When I worked on Multilevel I had a simple thing in mind: the usage of this software had to be kept as simple as possible and the access to important parameters had to be as quick as possible - very much like in the cockpit of an airoplane, where the pilot has to be able to obtain the essential data in a very short period of time. However, the software was to incorporate powerful tools for reliable planning of even very complex mixed-gas dives. A further emphasis was put on the ability to reveal the mystery that is normally associated with decompression algorithms - to enable the diver to understand what is going on in the "deco model black-box".
Therefore I decided to display additional information like inert-gas partial pressures and individual tissue-ceilings. By doing so, the user is able to take a look behind the theoretical foundations of the decompression-algorithm and can decide for himself, wether he trusts in a simple mathematical model or not.
The concept behind Multilevel is different from nearly every
deco program on the market.
It is a dynamic rather than the usual static approach - much like a real dive computer works. During each stage of a dive, the diver can experiment with different settings and immediately learn, how a modified gas mixture affects the decompression.
Important functions like gas calculation, ceiling and tissue status are incorporated. Multilevel relies on a modification of the famous ZH-L16B algorithm by Prof. Bühlmann (ZH-L17BTS).However, the user can still calculate decompression schedules by using the basic algorithm, if intended. Therefore he only has to switch off the low helium tissue tolerance and the deep stop function. However, empirical testing has without doubt demonstrated the benefit of deep stops to the diver's health.
I only want to briefly mention that deep stops in Multilevel are generated using a tissue No. 0 with a low inert gas tolerance and a half-time of only 2 min. This comes very close to the empirical concept of Richard Pyle.
The safety factor of Multilevel (preset 30%) is generated by diminishig the famous coefficient a, which is responsible for the calculation of the tissue tolerance. The preset of 30% was chosen to reproduce the old Bühlmann tables which incorporate a lot of empirical testing. Only tissue tolerance is used to add safety. On- and offgassing is not affected. No such thing as a so called "adaptive algorithm" is used. In modern deco computers "adaptive algorithms" are used, to simulate, how offgassing is retarded during the final part of the dive. This is based on the assumption that physical exercise and body temperature are normally at its highest level during the first half of the dive, while during deco a diver is normally at rest and the body temperature drops considerably, thus leading to a prolonged period of time for the inert gas elimination. This process is normally simulated using prolonged tissue half-times for tissues like muscles and skin. However, even by the introduction of "adaptive algorithms" the number of DCI has not been diminished significantly. Mostly, the effect of "adaptive models" is resctricted to a considerable increase in shallow stop time, while observations during extreme exposure dives indicate that deep stops are a much better choice (to quote Georg Irvine: "They bend you by not doing the deep stops and then treat you by extending the shallow stops"). Apart from a lot of missing scientific data, regarding "adaptive models", many software programmers like to incorporate a bunch of "workload-modifiers", "temperature-factors" and so forth, having only a faint correlation with real-life situations but a very strong correlation with the programmer`s need to incorporate fancy but unproven gimmicks. But it is your choice: if you think that "adaptive algorithms" are a state-of-the-art thing you can roughly reproduce the good old Aladin deco with Multilevel by simply increasing the inert-gas percentage during the final stage of the dive by at least 10 % (compared to what your true inert-gas percentage during the dive would be).
3.2 General overview on the functions
The fundamental concept of Multilevel is based on the reduction of the complex real-life dive profile to square profiles - so called levels. This means that the depth/time curve of a real dive is split into several smaller levels, the number of which is depending on the need of the individual diver.
While small intervals come very close to the real dive profile, they lead to much more in-water taskload. Greater intervals lead to a much more simplified dive schedule but have the important disadvantage of an increased deco penalty. A more detailed discussion of multi-level dive schedules , including many practical examples can be found in chapter 4.
All of the three classic dive gases (oxygen, helium and nitrogen)
be used. It is sufficient to only enter the percentage of helium and
since the oxygen percentage is automatically determined.
The important modules of Multilevel can be seen by taking a closer look at the graphical user interface (GUI). At the top you can enter the basic paramaters of every dive - depth and time of a particular level, followed by the safety factor (preset 30%, leads to a linear reduction of factor a for all 17 tissues) and the altitude above sealevel (preset 0).
Below you find the percentage of nitrogen and helium (preset is compressed air: nitrogen 79%, helium 0%).
To the right the three function buttons (explained below) and the current level and the runtime are shown.
Now you can choose between four basic modes by clicking on the mode
rollup window (down to the right): Tissue-, Profile-,
Mix and Passive RB-mode. When beginning a new dive you have to be in the Tissue-mode (you can't dive new levels or reset Multilevel while operating in Profile- or Mix-mode).
I assume, you are eager to calculate your first dive profile with
so here we go:
Let us just simulate a "normal" open water dive, using compressed air.
Enter a very straightforeward profile, e.g. a depth of 40 m and a dive time of 20 min. Altitude above sealevel is still 0 m. We are fine with the nitrogen preset (79 %), so all you have to do (you may have guessed that it can be done that simple) is to click on the OK - Button and tell Multilevel that you want to dive with these settings. The most apparent change is that immediately after the mouse-click the big status-window on the left depicts 17 different tissues and the respective saturation status.
Note: the saturation status is visualized using nitrogen and helium partial pressures. The partial pressures are given in the SI-unit Pascal (please don't blame me for using the international units). Most of you may be more familiar with ATA or bar. However, it is very simple to convert the units. 1 bar equals to 100 kPa (which means kilo Pascal). In our example a diver descends to a depth of 40 m. There he experiences an absolute pressure of 5 bar or 500 kPa (1 bar atmospheric pressure plus 4 bar water pressure).
If you take a closer look at the nitrogen partial pressure column you can see that the first tissues (the fast ones) are more or less saturated while the degree of saturation is dramatically diminshed in the slower tissues. At the moment no helium is present in our model tissues (as you may have expected this is going to change very soon). So, the next column "pIg" which displays the total inert gas pressure (nitrogen plus helium) can at this very moment only repeat the contents of the nitrogen partial pressure column. The most interesting column is the one to the right. Here the so called "ceiling" is depicted for each single tissue. Ceiling is a theoretical concept which means the depth to which one can safely ascend without risking a bubble-induced damage to a particular tissue. From this it follows very clearly that decompression is always dictated by the tissue with the deepest individual ceiling. In our case (where we have accepted the deep-stop preset) our first stop is without doubt determined by tissue No. 0 (the deep-stop generating 17th tissue in the ZH-L17TS model) and means a first stop at 21 m. This tissue is not present in the classic Bühlmann ZH-L16 model. It can be ignored or (what I suggest) used to conduct safer dives without the need for dramatically increased deco time.
What other information can we obtain from Multilevel,
our first dive?
To the right the ambient partial pressures of all three gases of our breathing mixture are displayed. These partial pressures are the driving force behind the saturation of our tissues. You may have noticed that even the very fast tissues have not quite reached the Nitrogen partial pressure of the breathing gas at a depth of 40 m. This is due to the fact that by entering the lung the partial pressures of all gases are slightly diminished because at a body temperature of 37°C the water adds to the total pressure.
Below, you see the dimished breathing-gas volume in our tanks (we started with a preset of 8000 liters which is the equivalent of a nice double-twenty filled to 200 bar). Now at the end of the level 5000 liters are left which means 63 % of the initial supply. In order to calculate the gas consumption, Multilevel needs your respiratory minute volume (RMV). You see that the preset is 20 liters, but it can be easily changed during each stage of the dive. The deco stop interval is preset to 3 m (the classic distance between conventional deco stops) and the tank pressure is only needed if you like to blend some mixes and this will be discussed below (Well, it is true, we are approaching trimix land!).
Now the million dollar question is: how can we calculate a professional deco schedule with only three major buttons? I told you, it was going to be simple: OK ist for diving a particular level, Deco Prognosis allows for a rapid info on your next deco stop and Reset can clear all your (model) tissues at once and lets you start another dive.
So it is enterly clear: you have to click on Deco
Prognosis to start your ascent. By doing so Multilevel
changes your actual depth (40 m) and time (20 min) in the upper profile
windows to the depth of your next deco stop (21 m) and the time you
to hang there (2 min) until you can ascend to the next stop (the
of which is dependent on the respective parameter window).
Note: the deco prognosis is only a proposal. You may enter a lot more different levels, as long as you don't hit the ceiling. Remember what the name of the program was? You are right, this is true multi-level diving, so in theory all you have to be aware of is your ceiling (and your gas supply!).
Before you are starting to get to euphoric about the possibilities of Multilevel please be reminded that this is only a mathematical model with certain limitations. The more Yoyo-profiles you incorporate in your plan the more uncertain your dive gets. This is the plain truth and it is completely your decision what to do (please don't blame me for your bends!).
So now we decide to accept the deco prognosis. To do so we have to confirm our first deco stop as level No. 2 by clicking OK. This leads to a further change in the tissue saturation status. You see that the fast tissues have started to eliminate nitrogen. The good thing about Multilevel is that there is no strict boundary between the working phase and the deco phase of a dive (guess why it was developed by cave divers who often had to overcome difficulties with nasty dive profiles?). A "normal" deco program would ask you to enter all levels at once and then start to calculate your deco. However, as you don't know your actual ceiling with this approach you may often end up with endless repeats until you have got it sorted.
The other good thing is that you can play with several different gas
contents during a dive.
I will show you how it works: let us just finish our test dive. Click on Deco Prognosis to get your next stop depth (15 m) and your stop time (2 min) displayed. Confirm by clicking OK and repeat this procedure until you have worked your way up to 6 m (please don't confirm your deco at this level). You see that your stop time at this level would be 9 min. Now we are interested in the stop time with pure oxygen. Change the nitrogen content to 0 % and hit the Deco Prognosis - Button once again.Your deco time changes to 4 min. This is only a prognosis and does not affect your tissues - so you can play with several mixes until you have found the one which suits you best. Eventually, you have to confirm your settings by clicking OK .
Now is the time to start a mixed-gas dive. What about a little wreck
dive to 80 m?
We will split the dive into three major levels. We will use nitrox 30 as travel gas and trimix with 15% oxygen and 50 % helium als bottom mix (TMX 15/50/35). Just enter a depth of 40 m on nitrox 30 (nitrogen 70%). Please note that Multilevel calculates true rectangular profiles. This means that the program assumes that you stay at this given depth for the whole time of the level, which is normally not the case, since you have to include time for your descent or ascent. You can either add separate levels for ascent and descent or just use the rectangular levels alone but then you need to manage your ascent and descent within the level time.
In this case, you assume that you can descend to 40 m at a rate of 20 m/min. You add a certain safety margin and enter a level time of 3 min. Then you change to trimix. You enter a helium content of 50 % and a nitrogen percentage of 35 %.
Multilevel includes a mixed-gas blender. After having entered the gas contents you can click on the mode rollup window and change to the Mix - mode. Automatically a complete blending protocol for your trimix is displayed. Please note that the settings End Pressure (preset 200 bar), Start Pressure (preset 0 bar), Remaning He (preset 0 %) and Remaining N2 (preset 79 %) have a direct impact on the blending protocol. At the beginning Multilevel assumes that you want to fill a completely empty tank and thus Start Pressure is set to zero. If you however want to refill a tank with a left-over mixture then please enter the correct values for Start Pressure, Remaning He and Remaining N2.
To confirm your level you must now switch back to the Tissue
- mode, enter the time at 80 m and click OK.Then
switch to nitrox 30 at 40 min and pure oxygen at 6 m - and that was
If you want to do all your deco at the 6 m stop then change your deco interval to 6 m as soon as you have reached the 6 m stop. This works excellently!
Oh, I forgot, you can jump backwards in time by entering a negative time (by doing so, you can undo your last entry).
Last not least: you can switch off the deep-stop option by deactivating the DS box (but I would like to discourage you to do so). The same can be done with the low helim tolerance option (low helium tolerance means, that helium is treated like the low tissue tolerance inert gas nitrogen, where a lot more data is available).
To see your dive profile you have to switch to the Profile - mode were the dive history is displayed. You can easily print or save this table by marking the whole text with your mouse and copying it to the clipboard with CTRL + C (this is the windows combination but it may be different on other operating systems, on the "Zaurus" use the "function" key instead of CTRL). Then you can insert the table in a text editor using CTRL + V.
I hope you have enjoyed your first computed mixed-gas dive with Multilevel!
Moreover, the dump factor has a linear impact on your gas
as you will soon discover when playing with this utility.
Rebreathers are gas-extension devices and thus quite clearly you want to see a diminished gas consumption in your dive plan when using Multilevel. Here we go!