I had been thinking about rebreathers for quite a while, but never got round to doing anything about it. I sat down and analysed my requirements. Photographically, I liked the idea of less bubbles. The main thing of interest to me was the prospect of getting closer to big fish.
I already carry more than enough camera and diving equipment on foreign trips without having to ship a rebreather as well, so availability of rebreathers as local rental equipment was a prerequisite if I was to put rebreather training to good use in the future. In this respect one type of rebreather dominates, the Dräger Dolphin semi-closed circuit rebreather (SCR).
Having analysed my objectives I ventured to Bonaire to do some training. My plan was a few days open circuit diving, then do the SCR training, followed by few more days diving so I could practice sneaking up on fish and evaluate the rebreather in real use.
Bonaire is an island almost completely dedicated to diving. Even on open circuit scuba I was able to get close to many of the fish. By the end of a few days easy diving I was feeling a little bit sceptical. Would a rebreather really make a difference?
Time came for the course. Entry requirement for the course is a basic nitrox qualification. With students at this level the theory can take a good long day. The course has to effectively cover advanced nitrox theory in addition to all the rebreather specifics. I have been teaching technical nitrox courses, so with me as the only student the classroom work was well covered in just one morning.
I was amazed at just how simple the whole thing was. Within the hard shell are exhale and inhale bags connected via a scrubber canister and what is effectively a second stage demand valve with a controlled leak into the inhale bag. This is fed via a first stage and a cylinder of nitrox that lies across the diver's back.
The principles upon which a semi-closed circuit rebreather works are correspondingly simple. A single cylinder contains a nitrox mix. As long as carbon dioxide is removed from the exhaled breath, a diver can re-breathe such a mix again and again. To compensate for consumed oxygen a constant trickle of fresh nitrox is added to the breathing loop. As nitrogen builds up an exhaust valve automatically vents excess gas. In practice an equilibrium is reached with a fairly constant nitrox mix. For example, with 50% nitrox in the cylinder a diver making a relaxed swim would end up breathing a 40% mix.
The only electronics was an optional oxygen monitor that would warn of high or low oxygen levels. It is not needed to make the thing work, but it did seem like a nice idea to have something to reassure me that the mix I was breathing was within safe parameters.
We set off to the reef so I could get used to the system. As this was my first dive on a rebreather I had decided to keep the photographic side simple. I had a very wide-angle lens on my camera to get some basic diver shots. With those out of the way I could go fish stalking with a close up lens after I had a few dives practice.
Lining up for a shot by a nice clean dome of brain coral I breathed in to rise up slightly and nothing happened. I had forgotten about the quirks of buoyancy control on a rebreather. With the rebreather holding exhaled air all buoyancy control has to be achieved by fine tuning the air in the stab jacket.
I hastily scrabbled for the stab control to get some air in and avoid denting the reef. Coming round again I could really have done with a third hand to control my buoyancy whilst the other two were busy with my camera.
I was just lined up for the shot and the audible alarm in the optional oxygen monitor went off. At just 15 metres my oxygen partial pressure had reached 1.3 bar. We ascended a couple of metres to reduce the oxygen partial pressure and clear the alarm before repeating the attempt with the same results. It was OK while I was swimming along and metabolising more oxygen, but as soon as I just floated there to take a picture I stopped metabolising oxygen and the oxygen percentage I was breathing crept closer to that of the supply cylinder.
Apparently these monitor and alarm accessories can be set for less frustrating levels, but they were a relatively new piece of kit and had not been reprogrammed yet.
Returning towards our starting point I found a nice looking grouper resting in the shade of a soft coral. Under normal circumstances I would have used a longer lens, held my breath and floated in for one shot before backing off to avoid scaring it with my exhaust bubbles.
We crept in and got close enough for a photo through my 14mm lens, a lens not designed for fish portraits of anything smaller than a whale shark. The grouper flinched at the flash but stayed put. I continued breathing without any exhaust bubbles to scare it off. There were now two divers on rebreathers much closer than a single diver could have got with open circuit scuba.
My conclusions from this limited experience: I am wholly convinced of the advantages of rebreathers for getting closer to fish. I have the pictures to prove it. Maybe I could have got as close on open circuit, but not with two divers for repeated photographs. Bubble free diving would be even more of an advantage to divers with video cameras.
The minus was not being able to fine tune buoyancy with my own lungs. Sneaking up on a fish I needed two hands to hold my camera and a third hand to play with the stab controls. Established rebreather using underwater photographers must have developed techniques for stalking fish that avoid buoyancy adjustment when lining up a shot.
Another minus was the over sensitive alarm on that annoying oxygen monitor. An optional extra I could have happily done without. The sooner it is reprogrammed to a less frustrating level the better.