted safely? Most important is the fact, that the gas management has to be very conservative. Some might think "no problem, all important gases have to be redundant, gas management relies on the Third's Rule and off we go!" But be careful, it isn't always that simple! The Third's Rule is a basic cave diving principle in areas like Florida and Yucatan, where the cave divers are able to work together as a team under excellent conditions. In an emergency situation even the total loss of gas of one of the divers involved can be compensated by his buddies who will do their best to provide him with gas via their long-hose.

However, this gas management rule changes completely for solo-dives which are often conducted when sumps are heavily silted and a communication between buddies is not possible. In this case, the diver is forced to rely completely on himself. Therefore he can not use the Third's Rule in a comletely submerged cave, using independent doubles. Otherwise a worst case szenario may happen like the malfunction of one of his breathing systems just at the turn of the dive, where one third of his gas supply has been consumed. Now exactly the same amount of gas, that has already been used for the way in, is left for the exit. But on the exit the diver is confronted with bad visibility and the higher respiratory minute volume (due to the high stress level).
Obviously, his chances to make it back are not very promising.

Therefore he should have either used a Quarter's Rule or - better - the Third's Rule with three independent tanks. The third tank can be attached to the body or staged along the way.

Using Multilevel, the gas consumption can be excellently evaluated. The outcome of a breathing system malfunction can be calculated during all stages of the dive.

How shall the real dive profile be transformed into a multi-level dive profile, that can be processed by a decompression program? It is quite clear, that by reducing the profile to only a few levels, I will get an overly conservative and time-consuming deco schedule, but the in-water handling is very simple. However, by using a dive schedule with a lot more levels, I will get a shorter decompression time at the cost of a very complex schedule which adds to the task-load.

Here is an example of a real mixed-gas cave dive (datatrak curve):

In the above case the cave diver opted to split the real depth/time curve in 3 levels, which can be easily dived. And here is the 6-level solution:

Only an underwater mixed-gas computer can do a better job. But even then, the dive has to preplanned, using a powerful decompression software.

Multilevel calculates true rectangular profiles. This means that descent and ascent have to be entered separately or treated as part of the time at a bottom level, which shortens the true bottom time. Whichever way you choose, you can tailor the profile handling to your personal needs .

The safety factor of Multilevel is being created by a pure reduction of the tissue tolerance (figure a). In case you are aware of  risk factors like cold or physical exertion I suggest you increase the inert gas fraction by approximately 10 % during decompression.

A safety factor of 0 % without additional features like deep stops or reduced Helium tolerance is equivalent to a pure Bühlmann ZH-L16B algorithm. By reducing decompression time by 4 - 5% the first type I DCS - symptoms can be experienced by some individuals.