SSteptepping into rebreather diving By Paul Beenaround Alex had always been interested in learning to dive with the new breed of closed circuit rebreather’s that had appeared in recent years, so when he was suddenly faced with the choice of a dance weekend in chilly Munich or a week learning to dive with them in the Red Sea it wasn’t a hard decision to make. He was soon on his way with Easyjet flying over the Swiss Alps en-route for Sharm el sheik. Everything went very smoothly and he had soon settled down in the Camel hotel, a slightly upmarket one in the centre of Sharm which was not far from the Red Sea College where he would be doing his rebreather course. The next day he met Sherif, an impish Egyptian who was going to be his mentor for the combined basic and advanced four day course. After some classroom work he was shown the intricate workings of the Poseidon MK6 rebreather, the only one recognised by PADI for recreational sports divers on their recently introduced rebreather course. It was a very complex piece of kit, largely controlled by computer software, which took away the risk of diver error. This is unlike many other versions on the market that had caused accidents and deaths, usually through the diver making a mistake. As nice as it was to know that it was keeping him alive, the Poseidon rebreather’s had had many software problems that largely prevented you either starting a dive or necessitated aborting a dive halfway through for often non-existent faults. Hopefully, though, these had been rectified with the recently introduced MK7 version. The PADI diving manual even suggested having a spare open circuit scuba set handy just in case. Alex’s unit seemed to be ok, but over the 2 coming six days diving he was to see other divers experiencing problems with theirs. Alex’s first two days of diving were from the nearby beach, opposite the dive college where he had first been talked through the complexities of buoyancy control and bailout procedures, so very different from open circuit scuba. The rebreather unit on first sight looked like a standard divers stabilizer jacket with all the rebeather components attached to it. Once under the surface using the rebreather felt very strange, more like blowing in and out of a paper bag than the power assisted blast of life giving air he had become accustomed to from his scuba set. Added to this was the total silence and just a slight awareness of his breath passing back and forth through the breathing loop coupled with the slight expansion and contraction of the counter lungs on his shoulders which had some similarities to an astronauts life support system. Put simply, he was breathing unassisted through a breathing loop. The loop started from one side of the mouthpiece, from where exhaled air passed through a non-return valve, along a corrugated tube into a water trap attached to a bag called a counter lung, before entering a two litre canister called a scrubber. The scrubber was filled with co2 absorbent that looked like cat litter, which removed the carbon dioxide from each exhalation. The air then passed back into a second counter lung and water trap, along a second corrugated tube back to the mouthpiece to provide another life giving breath of carbon dioxide free air. The whole process is driven by the power of the lungs. However, to put it 3 not so simply, the rebreather had oxygen sensors that automatically activated injectors that put just the right amount of oxygen back into the breathing loop to keep the life supporting air at a maximum partial pressure of 1.3 or less, depending on depth. This is one of the reasons why rebreather divers can stay down for hours underwater without getting the bends - as the unit is always replacing the nitrogen content of the air with oxygen, the cause of bend attacks, with just the right amount of oxygen falling short of an excess that would cause oxygen toxicity and a blackout. Alex found the random hissing of the oxygen injectors strange to listen to, something the on-board computer was totally in control of as it did its job. Air was also injected in the same way from a second small cylinder to maintain a full breath in the breathing loop. This could be needed due to either a loss of air from mask clearing or descending to a greater depth. In some non-recreational rebreather’s these actions are monitored and controlled manually by the diver. Alex’s rebreather had a red light mounted on top of the mouthpiece that flashed every five minutes to remind him to check his consul for partial pressure, cylinder contents and battery life, apart from the usual diving necessities of depth and time down. The battery was contained in a transparent perspex pod along with a computer, effectively the brain of the rebreather. The computer came in different levels of function depending on the kind of diving intended. Alex’s was the most basic one, limited to 130ft with no decompression ability. The two things that the Poseidon rebreather’s couldn’t protect a 4 diver from was an exhausted scrubber unit (The scrubber unit was only good for three hours) or faulty non-return valves in the mouthpiece. In both cases the rebreather would allow the diver unknowingly to continue breathing from the loop without the carbon dioxide being removed, which would eventually lead to unconsciousness and drowning without prior warning. Carbon dioxide sensors are being developed for the future. Before entering the water the final check is a five minute pre-breathing test taken with a pinched nose whilst sitting down, just to make sure that the set is safe to use before the dive. The other big no no when using a rebreather is removing the mouthpiece whilst in the water without first switching over to the built in bailout valve, which automatically closes the breathing loop from any water ingress. Flooding of the breathing loop would destroy the scrubber contents and create an un- breathable caustic cocktail, necessitating an expensive cleanup afterwards. The bailout valve on the mouthpiece is exactly what it says and automatically supplies air from the small air tank incorporated in the unit. Because the capacity of the tank is so small, diving without an external scuba bailout tank is limited to sixty feet, because the small air tank contains just enough air to allow the diver to reach the surface from that depth. This was where the advanced part of Alex’s course came in, though he did start early by carrying an external bailout tank after his first dive. Effectively he was given a seven litre scuba unit without a harness, which was clipped under his arm to the stab 5 jacket and used on all future dives. He never did like carrying it, even though it was essential on all dives over sixty feet, as well as to provide a backup for his buddy diver. It was uncomfortable to carry as it often banged against the other equipment and sometimes made his position in the water lopsided. It was also difficult to remove and replace underwater, which he was made to do often as part of the training course. He spent half of his second beach dive learning the skills that required for him to go on to the bailout cylinder without a facemask and then ascend to the surface. He was then taken on a short tour of the nearby reefs where he saw an enormous five-foot black and white grouper, but had no camera. It didn’t all end once the days diving was over as he had to spend each evening studying the PADI dive manual in preparation for the two written exams he would have to take. On day three he graduated to boat diving with a mixed crowd of divers using rebreather’s and scuba equipment, and their first dive was on Woodhouse reef in the Tiran straits. Assembling the rebreather on a rolling boat with the distraction of other divers was a lot more difficult than doing it back at the dive centre. He had a very long checklist to follow, much like a pilots, and if interrupted he had to start again. Alex’s set had fifty five self analyzing checks to do whilst expecting him to acknowledge and respond on request by way of the monitoring consul, after he had analysed the gas contents in each of its two small cylinders to make sure the contents were correct before turning them on. If any of these checks failed Alex had to go over the whole assembly and start 6 again. Usually this only happened when he had missed something out or done it in the wrong order, but at least it was keeping him alive even if it often looked like it was trying to stop him from diving. Perhaps a short prayer and the sign of the cross should have been added to the checklist. The very last check to do was the pre-breathing one, which was left until just before the dive. The bailout bottle was clipped on before he jumped off the boat. After entering the water each diver had to do a complete rotation at twenty feet as his buddy did a bubble check on him, and if a leak was found the dive would be terminated. Sherif had told him to bring his camera, but even though he took it once he had no chance to use it as he was kept busy repeating the same survival skills over and over, including the difficult bailout tank removal and replacement whilst hovering over a drop-off.