What exactly is a closed circuit rebreather: a VERY brief overview?

The only constant in the diving industry seems to be that technology is constantly changing.  New dive computers are more feature rich, training standards evolve, and companies come out with yearly updates to their regulator product lines.  One thing which has been around since the beginning in one form or another, however, is the rebreather.  Recent technological innovations in the closed circuit rebreather (CCR) market have made these rebreathers not only attractive to technical divers, but also to recreational divers seeking the variety of benefits which CCRs provide.

In contrast to open circuit scuba equipment, a CCR recycles the gas which is otherwise wasted when a diver exhales through a standard regulator.  Have you ever watched your exhaled bubbles shoot to the surface and wondered how much of that gas your body actually used?  The answer is very little, which is one of the reasons CCRs are so appealing.  In contrast to a scuba regulator, a CCR takes a diver’s exhaled breath, scrubs out the carbon dioxide, adds a little bit of oxygen to compensate for that which was metabolized by the diver, and then allows the diver to re-use that breath.  This is how CCRs got their name, they are essentially a closed system from which no gas escapes.  Thus, a standard scuba tank becomes less efficient the deeper you go because you are “wasting” a larger volume of breathing gas via exhalation, because of the increased ambient pressure.  A CCR, in contrast, becomes more efficient the deeper you go because you are simply recycling the portion of breathing gas which you would otherwise exhale.  Thus, by staying within recreational diving no decompression limits and diving the CCR properly, a diver will never have to worry about running out of breathing gas, regardless of depth or time.

In addition to becoming more efficient the deeper you go when compared to open circuit scuba, a CCR allows one to experience significantly longer no decompression limit bottom times on a typical multilevel dive.  When diving with a nitrox tank recreationally, a diver enjoys the maximum allowable no decompression limit bottom time when the partial pressure of oxygen equals 1.4.  (An oxygen partial pressure of 1.4 being the maximum allowed recreational diving partial pressure of oxygen when diving a given nitrox mix.)  Presumably, this only occurs during the bottom portion of the dive because the partial pressure of oxygen begins to drop as the diver ascends.  As the partial pressure of oxygen drops, the partial pressure of nitrogen increases and subsequently decreases the allowable no decompression limit at that depth.  Theoretically, if a diver were able to switch to a tank of nitrox having a partial pressure of 1.4 at each depth on a multilevel dive, they would be maximizing their no decompression stop bottom times.  Well, a CCR lets a diver to do just that.  A CCR allows a diver to select a partial pressure of oxygen for a given dive which the CCR will electronically maintain throughout the dive.  So, a CCR diver will be breathing the optimum nitrox mix for a given depth automatically.  For example, if the diver selects an oxygen partial pressure of 1.3, the unit will maintain the partial pressure of oxygen at 1.3 regardless of depth.  In order to accomplish such a feat with open circuit scuba equipment, a diver would need an infinite number of nitrox tanks.  The net effect, as discussed above, means that the diver can enjoy longer no decompression bottom times at each level of their dive.

Given the benefits, it is easy to see why recreational divers are increasingly adopting CCRs.

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