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An occasional dream

Scuba cylinders

ISSUE 14 ARCHIVE - AN ARMCHAIR DIVER'S GUIDE TO TECHNICAL DIVING: TIME

Paolo Vincenzo Toomer

I write for this magazine every three months, and every three months I get an email from this wonderful lady called Juliet telling me that I need to get ready for my deadline. And EVERY single quarter she has to send me a reminder email that my article is late! It’s all about time!

Time is such a horrible little thing isn’t it? I mean, I was young once! Have you thought about how slow time goes when you’re bored or when things are not so good? But, boy, you better believe, the minute you start having fun and enjoying yourself, well, time races by. What the hell is that all about?

Anyway, let s apply my analogy to diving. After all, this is a dive magazine no?

In diving, TIME is your number one asset. Time lets you stay on the dive, it lets you help someone in trouble, it allows for minor delays. For once, time is a good thing. But time relies on one primary resource when diving, and that’s breathable gas! No one ever died from too much gas is one statement that most successful divers adhere to. So this month I will be discussing how NOT to run out of gas, or at least minimise your chances.

Remember this one thing: a technical dive does not begin until the decompression starts. Executing a decompression plan is easy, as long as you have the gas available to complete your obligation.

Everyone knows that we need to get back to the surface with some gas, blimey, I learned that on my Open Water Diver course. But doing this while you are overhead diving, whether it be a real (cave, wreck, ice) or virtual (decompression diving), can be difficult, and believe me, this is one of the most important parts of dive planning.

The first thing we need to know is how much gas we consume per minute so we can add this into our decompression plan and then work out exactly how many litres of gas we shall need for each level of the dive. This is called an SAC rate or Surface Air Consumption Rate.

To calculate your SAC is really easy. Jump into the water and descend to a steady depth. Around 10 metres should be fine. You need to stay level for the whole time, as fluctuating depth will give you an inaccurate reading, of course. Grab a slate to help you record vital information for the SAC calculation. Once you are steady at your desired depth simply time yourself and record how many bar of gas you used during the exercise. You need to swim at three different paces, one is very relaxed
just hanging in the water, this will be used
for your decompression SAC rate. The second will be at your normal swimming pace and the final one is at an accelerated rate signifying a hard working phase.

To calculate the amount of gas used multiply the bar used with the size of the cylinder used. Then divide this by the time taken to do the skill and then finally divide this by the depth converted to pressure.

If you used 15 bar in a 12-litre cylinder over 5 minutes at 10 metres it should look like
this: 15 x 12 = 180 litres, 180 litres ÷ 5 minutes = 36 litres, 36 litres ÷ 2 ata (10 metres converted to atmospheric pressure) = 18 litres per minute. Simple!

Now all you do is use the SAC rate against each level of the dive, multiply it by the
time spent at each level and Hey Presto: you know how much gas you are going to need to do the dive! Unless you have a gas delivery system failure you will have enough to complete the decompression schedule as planned.

The next thing we need to look at is the reserve rule followed by most tech divers. This rule was actually designed for and by cave divers. Unfortunately, the school of hard knocks taught cavers that one diver having a problem deep inside a cave will actually jeopardise all the team members’ lives.

The worst-case scenario is one where the team/diver reaches the turn point (only has enough gas to get back) and then has a failure. There is now a situation where if the divers share gas they will all run out on the way back. For example, the dive requires 2,000 litres of gas to complete. 1,000 litres used on the way in and 1,000 litres on the way back out. If one diver has a gas failure as they reach the turn, their buddy does not have enough (in fact any) gas to bring them home.

This problem was addressed much like helicopter search pilots address their fuel supplies. They use a system called the
rule of thirds. This system is not the most conservative, but it is at least a starting point. It works like this: If the diver needs 2,000 litres of gas to complete the dive, we multiply the 2,000 by one and a half, which gives us 3,000. We now have an extra 1,000 litres of gas spare.

So, unlike the situation in my previous example where the divers ran out of gas, now the divers have an extra 1,000 litres to get the ‘out of air’ diver safely to the surface. I mentioned that this was a minimum rule as if you do the maths, you use 1,000 litres in, 1,000 litres to get out. If there is a failure right at the turn point, we now have an extra 1,000 litres to share, but this leaves us with ZERO bar when we arrive at the surface! And that is seriously the “EDGE”. If you build in a little extra then this all becomes safer.

The most important rule once you are diving on ‘thirds’ is that no diver EVER goes past their turn point. If they do, they will be signing everyone’s death warrants if they have a failure.

I will never understand divers that break
their gas rules; remember what I said earlier – no one EVER died from too much gas.

I believe that diving is the most wonderful pastime, an activity in which I participate on a daily basis. But my job still has one underlying clause and that is ‘to go home to my family safely’. Anything other than that means I have lost control and I am the ultimate control freak and that is an understatement, ha ha!

I hope my articles give you a little more of an idea of how much pre-planning we do in order to execute a safe dive. I do this for every dive and after 5,000 plus dives I am still here enjoying myself. There must be something to these gas management rules, then.

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.

Paul is an Instructor Trainer and Course Director. He owns Diving Matrix Tec Lab with facilities in London and Malta.

If you have any queries, Paul can be contacted on email or at the Diving Matrix website.

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