DIVE LOG JUNE 2025 issue 412

Rebreathers allow us to do the kind of dives we could never do on open-circuit scuba, but they also expose us to new risks. This is why it is important for divers making the switch from open-circuit to closed-circuit to have proper training that includes lots of practice in avoiding and handling simulated emergencies, especially situations which they would never encounter on open circuit. One of the major dangers of rebreather diving is a phenomenon referred to as shallow water blackout or hypoxic blackout. Without oxygen, we cannot live, but too much oxygen is bad for us, and so is too little oxygen. We can function normally when we breathe oxygen at partial pressures between 0.16 and 0.5 atmospheres, but if we are exposed to partial pressures greater than 0.5 for extended periods, oxygen can cause us harm. And, if we breathe gas with an oxygen partial pressure below 0.16, we don’t have enough oxygen to keep us conscious, and we black out. The Risk of Shallow Water Blackout in Rebreather Diving Here’s a quick summary of the issue. It is all to do with physics and physiology.

to drop. Let’s assume that the free diver remains at 10m for some time, and, during this period, their oxygen partial pressure falls from 0.42 to 0.28. They will be fine at 10m because this is still a level that sustains human life. But what happens when they ascend? An oxygen partial pressure of 0.28 at 10m (2 atmospheres of pressure) equates to a partial pressure of only 0.14 at the surface, where the ambient pressure is one atmosphere. As they head for the surface, the partial pressure of the oxygen in the free diver’s body will become lower as the water pressure around them falls, and eventually, it will drop below 0.16 atmospheres. When this happens, the free diver will black out abruptly. There will be no warning signs. If help is not close at hand, they will drown. Rebreather divers can encounter similar issues when they ascend from a dive. Most electronic rebreathers maintain the oxygen level in the diver’s breathing supply at a fixed level, pre-set by the diver. As the diver ascends, the partial pressure of oxygen in the diver’s breathing loop drops along with the reduction in the surrounding water pressure. When the rebreather’s electronics detect that this is happening, they act to deliver a fresh injection of oxygen into the breathing loop to maintain the partial pressure at the desired level. Rebreather Diving

Free Diving

Free divers know all about shallow water blackout. It is a leading cause of free-diving fatalities and the sport goes to great lengths to protect participants from succumbing to it, especially in training and competitions.

This is the problem, as it affects them.

Before they descend, a free diver usually takes one or two deep breaths of air, which has an oxygen partial pressure at the surface of 0.21. Then they dive. When they arrive at 10m, the percentage of oxygen in the air in their lungs is still 21%, but as they are now at an ambient pressure of 2 atmospheres, the pressure of the air in their lungs has now doubled and the partial pressure of the oxygen in their lungs is now 0.42.

Over time, their body metabolises some of the oxygen, converting it into carbon dioxide, and their oxygen level starts

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DIVE LOG Australasia #412 - April‘25

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