Dräger Ray

While technically not a review (yet?), I suggest you bear with me (think of it as a "first impressions" piece or something). Earlier this week, I and eight other people got the chance to test dive semi-close circuit rebreathers (Dräger's Dolphin and Ray) in the local pool. Having deleted the big wall of text previously occupying this area, what with the probably all to technical description of dive physics and all, I will try to keep it brief. A rebreather is a scuba diving system that reuses the diver's expelled breath, scrubs away most of the carbon dioxide and adds an amount of oxygenated air. Unlike a open-circuit system which only works in a linear direction so to speak (from cylinder, through lungs and into the water), a rebreather has a "loop" system which recycles the exhaled breath and hence requires a mouthpiece with two hoses - one for air to the lungs and the other for air from lungs.

Closed Circuit Rebreathers (CCR) does not expel any bubbles from the breathing loop and requires two cylinders, one with a diluent gas (for volume control) and the other with 100% oxygen. Using a very advanced electronic system, a CCR calculates the oxygen content in the loop and adds oxygen mixed with the diluent gas for a mixture optimal to the current depth. Military CCR's may also be oxygen rebreathers (which operates only with 100% oxygen), but since oxygen gets potentially nasty at 1,4 bar and probably nasty at 1,6 bar the operational depth is only around 4 to 6 meters (susceptibility to o² poisoning varies depending on the individual, physical state, exertion and a whole bunch of other factors).

A Semi Closed-circuit Rebreather (SCR) on the other hand only has a single bottle of usually Nitrox (oxygen enriched air) - not counting the obligatory open circuit bail-out system, which technically is not a part of the rebreather. A SCR does expel some bubbles, but only from the back (which can be seen in the image below) and usually at a very slow rate. The valve is adjustable and very similar to the air exhaust valve on a dry suit. The SCR operates by constantly and slowly letting air from the cylinder into the breathing loop which then is bled out of the escape valve as the system gets pressurised. A pleasant side-effect of the scrubbing process is that the air gets a little bit warm and still retains its moisture. In contrast, breathing from a open-circuit system the compressed air is bone-dry and rather cold (a side effect of decompressing the air) which makes it unpleasant to breathe for extended periods.


















Unlike a open-circuit system, which can be used directly after assembly, a rebreather requires a little bit of preparation. After assembling the rebreather, and assuming that you're ready to dive, you slowly open the cyliders k-valve and let the nitrox into the system. At this point the counter lung (a "breathing-bag" which is connected to the loop) should be filling. To fill the system with nitrox (and remove any ambient air from the loop) you open the mouthpiece valve and breathing in from the mouthpiece and exhale though your nose. Then the scrubber material must be warmed up, this is important since the material must be effective even at depth. This is just a matter of breathing though the loop for a few minutes (3, I think). At this time the rebreather should be ready for your dive. Turn on oximeter, dive computer, check the bail out system and fill the vest (or BCD part of the rebreather). When entering the water, the rebreather's mouthpiece valve should be closed, if not water pressure will compress the counter lung and air will escape though the mouthpiece. The diver will loose buoyancy, possibly even sink and if water enters into the scrubber a not so fancy drink called "caustic cocktail" will be brewing in the loop. To begin the dive, put the mouthpiece in your mouth (not closing your lips around the mouthpiece completely), open the mouthpiece valve, put the mouthpiece in your mouth and descend. Apart from using the BCD to reduce buoyancy, a diver can also exhale though the nose which allows air to escape the loop thereby reducing buoyancy.

The first thing you'll notice while diving with a rebreather is the strange sensation of breathing through a counter lung system. I'll say that it probably feels kind of like breathing air from a bag underwater, if that makes any sense. As I waited on the bottom of the pool for my dive buddy, I couldn't help to feel that the system delivered to much air - not that I needed to breathe it in, excess air is bled out through the aforementioned valve, but the pressure kind of felt unnecessary high - as if the water pressure was pushing on the counter lung. There was some problem with my dive Buddy's equipment so the instructor signaled that I could swim around for a while. As I moved from kneeling to swimming horizontally, the excess pressure I previously felt from the counter vanished. Breathing became much more natural as the pressure was now more evenly distributed on the counter lung and the mouthpiece was no longer the highest point. Soon my dive buddy was in the water and we could swim around at our leisure. Unlike a open-circuit regulator, there was no noisy on-demand valve, delivering cold and dry air while creating a torrent of bubbles around your mouth. Everything was quiet. Perhaps to quiet. Somehow I've gotten used to the noisy sound of air circulating, being able to listen for abnormalities, leaks and other warning sounds. Now, the only thing I heard was the occasional hiss of air being added to the counter lung if it was running low. The counter lung also pretty much throws the old principle of buoyancy control using lung volume out the window. Inhaling to ascend does not work anymore since the total amount of water dispersion is the same - inhale and air is taken from the counter lung, exhale and air enters the counter lung. Pretty much zero-sum all around.

We each had about 10-minutes in the pool. I dove with the Dräger Ray, pretty much for the simple reason that I warmed up the Ray's scrubber - might as well dive with it. The Ray is more compact then the Dolphin but also has a shallower maximum depth. It was too bad that we didn't have a chance to try both systems. I loved the Ray, but unfortunately (apart from being pretty damn expensive) it has a maximum depth rating of just deeper then 20 meters (depending on nitrox blend and modifications to the Ray) where as the Dolphin can go as deep as 40 meters (if my memory serves me correctly).

Anyways, if there's one thing you should take with you from this, what ever it is, it is this; If you get the chance to test-dive a rebreather - take it. Just make sure that you do so in confined water under instructor supervision.