In depth knowledge about deep diving. Extracted through an interview with Jani Santala, Technical diving Instructor Trainer, TDI & SSI

The deep of the ocean can be tempting. “There are hidden gems of history down there, encouraging some of us to come and visit. For me one of the most memorable of them all is the Kasuuni wreck.” Jani Santala, Technical diving instructor trainer.

She lays in the Gulf of Finland, amazingly well preserved by the cold water and low oxygen conditions. The history behind the wreck is still unknown, but it has been dated to late 18th -early 19th century based on the pottery and marine items on deck. The structure of the ship is still almost intact, and a broken stump of one of the three masts of her frigate rigging still remains. The canons are out and some of the gun ports are still open, indicating that she may have been in battle when she went down. Descending onto the ship is like entering a time capsule - everything is kept as it was several hundred years ago. The condition of the wreck is incredibly good.

A wreck like this makes it worth it to go deep for some of us. The Kasuuni lays on the 60 meter level, which makes it within reach on open circuit. Still there is a lot of things to think about before jumping into the water and starting the adventure.

Adequate training is key to reduce risk, and it makes the dives more enjoyable. For a dive like this you need to complete the mandatory checklist.

Make a proper dive plan
Have a backup plan
Do thorough gas calculations (also in case of bail out scenario)
Mentally ready
Physically ready
CCR Checklist
Buddy check before jumping
In-water check

There are clear benefits of having a high level of experience in OC before switching to rebreather. After years of teaching I have noticed the change in the basic experience level of the students.

Generally, divers are switching to rebreathers earlier than previously. This arises a few potential problems. If you go straight from recreational open circuit diving to CCR you have no practice in deep Open Circuit diving. So, what can happen?

When we started the dive, it looked good. It is a sunny day with flat sea. I find myself at 60 meters, admiring the Kasuuni wreck. She is 29,5 meters long, 3 masted frigate with 6 canons and 12 gun ports. While we were descending, we got a good overview of the wreck, but suddenly the current picked up and visibility was lost to only 1 meter. We can hardly see each other in the bad visibility now, and the current is so strong that we cannot manage to follow our original dive plan. Fighting against the current, drifting further and further away from the wreck.

My breathing rate goes up. I can hear the pulse pounding in my ears. I don’t know where I am. It feels like the heart will burst out of my chest any minute, and I am fighting to get enough gas… This is the crucial moment - What do I do? What bailout gas do I have? Where is it? Can I control my breath long enough to switch to my OC bailout?

Typically, in the cases where divers go too fast into deep diving, the bailout ascent from 60 meter or 100 meter takes them by surprise. Breathing at deeper depths is different. The gas mixes are made to take away narcosis and keep your head clear.

Some divers want to save money and decide that air is sufficient bail out gas for 60 meter dives. In this situation you may have a CO2 hit, inducing severe anxiety or even a panic attack. If switching to air at 60m you will have a very high gas density resulting in increased breathing work, and high level of narcosis further enhanced by poor visibility. All this combined may lead to a disaster.

When you are accustomed to diving with CCR you normally have “unlimited” amount of gas. Still, rebreathers are great to dive up to the point when they are not. Although today’s RB’s are very dependable and this happens very seldom, it’s still an ever present possibility.

If you have no experience in OC deep diving there is potentially an increased risk related to the feeling of “being safer on the loop”. In several cases over the years divers have chosen to stay on the loop because they had a feeling of false safety. It is feeling more safe to breathe something on the loop, the problem is that you don’t know what you are breathing if the RB fails. It’s the quality of the gas that matters, and the buildup of CO2 in your body. If something happens to your rebreather and you need to bail out to open circuit gas - what is the density of your gas? What will the narcosis level be?

If the bailout gases are chosen correctly the density of the gas is not too high, but even with an optimal gas choice it feels completely different to breathe the open circuit gas at depth. A small thing as bubble sounds in your ears can stress you out.
High helium content in the gas makes a whistling sound that you are not accustomed to if you have minimum experience on OC.

On open circuit you would typically choose 21/35 as a standard gas for this kind of dive. If you are choosing a bailout gas however, the next standard gas 18/45 would be a better option. Less gas density, less narcosis - and on a CO2 hit the higher level of helium is helping on the ventilation and removal of CO2 from the tissues. Further, when switching to OC bailout during a RB dive, the fundamental idea is to get out of the deep water as quickly as possible, because at the depth your bailout will run out surprisingly quickly! In shallower water you will have more gas at your disposal, relax and concentrate in performing a successful decompression profile. In order to achieve this, you need to be as clear headed as possible, and be under least possible stress – to avoid a CO2 slammer, narcosis hit or the like!

The cost factor should not be an element that is taken into consideration if you want to do this kind of diving. To save a couple of euros on the helium bill is not worth the added risk. The bailout gases are not supposed to be used unless something is wrong - and if that happens you better have the best possible gas mix to bring you safely to the surface. How much do you value your life, 20-30€? It’s got to be worth more than that, don’t you think?

I recommend my students to complete the Normoxic trimix on open circuit before continuing with CCR. The experience on open circuit deep diving might be the thing that saves you in the rare occasion something goes wrong in the deep. To be able to enjoy diving, it is important to think about the risks, and realize that human error is usually what goes wrong in diving.
Practice, practice, practice.

Stay tuned for the next episode in the "Stay in Rec limits or Go Tech" series, The caves are calling!