(15-10-09) Its quiet sailing, the almost absence of bubbles and prolonged dive times have made Rebreathers (RB) essential for military and professional divers since many years. Lately, most accessible prices and their use in recreational diving have made them be more attractive for an already important niche of divers worldwide.

    Although it can seem a modern system, the truth is that the Rebreather (recycler) is a much older invention than it seems. The first one was conceived in the 17th century by the Neapolitan “Giovanni Borelli”, a physicist and mathematician who made important contributions to the field of the Medicine.

    His first project consisted of a leather sack that the diver carried at his back, and its buoyancy could be controlled. This system did not work, but it was the starting point for other scientists to base on to continue improving it.

    The first systems of independent breathing were designed by Henry Fleuss in 1878, and they were closed circuit, so that the recycler is previous to the first independent open circuit systems. At that time, the existing technology allowed that recycling the air was easier that compressing it to high pressures to have the same autonomy.

    The first absorbing CO2 was used by Khotinsky and Lake in 1881. The company Dräger produced systems for navy before the I World War, using oxygen. The systems with Nitrox were used during the II World War and they are still being produced.

    In recent history, the company Electrolung produces since 1969 the first electronic control for RB. Progresses during the 20th century and mainly the Gagnan-Cousteau regulator (open circuit system) appeared in 1942, made closed circuit systems be ostracized.

    During decades, only the Navy, and especially combat divers in concealed operations, have had this system, since, as they do not emit bubbles outside, it is very difficult to detect their presence.

ADVANTAGES IN THE RB USE

    Longer dives: it is one of the most of the important reasons to use RB in dives. Perhaps the main advantage of RB CCR (Closed Circuit Rebreathers) and, to a lesser extent, RB SCR (Semi Closed Rebreathers), is the increase in gas efficiency. Under normal circumstances, a diver only uses a small fraction of oxygen in each inspiration. When he exhales, most oxygen leaves the lungs hardly being used.

    As we already know, our body metabolizes less than 21% of O2. When the depth increases, this limitation of open circuits multiplies. Due to the increase of the pressure, to deeper depths, more gas molecules get lost in each breath.

    A RB, however, retains most of it and even all the exhaled gas processes it, and it gives back to the diver. In the closed circuit systems, as exhaled bubbles do not exist the efficiency of the gas to depths does not change. It is that to greater depths, the use of RB CCR will be more advantageous, from the point of view of the efficiency of the gas.

    For example: a bottle contains enough air to maintain a diver standing in surface during 1 hour and a half approximately. The same bottle will only last 45 minutes 10 meters deep and less than 10 minutes 90 meters deep. But if the same bottle is loaded with Oxygen and used in a RB CCR, the diver will be able to be underwater much more time without being concerned about the depth!

    Of course, a RB does not make the diver immune to narcosis of DCS and Nitrogen (N). Those risks continue existing although the most sophisticated RB CCR more can fit their mixture of the gas to reduce the DCS can fix the mixture of the gas to minimize the DCS risks. The advantage of the RB is the longer duration for most of us.

    Quiet navigation: Your traditional open circuit equipments, whenever we breathe, we released great amount of bubbles that are noisy in aquatic environment. This hits on wildlife of the surroundings, making that, for example, fishes or mammals are reluctant to our presence.

    No Bubbles: The RB emits few or no bubbles, according to the type of equipment we use. The roar of the bubbles is not heard and the fish do not hear them either. That allows us to be closer to marine life, and even fish they approach us with no fear, reason why RB are more and more popular among professional photographers and in the world of researchers. One does not become invisible, but becomes more unnoticed, so the moment to approach them will seem less alarming.

    Warmer and humid breathable gas: Chemical reaction in CO2 purifier heats up and dampens the gas we breathe, and body temperature we exhale the air. The diver with RB has a warm sensation in the mouth and he won’t feel the dryness of the air we feel when breathing with an open circuit.

    It is possible to add another reason that is very important for divers who dive in cold water: as the inhaled gas is at corporal temperature, we avoid some cooling in our body. It is a reason extremely important when choosing an alternative to dive in cold areas like high mountain lakes or the poles.

    Optimal gas mixture: The most sophisticated RB constantly supervises the partial pressure of oxygen in the mixture we breathe. They can maintain their Ppo2 constant without being concerned about depth or the effort, or change it according to needs such as decompression. The advantage can be less nitrogen absorption and less time to exhale it. In other words, more time with less risk of DCS. It is, it allows us to breathe the optimal mixture in each level. The RB were not created all equals, nevertheless, the less expensive designs do not have this ability.

    Decompression efficiency: Other pluses are that an optimization is got in the decompression. This advantage is only applicable to RB CCR, but not to the SCR. The reason is related to the differences in the dynamics of the gases that are breathed in both types of equipment.

    The RB SCR maintain a more or less constant FO2 (Fraction of Oxygen) in the gas that we breath, whereas the CCR maintain a constant Ppo2 in the gas we breathe. In these last ones, the concentration of O2 in the breathed gas  is at maximum level of security possible during the whole dive, which implies that the complementary portion to the O2 in the breathed gas remains to the minimum. All this makes that the diver can be more time in the seabed without needing decompression, or accelerate the decompression process when it starts.