Initially, there was an opinion that technical diving, and especially diving on trimix with subsequent decompression on oxygen enriched mixtures, must be completed only on the basis of previously prepared plan, which must be calculated according to the tables or with help of specialized software, and it must be recorded in a slate. To me, such approach seemed indeed reasonable just a couple of years ago. However, some special types of dives, for example, cave dives, rack dives and, generally, any dives with almost unpredictable variable profile cause serious difficulties if planning is done in the described way.

Therefore, as soon as multi-gas computers suitable for technical diving became available, I started to take interest in the practical aspects of their usage. Without going into details, which are beyond the scope of this article, at first I will note that from the moment of my first dive with VR3 computer, the undoubted leader, in my view, in this thriving developing market, I have been using it in all my dives and, moreover, I recommend this device to all my students and no one yet has been disappointed and, for sure, no one returned to an old-way “by slate” diving. Why? Simply because, it is very comfortable to dive with VR3. Besides, during the last year I created my own concept of gases selection for extremely deep diving on trimix, which is aimed to maximum shorten the time of decompression with simultaneous control of negative processes, caused by counter diffusion.

And finally, the third postulate. It is possible to ascent very quickly before the first micro-bubble stop (which is always 2 minutes with VR3). Numerically – the speed may be about 30 meters per minute, if not higher. After that, we maintain the speed recommended by computer.

The two mentioned points make up the basis of my new procedure for technical dives organization and planning, which I submit for your consideration. Of course, everything written here is not only the result of theoretical researches, but it is also proven in practice. In particular, during recent series of extremely deep dives, carried out in the period from the middle of March to the end of April 2005 and finished by achieving 192-m mark. None of us had any signs of decompression illness after any of tens of dives in those series. I focus your attention that dives were not conducted in overhead environments or in cold water.

 

So, let us start with the planning. First of all, I underline that I consider my own empirical experience as a basis of planning for any dive. In other words, if you are planning to reach 130 meters, it means that you dived to 120 meters on several occasions and such questions as gas flow rate and estimated ascent profile should not be of serious concern for you, if for no other reason, than because you have an idea how it was in the previous time. Under no circumstances it means that you should not use your computer planner. You certainly should, but with understanding that it is simply a convenient way to do formal calculations, and by no means it can substitute your own experience. Increase of depth (run time, length of run in overhead environment etc.) must be conducted gradually. Concerning backup options, I consider two alternative variants – the second VR3 or Bottom timer together with plan in a slate. You make the choice. The first option is better for diving with unpredictable profile, the second one is more reliable for regular trimix diving, and obviously, this option is considerably cheaper. For successful exploitation of the first variant I recommend to start with practicing the mixtures switching on two VR3, this is not difficult given you have the skill. Break-down of one of your computers in this option (just as computer break-down in the second option) is definitely a signal to abort diving and start ascending. Here I do not touch the question of quantity and strategy of backup plans development (exceeding time limit, depth etc.), as I think there is no universal answer to this question and each competent diver is capable of projecting some possible complications of a particular dive and choose what is necessary. I consider the presence of second VR3 as the most universal backup plan.

 

Now, the method of mixtures selection. For simplification, I will give an example of particular calculation of touchdown-dive to 210 meters, with necessary comments. First of all, before a deep run on trimix I recommend to dive a several times on air for adaptation to nitrogen narcosis in order to amplify the planned depth later on. This is determined not only by economic factors, but also by real danger of still insufficiently studied NSHP effect, and by wish to increase the nitrogen fraction in the bottom mixture for reduction of subsequent counter-diffusion jump.

The partial pressure of oxygen for this particular dive I set as 1,6, which I consider absolutely reasonable taking into account the extremely short exposure. Generally, I do not like to dogmatically limit the partial pressure of oxygen by 1,4 number. It depends on a profile! If it is a touchdown, then why not make it higher?

 

Of course, if we talk about long exposure or cold water, then it is necessary to apply more conservative approach, but not in our case. So, we select the given depth of 50 meters, partial pressure of oxygen at 1,6 and the result is the bottom mixture Tx7/66. Its fraction of nitrogen is 0,27. If we follow the traditional ideas about struggling counter diffusion, it is obvious that all subsequent mixtures must contain no more than 27% of nitrogen. It is not difficult to select such mixtures, so we will not spend time on this. Everyone can easily practice on a computer planner.

 

For example, such set Tx7/66, Tx15/58, Triox35/37, EAN80. Now let’s upload this "correct" set, for example, in V-planner (I used the licensed version V-Planner 3,61 by R.Hemingway, VPM code by Erik C. Baker., Decompression model: VPM - B, Surface interval = 1 day 0 hr 0 min., Elevation = 0m, Conservatism = Nominal) and we will get the total run time equal to 176 minutes. In this case we will need 6453 liters of helium (you can reproduce my calculations in V-planner, simply accept RMV=17 liters per minute, descent rate = 25 meters per minute, ascent rate up to 100 meters = 20 meters per minute, from 100 to 6 = 10 meters per minute, from 6 to 0 = 5 meters per minute, time on the bottom = 36 seconds, stop size = 6m, last stop = 6m and all indicated above installations. Apparently, these installations do not change for further illustrations. Just do not forget that V-planner uses not the time on the bottom, but run time, so assign it as 9 minutes and you will obtain the desired time on the bottom).

And now let’s look at my proposition. So, the bottom mixture remains the same, Tx7/66. And now comes the most interesting part. I put forward the assumption that counter diffusion jump within 1,5-1,7 is normally endured by human organism (I emphasize that it is proven experimentally by our team, and therefore it is valid at least for sample of three people in more than tens of dives within the range of 120-192 meters). And, based on this assumption, I pick the rest of mixtures. Let’s begin with the latter one. I propose EAN 65.

 

Believe me, it is very practical mixture – it is easily mixed according to partial pressure even in the absence of pressing compressor and incomplete transport balloon (if you want to make it stronger, I do not mind, simply put the data in a planner; the gain would make even a cat laugh, then ask yourself – what’s the sense in torturing a blender?). So, we already have “a reverse jump” in nitrogen. In the last mixture the nitrogen fraction was 0,35. Let’s choose the transport nitrox. Let this be EAN 38 (nitrogen fraction is 0,62). So, we have the nitrogen fraction of 0,27 in the bottom gas, and 0,62 in the transport nitrox. The reverse jump – more than twice – is too much (and imagine what would happen if we switched to air! Poor....., but, ok, I will not continue). Well, and nothing awful, we must select a transport trimix yet. And here is the very essence of the method. We will select it in the way to maintain the equality of reverse steps, with breaking one large step into two small ones – indeed, it is frightful to jump from 2,5 meters height, but it is ok to try two times by one meter, isn’t it? Mathematically, this is called the geometric average between numbers 0,62 and 0,35.

 

And we just have to find the fraction of oxygen. But now this is solely a question of gas flow management. From my experience, I will say that the optimum depth will be, let us say, 100 meters, taking into account that we have a touchdown dive and will be very rapidly ascent to the first micro-bubble stop.

As a result, transport trimix came out as 15/45, and instead of one large counter diffusion jump we got two small ones – by 1,5 times. So, our set of mixtures - Tx7/66, Tx15/45, EAN38 and EAN65.

We load it in V-planner and receive the run time of 138 minutes, and we need only 4300 liters of helium. I.e. run time is reduced by 38 minutes, and the flow rate of expensive helium – by 2153 liters (in the prices of Dahab it will comprise approximately 150 dollars, and it is all apart from helium for accompanying guide!)

 

But, we have not yet increase the ascent speed up to the first micro-bubble stop, which VR3 will give us somewhere at 150 meters mark (it is necessary to look up in Pro-planner, I simply do not have the license version ready at hand, and I consider unethical to use something else for publication)! Check yourselves, the result will be a nice surprise for you.

Once again, I will emphasize that all above is tested by our team during two runs of deep diving, conducted in Dahab in the period from the middle of March to the end of April 2005, which were published in the Internet, on the forum of Underwater Portal of Tetis company and on the website of Russian club in Dahab.

Well, and in conclusion I would like to say several words about the behavior of my favorite VR3 and its usual breakdowns. Firstly, I will say, if it works, then everything is wonderful – I do not have any claims to the interface.

Concerning algorithm, this is a Pro-Planner, and we will leave the mathematical analysis of algorithm for specialists. As a practitioner I will say – I like everything, especially the concept of variable ceiling! I will not go into details as retelling of computer operating instructions is not included in my plans. The malfunctions. In my experience, a couple of times I had pixels disappearing in the center of the screen, one time the button fell out and one time the program screwed up, it happened during my and Igor’s dive to 192 meters. Computer continued to correctly show time, but it showed the depth with the error by 5 meters and completely absurd chart of decompression. The duplicating device rescued us. Here is the complete list of malfunctions, which I encountered during a year. For the sake of fairness I want to note that “Delta P” company fulfilled all guarantee obligations.

 

Andrey Chistyakov

Instructor TDI, Instructor PADI