LUK Technical Diving

Planning a technical dive

How would you go about planning a technical dive? Have you ever planned a proper technical dive?

Well, if you are a certified technical diver for sure you would’ve done it at least during your technical diving courses, but if you are still not a certified technical diver and you haven’t done the panning yet this article will bring you closer to the basics of planning such a dive.

This article will be an introduction into the planning phase of a technical dive. We won’t go into numbers and calculations, not this time.

So, how would you do it.

You would start by deciding where to go (location), what to do (perhaps explore a wreck), your max depth during the dive (72 meters), and how long you would like to stay at depth exploring the wreck (bottom time).

So, you are planning a dive to a plain wreck with maximum depth of 72 meters.

A wreck is in a protected area and penetrations are prohibited; therefore, you are going to explore the wreck from the outside. Take few photos of the soft coral species that live on one of the propellers.

So, let’s say you decided to go to a plain wreck which lays on 72 meters depth. A plain is in a protected area and penetrations are prohibited. Therefore, you would just explore from the outside, take photos of the coral species that lives on propellers.

You and your diving partner decided for a 20 min bottom time. This is how much time you will need for descent, swim around, and to take those few photos.

With all the above information the first part of planning is done.

Now you are going to decide what gasses you are going to breath during all stages of your dive. Travel gas (if any), bottom, gas, stage gasses and decompression gasses.

We said you were going to 72 meters. Because of the depth this dive cannot be done on air as breathing gas, as potentially we would be facing two problems:

Problem No.1 – oxygen toxicity – the partial pressure of oxygen at this depth would be over the maximum tolerated, so there is a potential risk of oxygen toxicity which could potentially lead to a seizure which could potentially be fatal underwater.

Problem No.2 – nitrogen narcosis – partial pressure of nitrogen would be too high at this depth while breathing air, which would give us very strong narcotic effect, which might have dangerous consequences.

Also, decompression on air is insufficient and it would take too long for us to decompress on air from such a dive.

So, we would need to use mixed gasses.

For bottom gas a good choice would probably be something like 16/50. Which means 16 % of oxygen and 50 % helium. This mixed gas we call trimix as it contains three gasses: oxygen, nitrogen and helium.

But why helium, and why 50 %?

Well, by adding helium we are reducing level of nitrogen in the mix, therefore reducing nitrogen narcotic effect at depth. We are also reducing level of oxygen in the breathing mix because as mentioned above 21 % of oxygen would be too high at such depth.

But why 50% of helium?

50 % helium is not just a random number. It was calculated according to the level of nitrogen or nitrogen narcotic effect that you are willing to take during the deepest part of your dive. It would depend on your personal preference of course.

You would make this calculation by using END or equivalent narcotic depth formula.

With 16 % of oxygen and 50% helium we have remaining balance of 34 % of nitrogen. This 34 % of nitrogen will give us the same narcotic effect at 72 meters while breathing 16/50 trimix, as if we were breathing air at 31 meters depth.

So, we are going to use trimix 16/50 as our bottom gas.

Now we have to decide what gasses we are going to use for decompression.

We are going to need several stage tanks with different mixed gasses in order to decompress efficiently from such a dive.

I would probably go with Nitrox 36 making gas switch at 34 meters. I would also carry Nitrox 50 making the gas switch at 21 meters, and finishing decompression with 100 % oxygen making the switch at 6 meters.

As the bottom gas is hypoxic (low on oxygen) it would be wise to use one of the stage gasses, probably Nitrox 36, during the surface and first 10 meters of our descent. We call it “travel gas”.

Dive profile would look something like this:

Example of a technical dive profile

So, for 20 min bottom time at 72 meters, you will spend 40 minutes decompressing. That is the “price” you have to pay in order to take those few good photos at 72 meters.

To make this dive plan we would use one of the dive planning programs. There are several programs available but we won’t go there now, you will learn all about it during your technical diving courses.

Now, depending on your surface air consumption or SAC rate you will have to determine volume of each gas individually, for each step of your dive. Making this calculation you will know how much gas you will have to take on this dive. So how many and what size of cylinders.

By this stage you will know very well your SAC rate, you would have done the calculation during your Intro to Tech or Adv. Nitrox course.

When this planning phase is done, you would start organising all your equipment, fill and analyse all the tanks, properly mark them, make sure all the equipment is serviced, checked and working fine, including your regulators and SPG’s, you would set up your dive computers, charge your torches, and so on.

Above planning was done as if you were going to execute this dive on Open Circuit.

Therefore, we could execute this dive using closed circuit rebreather. That’s how I would have done it.

To reduce the cost, we could execute this dive using closed circuit rebreather. That’s how I would have done it.

Now days not many (probably none) of deep dives such as this one would be done on open circuit. First of all, prices of helium are enormously high, and secondly when using rebreather you will have optimum decompression, therefore much shorter decompression times. There are many other advantages using a rebreather too. But on how to plan a rebreather dive, maybe some other time.