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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. These devices 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 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 being 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, and with the umbilical and 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 device recently, and they called him to try it. He tried to go down with his device, but he was not a diver, he was an engineer and 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, he walked 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 rebreather.
Until the end of the 1990s the main use of rebreathers was in military diving when a reliable electronic rebreather was commercialized.
What Is A Scuba Rebreather and How Does It Work?
The basic idea is that we do not use all the O2 with each breath we take, in fact, we only use approximately 5%, since air has 21% O2 we could use that same breath several times. Imagine that we breathe into a closed bag, taking air from it and exhaling back. What would happen? It would increase the level of CO2 in the bag since we would exhale the stale air in it and at the same time the amount of O2 would decrease as it is used by our body with each breath. To eliminate the excess of CO2 and its toxic effects on our body, we would put a chemical compound in the bag that would react with CO2, absorbing it. Now we only have to worry about filling in the O2 that we are using on each breath. The way to fill the bag with O2 determines the type of rebreather.
Scuba Rebreather: we will start with the oldest and simplest.
The first was, as we have discussed, that of Henry Fleuss, which was a rigid rubber mask with two tubes and a pure O2 injection system with a 30 bar/ 435 psi cylinder. Going back to the example of our bag we simply add pure O2, the problem would be that in the end we would be breathing pure O2 and we know that we cannot breathe pure O2 deeper than 6 meters/20 feet without suffering the effects of O2 toxicity that can to cause seizures and subsequent drowning.
Therefore, these rebreathers have only military use (night interventions without blowing bubbles at shallow depth) or cave diving when we find flooded siphons of no more than 6 meters/ 20 feet deep.
So, the semi-closed rebreather was invented. Instead of putting O2 in the bag, we put gas with more O2 content, Nitrox, and we breathe it several times until the O2 content drops too low and we inject this enriched gas again.
The more enriched the gas is, the more times we can breathe it and the more efficient it will be (the bottle will last longer), but the depth limit will also be limited by the partial pressure of the O2 and with a Nitrox 32, it is limited to 40 meters/131 feet, for example.
But we can already dive to a reasonable range with better use of the available gas. There are two ways to inject this gas: active and passive.
The active implies that we inject the gas through a special hole that introduces a fixed amount of gas into the mixture to make the O2 content adequate all the time, but of course, this implies that it is continuously spent even on the surface while the tank is open, and when putting O2 but also N2 we have to eliminate the excess gas that accumulates in our bag every so often since our body only uses O2. That is why they are called semi-closed, since every few breaths they release bubbles through an over-expansion valve, the same as the ones used by the BCDs to prevent them from bursting when they over-inflate. These systems achieve approximately 4: 1 efficiency. In other words, a tank of the same size would last 4 times longer breathing from it with this rebreather rather than witn an open circuit regulator. An example was the Draeger Dolphin that 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.
The passive use of a regulator functions with two bags 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 of the % according 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 in disuse, especially since their electronic cousins are becoming more reliable and lighter.
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.
Advantages Of The Scuba Rebreather
One of the advantages of 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.
As for 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.
Rebreathers are an extraordinary tool that with the 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.
Rebreathers are here to stay, but they won’t be the predominant form of diving soon. Only the most active divers or those who need them for specific activities, such as photography, exploration, etc. will take the step to use these devices.