Text by Paul Vinten, photos by Irena Tyfova, Kevin Moore & Simon Richards.
Images courtesy of Jetsam Technologies Ltd.
Over the past few years, there has been a noticeable increase in the number of divers jumping in the water without tanks on their backs, but instead carrying strange boxes or weird looking configurations of pony cylinders, hoses and cables. The Closed Circuit Rebreather is now an almost common sighting at most of the diving destinations around the world. While they started off as extremely expensive pieces of kit for just the hardcore technical and cave diving community to push back the frontiers of their diving exploration, the modern rebreathers on the market, while still by no means cheap, are more & more being aimed at the general diving population. Still more commonly purchased by the members of the technical diving community, there are increasing numbers of recreational divers discovering the advantages of silent, bubble -free diving. In buying a unit for myself, I looked at the various rebreathers on the market, their merits and limitations, their prices and, of significant importance to those in more far flung global locations, the ease of obtaining spare parts and servicing the unit. I finally opted for the Classic KISS unit from Jetsam Technologies Ltd. based in Canada. My decision was based on the fact that is a manual CCR as opposed to an electronic CCR and so there are less parts to go wrong, and when things do go wrong, it is easier to get spares as they do not necessarily have to come only from the factory – there are several parts which are more widely available, with some parts even available from any good camping & outdoor activities shop! Added to this was the fact that due to its lack of electronics, the price tag is far less than that of the electronic CCR. Of course there are certain limitations to the KISS rebreather such as the maximum operating depth, but it was nevertheless more than suitable to the majority of my diving needs. So just how good is it? Well before I try to describe that, perhaps a brief explanation of just what a Closed Circuit Rebreather is might be in order.
In ‘normal’ scuba diving when a diver jumps in the water with a cylinder strapped to their back, they are diving what we call ‘Open Circuit’. In other words, the diver breathes in from the tank via a regulator, and then exhales all of the waste gas into the water, hence the breathing cycle is ‘open’ to the surrounding water. A Closed Circuit Rebreather, as the name suggests, has a closed breathing loop which recycles all of the diver’s exhaled gases back into the unit to be re-breathed by the diver. The result of this is that the diver can make a dive with a much smaller gas supply (often just using 2 or 3 litre tanks) as they only require an oxygen supply to provide the oxygen which his or her body is using for metabolism and a second tank containing air (or trimix), called the diluent gas, which maintains the volume of gas in the breathing loop so the diver can get a full breath even as they go deeper and the pressure causes the gas in the breathing loop to decrease in volume.
A diver’s body metabolises between 0.7 and 1 litre of oxygen per minute. A diver with a Surface Air Consumption (SAC) rate of 20 litres per minute will be inhaling approximately 4 litres of oxygen and 16 litres of nitrogen per minute. The body will use about 1 litre of this oxygen and so the remaining 19 litres of gas per minute are simply wasted on exhalation. At 30 metres (4 atmospheres pressure absolute) the same diver will be inhaling 80 litres of gas per minute, and so wasting 79 litres per minute into the water as the body’s oxygen requirement remains unchanged regardless of the depth. At 50 metres, 119 litres of the 120 inhaled per minute will be wasted.
A Closed Circuit Rebreather keeps the gas a diver exhales, removing the carbon dioxide through a chemical scrubbing process, adds the tiny amount of oxygen the diver’s body requires, then feeds it back to the diver via the mouthpiece, hence the breathing loop is fully closed. The diver is breathing into and out of bags, called counterlungs which enable the breathing loop to contain enough gas to give a full breath every time, with the scrubber canister usually located between the two counterlungs. The counterlungs are connected to the diver through a breathing hose which is much wider than a normal regulator hose in order to minimise the breathing resistance to the diver. Then finally, somewhere in the breathing loop will be located oxygen sensors, similar to a nitrox analyser, in order that the diver can monitor their partial pressure of oxygen at all times and maintain the optimum breathing mixture for the depth they are at.
While all this sounds complicated, the end result is a much longer gas supply all in relatively small package when compared to twinsets and stage tanks, and all this combined with the fact that the diver will be breathing the optimum gas mix at every depth so maximising their no decompression time, or should decompression stops be required, the rebreather will enable accelerated decompression profiles without the need for carrying several stage tanks containing different decompression mixtures. In addition to this, because of the chemical reaction in the scrubber which removes the carbon dioxide, the diver is breathing warm, moist air rather than cold, dry air as in open circuit.
Add to all this the experience of bubble-free, silent diving and you really have a great piece of diving equipment.
The two general types of Closed Circuit Rebreather on the market are manual and electronic, with both essentially doing the same thing but with the electronic one monitoring the oxygen partial pressure and, through a computer, deciding when it needs to add oxygen automatically to maintain the diver’s breathing mixture. With a manual one, the diver still has the displays showing the partial pressure of oxygen, but it is up to the diver to decide when, and how much oxygen to add into the loop. This has advantages and disadvantages, like most things. The advantage is that there are no computers, solenoids (which control oxygen injection) and other electronic components to fail, especially in the event of water getting into the loop. Also the diver has full control over their oxygen partial pressure, being able to increase or decrease it at will without the need to reprogram computers under the water. The disadvantage of this is that the diver must monitor the unit closely, knowing there is no help in ‘flying’ the unit from an onboard controller and so this may increase the task loading on an already stressful dive. There is also a shallower maximum depth limit when compared to electronic units due to the oxygen injection method, although this is really only of great concern to those deeper trimix divers.
So why doesn’t everybody use a Closed Circuit Rebreather if they are so good? Well obviously the price is a big factor for the more infrequent diver, with prices ranging from in the region of 5000 euros for a manual rebreather up to in excess of 15000 euros for the more expensive electronic units. Costing aside, a rebreather diver also needs a different attitude towards their diving. It is no longer possible to just put a regulator on a tank, check it’s full, then leap into the water in the space of a couple of minutes. Setting up rebreather requires several steps and a number of pre-dive safety checks and tests. All this takes time and dedication, so if you are not prepared to spend time setting up your unit and doing the checks while others on the boat are sunbathing or getting a cup of tea or stripping your unit completely after a days diving while others are watching TV, then rebreather diving is just not for you.
So I finally decided on the Classic KISS rebreather, a manual unit with a manufacturer’s depth rating of 75 metres. It is a small unit when compared to certain other rebreathers available – another thing which attracted me to it in the first place, and has a price tag at the cheaper end of the rebreather market. When it finally arrived in my hands via numerous shipping methods to the UK then Egypt it was in several small and not-so-small pieces. Having already downloaded the user manual from the Jetsam website I knew what to expect, and coupled with the DVD supplied with the unit, putting the right ‘o’-ring in the right place and bolting things together was a very simple and relatively quick (about half an hour) process. Admittedly, being a technical diver for a number of years and having worked with rebreather divers for much of that I probably had a significant advantage in at least knowing what everything did. For the diver who is completely new to the rebreather world, I would strongly recommend waiting until you make your course to assemble the unit so that your instructor can explain parts as you go along, and also ensure correct assembly. Obviously under no circumstances should a diver take the unit under the water without completing a proper course on the specific rebreather they are using. The large warning on the top of my unit said it all – ‘This device is capable of killing you without warning’.
The Classic KISS rebreather has back mounted counterlungs – meaning that they behind the diver, located in a solid case to protect them. Other rebreathers, such as the Evolution, have over the shoulder lungs and still other units have front mounted ones. The basic difference between the different counterlung locations is the work of breathing (WOB) as the gas in a breathing loop will always try to migrate to the highest point in the loop. The trick is to try to keep the counterlungs at the same depth as the diver’s lungs in the water, so slightly altering the attitude of the diver in the water while swimming.
Oxygen injection on the KISS is via two methods – there is a constant flow orifice which very slowly feeds oxygen at a fixed rate set slightly lower than the divers metabolic oxygen requirement, then all that is needed is for the diver to manually add small amounts of oxygen from time to time by pressing a button similar to a BCD inflator button in order to maintain the correct oxygen partial pressure in the breathing loop. There are three partial pressure displays coming from three separate sensors in the rebreather giving the diver complete redundancy in the event of one sensor failing or flooding and it is up to the diver to take the average reading or to decide if one reading is out.
I so it was time to get on with the course with my instructor, Mickael from Tekstreme in Sharm. The theory sessions, as would be expected for something as complicated as a rebreather, are quite in depth covering all manner of things from rebreather maintenance and check lists to diving on fixed partial pressures as opposed to open circuit diving on fixed gas fractions. It’s a totally different concept and one which is covered at length. The diving started off with two confined water sessions lasting an hour each in which all the emergency procedures were covered, as with any diving course. Most emergencies on a rebreather are to do with the oxygen partial pressure being too high or too low, resulting in oxygen toxicity problems or hypoxic situations. For this reason, rebreather divers always carry some form of ‘bailout’ option which enables them to get off the rebreather loop and on to an open circuit option to get them back to the surface. In addition, skills were required to adjust to a different form of buoyancy control. In open circuit diving, the diver’s buoyancy changes with every breath as they inhale & exhale – a fact which is used in fine tuning that perfect hover. On a rebreather, the diver is not changing the volume of gas in the breathing loop as they breathe – they are simply moving a fixed volume of gas around a loop of which the diver’s lungs have become a part. This therefore means that the buoyancy control is done through the BCD alone, with a second consideration being the breathing loop volume which must be maintained on descent and ascent.
Following the confined water sessions, a further 6 hours of diving were required in open water at progressively deeper depths down to a maximum of 40 metres to practice the emergency skills and to get used to diving on a rebreather. It was quite clear from the start that becoming proficient on a rebreather was going to take a lot more diving and practice than the course could offer in just 5 days, a fact which was stressed from the start by my instructor. In switching over to closed circuit diving, you must realise that you taking a step backwards in your diving abilities and treat yourself like a newly qualified open water diver. Just because you have been diving to 30 metres or deeper regularly on open circuit doesn’t mean you can get on a rebreather and continue with exactly the same dive profiles. This has been the reason behind a number of accidents, some fatal, on rebreathers. It’s important to realise that you have to start shallow again and slowly build back up to your ‘regular’ diving depths. After over 130 dives and 110 hours on my unit, and I am still finding improvements that need to be made to my diving, be it trim, maintaining a ‘minimum loop volume’ or making adjustments to my kit.
I have to say though that I have become completely hooked on rebreather diving, finding that even a shallow 9 metre dive is now far more enjoyable now just because of the silence and the fact that the marine life now comes right up to me with no bubbles being produced to startle the fish. Of course, on the deeper dives, the highly efficient gas consumption and reduced decompression times simply speak for themselves in the great advantage to be had, especially when looking at the open circuit technical diver weighed down with a large twinset and two stage tanks.
I try not to laugh too much though – I used to be just like that!