Scuba Rebreather: What Is It and How Does It Work?

We have been in the world of recreational diving for some time seeing how the concept called technical diving has slowly but surely installed itself in our community. Technical diving arose from having to use adapted techniques and procedures to be able to dive deeper and for longer than we could do with classic recreational diving techniques and equipment. In this aspect, the so-called rebreathers have stood out.

This article will discuss scuba rebreathers and innovative diving equipment that has become a game-changer for advanced divers.

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1. What are Scuba Rebreathers?

Rebreathers are diving equipment that recycles the exhaled air by removing the carbon dioxide and replenishing the oxygen content, allowing the diver to reuse the same air multiple times.

The basic idea is that we do not use all the O2 with each breath.

Air has 21% O2. Breathing, we only use 5% of this oxygen approx. Therefore, the hypothesis was we could breathe several times the used air. Couldn’t we?

If we breathe into a closed bag, taking air from it and exhaling in it again, the level of CO2 in the bag would increase and decrease O2.

We needed a machine capable of removing CO2 excess and refilling the oxygen.

2. History of Diving Rebreathers

Diving Rebreathers have only been accessible to amateur divers in the last 20 years, but rebreathers predate the regulator that we are so used to using in recreational diving and Cousteau invented in 1944.

The engineer Henry Fleuss invented the first functional underwater rebreather in 1878. He designed it to rescue miners in situations of toxic gas leaks so common in coal mines. In 1880, a tunnel was built below the Severn River to link England and Wales.

The tunnel flooded during construction and engineers needed to close a watertight gate so that pumps could drain the tunnel and work could continue. Divers tried with classic equipment, but the gate was 12 meters/39,37 feet deep and 300 meters/ 984 feet away. There was another problem. Divers had to dive inside the tunnel linked to the surface with the umbilical to breathe and it was in complete darkness (there were no underwater lights at that time). They could not drag the hoses far enough to be able to carry out the job.

Someone recommended that they speak to Henry Fleuss who had invented his rebreather recently, and they called him to try it. He tried to go down with his device, but Fleuss was not a diver. He was an engineer. So, after 5 minutes he panicked and went to the surface.

They asked him if he would let the main diver Alexander Lambert, nicknamed “the bull”, use his apparatus.

With very little instruction, Lambert went through the 300 meters/984 feet of the tunnel in the dark and was able to close the hatch. This was the first underwater use of a dive rebreather.

Henry Fleuss’s scuba rebreather had a container (bag), a rigid rubber mask with two tubes, and a pure O2 injection system with a 30 bar/ 435 psi cylinder, but it had a problem.

Going back to the example of our bag. If we simply add pure O2 to the bag, after a while, we will be breathing pure O2. All scuba divers know that we cannot breathe pure O2 deeper than 6 meters/20 feet without suffering the effects of O2 toxicity.

For a long time, the main use of underwater rebreathers was in military diving until the end of the 1990s. It was the year when the first reliable electronic rebreather was commercialized.

3. How Do Rebreathers Work?

So, the semi-closed rebreather was invented. In this scuba rebreather, instead of putting O2 in the bag, we put another gas mixture with Nitrox.

The diver breathes this gas several times until the O2 content drops too low, then, the rebreather injects this enriched gas again.

The more enriched the gas is, the more times we can breathe and the bottle will last longer.

Yes, we know that the depth is limited by the partial pressure of the O2. For example: with Nitrox 32, the limit is 40 meters/131 feet. But we can use other gas mixes, even Heliox.

Currently, a dive rebreather has a mouthpiece for the diver to breathe through, one or two containers depending on the model, two hoses (one to carry exhaled gas to the container and another to carry recycled gas to the diver), and a compartment filled with soda lime, where CO2 is eliminated. The device is connected to the air cylinder. The way to fill the container, also called the lung, with new air determines the type of rebreather.

4. Types of Underwater Rebreathers

There are two ways to inject this gas: active and passive.

4.1. Active Diving Rebreather

The active diving rebreather or semi-closed implies it injects a fixed amount of gas into the lung to keep the O2 content adequate.

There is a disadvantage. It implies that gas is continuously spent even on the surface while the tank is open.

When putting oxygen and nitrogen all the time, these rebreathers must eliminate the excess gas that accumulates in the container occasionally.

That is why, this kind of scuba rebreather is called semi-closed since every few breaths it releases bubbles through an over-expansion valve.

These systems achieve approximately 4:1 efficiency. In other words, a tank would last 4 times longer breathing with this rebreather rather than with SCUBA.

An example was the Draeger Dolphin which was commercialized in the late ’90s. Within this type there is a more evolved version of the manufacturer KISS, in which pure O2 is injected at a level slightly below the metabolic consumption of the diver, if the diver increases his O2 consumption, doing more intense exercise, he must add the missing O2 manually.

4.2. Passive Scuba Rebreather

A passive scuba rebreather functions with two lungs one inside the other. As we extract gas from the large one it compresses the small one, and in the end, the content of the gas from the small one is eliminated, replacing it with fresh gas.  The small bag is adjusted to the size of the large bag so that the percentage of renewal is adequate.

This system is more efficient than the previous one, achieving efficiencies of 10: 1 or 8: 1 depending on the design of the device. Examples of these devices are the RB80 and versions made by other manufacturers.

These systems do not need sophisticated electronics since we calculate the decompression with a lower percentage than the gas we inject. The manufacturer provides this drop in the % according to the unit design. For example, if we dive with a Nitrox 32 we will calculate the decompression with a Nitrox 28. This makes these units simpler, easier, and cheaper to maintain, but their efficiency is not enough, and they are bulky and heavy, so they are falling into disuse, especially since their electronic cousins are becoming more reliable and lighter.

4.3. Electronic Rebreather

Finally, we have the electronic rebreather, which is what we all think of when we talk about rebreathers. These use an electromagnetic valve called a solenoid along with an O2 sensor system and software. When the sensors detect that the amount of O2 is below the level the diver needs, they open the valve allowing O2 to enter until they detect that the O2 is at its proper level. This management is much more efficient reaching efficiencies of 40: 1.

5. Advantages and Disadvantages of Scuba Rebreathers for Advanced Divers

5.1. Advantages Of Scuba Rebreathers

• One of the advantages of scuba rebreathers is that consumption is the same regardless of depth. On the open circuit, consumption increases with depth. In a rebreather, we use only the O2 that the body needs according to the physical effort the diver is making, since we put the unused gas back into the bag.
• Another advantage, besides the duration, is that we breathe warm, moist gas, which reduces dehydration and cooling during diving, which reduces the risk of decompression sickness.
• Since the rebreather is applying the best mix at each depth, it also reduces decompression times and thus increases safety.
• Scuba rebreathers offer several advantages over classic recreational diving equipment, including longer dive times, the ability to dive deeper, reduced noise, and the ability to capture high-quality images without scaring marine life. They are also perfect for night diving or cave diving when bubbles can give away the diver’s position.

5.2. Diving Rebreathers Disadvantages

• We find the devices are more expensive, more complicated, and need more maintenance than traditional equipment and need training, also expensive, and above all a time of learning and adaptation to its use.
• Unlike an open circuit system, where if there is a problem, for example, shortness of air, it is detected immediately because you cannot breathe; In a rebreather, you can breathe, even if the air content is not adequate, and that can cause fatal accidents in the case of more inattentive or inexperienced divers.

6. Conclusion

Rebreathers are an extraordinary tool that with time, money, and dedication can give you a lot of satisfaction, but if you don’t dive enough with them, they are more difficult to control than traditional diving equipment.

If you want to enter this world, you should look for a good instructor with a good reputation near you, train and dive with the unit assiduously without being in a hurry to go deeper or stay too long, because with these machines it is very easy to succumb to temptation.

Diving Rebreathers are here to stay, but they won’t be the predominant form of diving. Only the most active divers or those who need them for specific activities, such as photography, exploration, etc. will take the step to use scuba rebreathers.