To understand I have to get into a little bit of how rebreathers are working. If you already know, just skip this part.

CCR is designed to recirculate the exhaled breath of the diver,

pass it through a special lime that removes the carbon dioxide. Then it reaches the sensors which measure the oxygen content and tells the unit to replace the amount of oxygen used up. With this method, you can dive for long hours with only two small cylinders.

In one of them, there is pure oxygen, the other one called the diluent which is air or trimix according to the depth.

In Open Circuit diving you choose your bottom gas to have a maximum of 1.4 partial pressure of oxygen on the bottom phase of the dive. With 16% of oxygen in the twin set, which is still breathable on the surface, you can dive up to 77 m. CCR divers are using usually 1.3 partial pressure. Long story short for the same depth they would use much less than 16% as diluent. The machine keeps up the oxygen content of the breathed gas anyway.

So what happened here:

After already finishing the pre-dive checks, the divers had to wait for quite a long time. In the meantime, the rebreather was running, using up the oxygen to keep the partial pressure steady on the surface.

The diver to make sure that his unit does not empty the oxygen tank – simply closed it.

This is a quite normal procedure, but in this case, it was not really necessary. Usually, divers doing it if they have a hybrid unit that gives a continuous O2 flow. The common units just stop adding oxygen after reaching a preset ppO2, so you can leave the CCR machine with the tanks open and they won’t empty themselves. What was definitely not normal that when they get the green light to go he forgot to open it. When he started to breathe from the unit, the oxygen level dropped rapidly. After probably a few breaths he just fell unconscious.

Now you would say: oh it should not have happened. You are right. However, with the heat sitting there in the drysuit, the rocking of the boat got him distracted. Also after the tough dive of the previous day, they were a bit too relaxed going on a “routine” dive.

The rescue

Luckily their training kicked in,

the rest of the team didn’t just rush down to the wreck, (as many times I have seen) they stopped to check on each other. Therefore they found the injured diver very quickly, and the prompt action with immediate CPR have helped the diver restart breathing.

In this case, the story has a happy ending. But it was a very close call. What would you have done differently? Be honest: have you ever had a close call yourself? 

There is no 100% sure that you can avoid accidents, either on CCR or on open circuit.

What you can do is get proper training, because there is only downwards in knowledge after.

Keep practising your skills, and play scenarios in your head, so if anything happens, you are as ready as you can be.

What is your method to keep your knowledge and skills fresh? When was the last time you went for a course or training session? Looking forward to hearing your stories in the comments.