By Chris Serfontein
Image by Chris Serfontein
Without the vibration of bubbles in the water, divers can approach marine creatures much more closely.
A question that is often posed to me when I discuss and promote rebreathers is, “I am just a regular scuba diver, so why would I want a rebreather? Besides, they are for technical divers, are they not?”.
The truth is that yes, a rebreather is a fantastic tool for technical divers as it allows for deeper and longer dives, less decompression and there is less weight to carry. Just as you can use a knife in multiple ways, like slicing through bread or using it to pry open a can of paint, rebreathers, too, can be used in multiple ways. A rebreather, as a tool, comes with its own benefits and challenges. As long as a diver religiously follows his or her training and understands the issues that are related to the use of the equipment, rebreathers are as safe as any other piece of equipment. A review of the “triggering events” which are common in diving accidents (including rebreather accidents) tells us that it is not equipment failure that turns a basic recreational dive into an accident, but rather user error.
So, regardless of the equipment that is being used, reducing or eliminating possible errors will reduce accidents.
Rebreathers, in general, provide four fundamental advantages over open-circuit scuba diving systems, namely the efficient use of gas, optimised decompression characteristics, warm and moist breathing air, and near-silent operation. These advantages are discussed in detail below.
Perhaps the most significant advantage of closed-circuit rebreathers (CCRs) is that they greatly increase gas efficiency. Under normal circumstances, a diver only uses a small fraction (around 3-4%) of the oxygen of each inhaled breath as most of the oxygen leaves the lungs unused when the diver exhales. When using open-circuit scuba diving systems, the oxygen and other gases in the exhaled gas are wasted in the form of bubbles. Rebreathers, on the other hand, capture these exhaled gases, chemically remove the carbon dioxide, and then replace this oxygen which your body has used. Typically, fully-closed rebreathers are equipped with two 3ℓ cylinders.
The first cylinder, or diluent cylinder, is usually filled with normal compressed air and the second cylinder is filled with 100% oxygen. The diluent provides the volume you breathe, air for your buoyancy compensator and even air for drysuit inflation in some cases. The oxygen cylinder provides the machine with a source of gas to replace the oxygen which your body uses during the dive.
Most literature states that, on average, your body uses 1ℓ of oxygen per minute during moderate to light exercise levels. With a 3ℓ cylinder filled to 200 bar of oxygen, it equates to 600ℓ of oxygen that is available for use during the dive.
Theoretically, you should then be able to use the rebreather for 600 minutes without having to refill it. In practise (due to the wastage during ascents, mask clears and buoyancy corrections) an acceptable time before you have to refill is around three to four hours of diving. This is not depth-dependent as your body does not use more oxygen when the pressure increases. So, ignoring decompression obligations for a second, you should theoretically be able to spend three to four hours underwater at any depth. As soon as you move into the technical diving field, you would fill your diluent with trimix, which is a mixture of nitrogen, helium and oxygen. As the price of helium is around R0.75/ℓ, a rebreather has a massive financial advantage over open-circuit scuba diving systems. For example, if you want to do a 100m dive, you would add helium into your breathing mix to reduce the percentage of nitrogen, which will reduce the narcotic effects of nitrogen, as well as reduce the amount of oxygen in the mix as too much oxygen would cause convulsions at depth. If you have replaced 50% of the gas in your diluent cylinder with helium, then you have halved the amount of oxygen and nitrogen. In a 3ℓ cylinder, 300ℓ of helium at R0.75/ℓ is R225 for the gas fill. To use the same mix for the same dive, but now putting it in a twinset of 15ℓ cylinders would cost you R2 250. It is 10 times more expensive to do the same dive on an open-circuit system as opposed to doing the diving on a rebreather.
A rebreather ensures that you breathe the perfect gas for a specific depth.
The second advantage of using a rebreather has to do with decompression optimisation. A rebreather ensures that you breathe the perfect gas for a specific depth at all times during the dive. Rebreathers are essentially “on-the-fly” nitrox mixing machines. They are capable of delivering the optimal nitrox blend for your specific depth. In other words, if you change your depth, the rebreather changes and adapts your breathing mix accordingly. This helps to optimise your bottom time.
“How much longer will I be able to dive?” is another question that I am frequently faced with. Well, if you dive on air to 20m depth, depending what tables you use, your no-decompression limit is around 30 minutes. On a rebreather, it is approximately 140 minutes. That is almost five times longer without any decompression. Bikini Reef is one of my favourite reefs in Sodwana Bay. It is in the 18-21m depth range and using normal scuba equipment, my dives usually last for around 40-50 minutes before going into decompression. This, I felt, did not allow sufficient time to fully explore what this reef has to offer. Since I started diving with a rebreather, I can now fully explore every nook and cranny of this beautiful reef.
WARM, MOIST GAS
A rebreather recycles warm, exhaled air, so divers continuously breathe warm, moist air, allowing them to retain warmth and feel more comfortable. This is in contrast to the cold breathing gas as experienced in open-circuit systems, which is cold due to the reduction of pressure at every breath just prior to inhalation. Think of when you receive your cylinder back from being filled; it is warm or hot (depending on how quickly the compressor operator filled it). The opposite occurs when the pressure is being reduced as the temperature of the gas drops and instant cooling is generated at every breath. In a rebreather, because significantly less gas expands, this means that less cold gas is introduced to the breathing loop. Our lungs contain a dense vascular system and much heat can be wicked away through exhalation. You will not expel your body heat into the water with a rebreather as the heat is maintained within the loop in varying degrees. Furthermore, during the chemical reaction in the scrubber that absorbs the carbon dioxide, heat is also generated during every breath and the gas is further warmed up in the rebreather. Additionally, one of the by-products during this chemical reaction is water, which means that the gas in the breathing loop is humidified and results in the diver not dehydrating as much as when they are breathing cold, dry air from a scuba diving cylinder.
With each exhaled breath, a diver who is using conventional scuba equipment releases a large burst of noisy bubbles.
The effect of this on the behaviour of marine life varies, but in most cases, due to the vibration of these bubbles in the water, fishes behave warily and are reluctant to allow a diver to approach them. Rebreathers essentially eliminate bubbles entirely. With rebreathers, divers are able to approach marine life much more closely, thereby reducing disturbance on their behavioural patterns. This is especially important for underwater photographic activities. With a rebreather, you become a part of the environment and not just an observer.
A RECREATIONAL MARKET
This November will mark four years since PADI launched its rebreather training programme for recreational divers and other training agencies have quickly followed suit, with most agencies now offering a recreational rebreather course as part of their course listings. The programme is based on a new class of machines, namely the so-called Type-R units, which automate many of the functions of rebreathers and in some cases even make decisions for the diver. This makes them easier to use and therefore reduces the potential for user error which can lead to accidents. These functions include automatically turning on should the user forget to, the inability of incorrect assembly and non-operation unless the scrubber canister is present or the gas cylinders are turned off. These rebreathers automatically calibrate the oxygen sensors, analyse the gases in the cylinders, and they have a built-in open-circuit bailout valve and a mandatory pre-dive checklist, to name a few. These units also require third-party testing, which minimises quality issues and ensures that they are limited to mainstream manufacturers. A recreational rebreather can be compared to a car that has an automatic transmission, anti-locking braking system, power steering, auto-lighting and active cruise control. We routinely rely on automation to make complex machines easier to use in the rest of our lives and recreational diving should be no different.
The Poseidon MKVI (which licensed its technology from Dr Bill Stone) was the first Type-R rebreather to be certified by PADI (in 2011) and represented a turning-point in rebreathers with a slick automated user interface, an automated pre-dive checklist and auto-calibration. Dr Stone also pioneered their patented dual-sensor “active validation” system, which yields a single partial pressure of oxygen value for measuring oxygen in the loop in contrast to all other CCRs which display three varying values – one for each sensor that the user must interpret. It was the first major innovation in oxygen sensing in more than 50 years.
With a rebreather, you become a part of the environment.
Another machine which is currently accepted as a Type-R rebreather is the Inspiration, which was launched by AP Diving in 1997 as the first production sport rebreather. It is now the world’s largest rebreather manufacturer and has been a pioneer in machine automation with the patented dual oxygen controller, auto-set point change, auto-calibration, auto-device detection, as well as being the first to offer a canister duration monitor in 2005. As a result, all of their rebreathers produced since September 2012 are compatible with the Type-R specifications by simply turning on a software key and modifying some plug-n-play hardware, such as removing the manual oxygen add valve (the equivalent to adding a governor to a car).
The Hollis Explorer was the latest addition to this group of rebreathers and offers the diver a price advantage with a 50-60% reduction in price when compared to the first two machines.
Being a solely semi-closed rebreather, it does not offer the buyer the potential to upgrade to become a technical machine, but it fills a valuable gap in the market for those divers who are not interested in doing deep technical dives. Hollis has completely split their recreational rebreather from their technical rebreather with two separate models (of which the Prism is their technical machine).
Both the MKVI and the Inspiration are upgradeable to technical level rebreathers. This feature forms part of their specific strategies and also marks the major difference when compared to the Explorer (which is not user upgradeable).
Rebreathers will never totally replace open-circuit scuba systems, but if you yearn to spend more time underwater exploring your favourite reef, photographing an elusive or skittish critter, or exploring reefs or wrecks that are hardly ever explored by scuba divers due to their depth, maybe it is time to look at a rebreather which can grow with you as you develop your knowledge and understanding of these wonderful machines.