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Tech Spec


Paulo Vincenzo Toomer

So here we are again, issue 13 (my lucky number) and going strong.

Now anyone who knows me or reads this column knows that I like having my body covered in art, I like riding motorcycles, but most of all, I just want to go diving. Just chuck me my rebreather and my drysuit and I am in my own little heaven. Whether it's walls, corals, deep wrecks, caves or even mud, I am HAPPY. The thing is, and I remember this from my Open Water diver course way back when dinosaurs roamed the earth that you have to "plan your dive and dive your plan". Now my God, this can be the most boring section of any dive, but number one, it's arguably the most important part of your dive and second, with modern computers and dive software, it can actually be quite cool. I have to be honest, considering school seemed such a waste of time to me when I was young. I now resemble one of the maths dweebs that used to discuss quantum fractions or some such gobbledegook during lunch break. When I give my presentations on the diver courses I conduct, this is way my favourite bit...
Travelling Diver

There are several important parts to dive planning:

Determining what the parameters of the dive are. So what are we planning to do? How deep? How long? This is totally you and your team's decision. This is obviously based on certification and experience level and is always controlled by the diver with the least experience. Remember this one thing, it is NOT the weakest link that determines parameters as weak links break. Inexperienced divers only break when pushed beyond their limits. This is normally due to peer pressure, ego pressure or something similar.

Decide what is the best gas to use on the bottom. Is it air, nitrox or perhaps trimix? The selection of the gas is mega important. Get this wrong and you could end up diving a gas that is dangerous in terms of its oxygen content or indeed its narcotic value. During this phase of dive planning you will become acquainted with one of my favourite aquatic characters, Mr Dalton. He is the man responsible for determining the partial pressure laws. Quite simply, the total ambient pressure of a gas is equal to the sum total of the pressures of all the gases contained within it. For example, at the surface the ambient pressure is 1 bar and air is made up of 0.21 bar oxygen and 0.79 bar nitrogen.

Therefore at 10 metres, or 2 bar pressure, the partial pressure of oxygen will be 2 x 0.21 which equals 0.42. This rule applies to all gases that a diver wishes to use on the dive.

Then we need to pick the right decompression gas. I like a deco gas that will allow me to accelerate my off gassing. I balance the best off gassing quality of a gas with the operating depth of the gas. The reason I do this is to be able to use my first deco gas at a reasonable depth so I have a bailout option too, should my back gas (twinset) be rendered inoperable. Again we will happily engross ourselves in Mr Dalton's laws to determine our best mix.
Once the gases and maximum depth are selected we can then enter the decompression planning phase. This is actually quite simple and if you can do this efficiently, not only will you get the best out of your diving, but you will become a deco nerd of note. And deco nerds rock! So, how do we start? Easy, pick a table, any table. Personally, I use decompression software by a company called V-Planner, but there are lots of deco software programmes available such as GAP, Deco Planner, iDeco, HL Planner etc. Just as a side note, I love technology and I have V-Planner for my Laptop, iPad and iPhone, so no matter where I am I can calculate and adjust my dive plan. Just type in your gas, depth and time along with the deco gases you plan to use and 'Bob's your Uncle and Fanny's your deranged relative', your dive plan will be displayed before you.

This now just needs to be written on to your dive slate so you can follow it precisely during the actual dive.

Oxygen often governs the depths of our dives and also the deco gases used. Not tracking oxygen can lead to serious injury, but fortunately tracking it is easy. We are primarily concerned with two forms of oxygen toxicity. One concern is Central Nervous System (CNS) toxicity, which is by definition a high dose of O2 over a short time period. The other is Pulmonary Toxicity, which is measured in Oxygen Toxicity Units or OTUs. By definition OTU toxicity is a low O2 dose over a long time period.

To calculate the oxygen implications of your dive, all you do is look at every level of your dive including the ascents and deco stops. Work out the partial pressure of the oxygen in your mix at each level, use a CNS/OTU look up table, find the OTUs per minute and the CNS% per minute values and multiply them out by the time spent at each level. Then add the two values up. Your CNS should not be higher than 80% without you having to introduce special procedures (air breaks) during the decompression. If your OTUs do not get higher than 300 per day you really do not need to worry about them. If they are more than 300 then the OTU table will give an allowable value per day that you are NOT allowed to break. Simple, no?

We are now on the final stage of planning. Gas planning.
Scuba Trust
I, like most technical divers, have a simple philosophy to decompression diving and that is the dive does not begin until the deco starts. I believe that any monkey can dive deep and stay there as long as they like. The problem comes in when they incur a high decompression obligation. Cold, currents, weather changes etc, these all affect your decompression. But the major concern is actually not having enough gas to complete your decompression obligation.

Fortunately for us, tracking this is also easy. Much like with oxygen tracking, we take each level, including the ascent, convert the depth we are at into a pressure format, multiply that by the time spent at each stop and our gas consumption rate and we will get an amount in litres for each step of the dive. Add them all together and you have how much gas you need to take on the dive. Again, easy. All you have to do now is work out your reserve gas. I have purposely left how we calculate gas consumption rates and also reserve gas for the next issue as I guess I have blabbed on enough now.

As with all my articles, these views are only mine and I do not wish to say that this is the only way to plan dives. As instructors and training agencies, we are continuously adapting, reviewing and changing the way we do things so training principles may be slightly different from instructor to instructor. I would be very surprised though if all the steps above were not covered.

I do hope this was informative and I hope that you enjoy being a geek as much as I do.

You can email Auntie Toomer with any of your dive queries and you might also like to check out The Diving Matrix.
Scuba Trust

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