CCR is Safer
By Ian Forster
PADI Course Director
PADI Rebreather Instructor Trainer
UWECO Instructor Trainer Director
Divers Alert Network (DAN) concluded at the end of the Fatality Workshop, April 8-10, 2010 that running out of gas was the major risk in diving fatalities, but that this risk could be reduced by gas supply monitoring and alarm engineering, including built-in, low-air alarms in dive computers. This “wishlist” expressed the workshop’s desire for divers to monitor their gas supply more frequently, as well as their hope to see the development of alarm systems in future equipment. In open-circuit diving the diver usually has to monitor his gas supply without any prompting, and although in-built alarms are available, divers usually opt out of this optional extra. When these divers forget to monitor their gas supply, they may only realise they are on their last breath when they are too deep to make a safe ascent to the surface.
CCR – The Most Efficient Underwater Life Support System
However, a modern, closed-circuit rebreather such as the Poseidon MKVI Discovery which comes with gas supply monitoring and five separate warning and advisory alarms as standard is simply the most efficient underwater life support system in the world. Many experienced rebreather divers actually feel safer diving their rebreather than diving open-circuit, just as 747 pilots feel, and are, safer in their aircraft than in their far less complicated automobiles. There are six major safety advantages that modern closed-circuit rebreathers have over open-circuit.
Just as good pilots use pre-flight checklists to ensure a high probability of a successful take-off, flight, and landing, so divers need to formalize the pre-dive process. In open-circuit diving completion of these checks is done by the diver, hopefully supported by his buddy. But because they are manual, and totally dependent on memory, it’s too easy for a diver to forget some of the checks. At best, this is an inconvenience when discovered as the diver enters the water – but too often it can lead to panic, injury or death when discovered underwater. However, in modern, closed-circuit diving all of the critical checks of the unit are completed by the electronics. The MKVI is the first truly auto-calibrating and auto-validating rebreather, performing 55 pre-dive test procedures, and prompting the diver to interact on 5 of them. Not only does this reduce the task-loading on a diver, at a time when they should be relaxed, but it simplifies the manual checks by the diver to those like whether the dry suit zip is done up, and the weight belt is on.
Safe Gas Mixture
Maintaining an appropriate partial pressure of oxygen (PO2 or PPO2) in the breathing gas is critical to ensure safe diving. For open-circuit divers on air this does not become a problem on dives shallower than 50 metres. However, below 25-30 metres all divers are increasingly affected by the high percentage of nitrogen in the air (79% N2 compared to only 21% O2), with their judgement and skills impacted by nitrogen narcosis. Divers can dive on gas mixtures which contain more oxygen and less nitrogen, but they have to be highly trained in oxygen pathophysiology, have to plan every dive using special oxygen exposure and toxicity tables, and constantly monitor their depth to ensure they don’t exceed the safe PO2 limits. However, modern closed-circuit rebreathers such as the MKVI do all this for the diver without the need for any special training, apart from monitoring the electronic display, and without the need for complicated maths or the risk of calculation errors. If the PO2 value varies substantially from the target PO2 (setpoint), the value will flash. If the value becomes dangerously high or dangerously low, the diver will be prompted to switch to open-circuit mode and terminate the dive. Throughout the dive, the rebreather delivers a safe gas mixture for whatever depth the diver is at, and decompression is also greatly minimized.
Much More Air and Time
Unlike open-circuit scuba, the rate of gas consumption on a CCR like the MkVI does not depend on the depth of the dive. Instead, the oxygen supply depends on how fast the diver consumes oxygen through metabolism. This particularly benefits divers with less muscle mass such as juniors and smaller ladies, who will gain significant improvements in dive duration with reduced risk. Because hardly any gas is used, CCR diving does not require fast reflexes or quick thinking, so the CCR diver has much more time to make decisions, and the risk of running out of gas is virtually eliminated. If the diver is monitoring his instruments and can accurately interpret the information, nothing happens fast. In terms of gas supply (decompression limits aside), the CCR has the capacity to support a diver on dives between three hours at 40 metres, and twelve hours at shallow depths, and at 5 metres, a diver can stay underwater almost indefinitely. Time pressure goes away, and dive planning revolves around adequate exposure protection rather than how long to dive, how deep to go, how much no decompression time is left or whether your rapidly-diminishing air supply is enough to get you back to the surface. You can take your time, yet never run out of gas, nor ever fear running out. This major cause of scuba diving accidents is eliminated on CCR.
Failures, malfunctions and stupidity do and will occur often. The open-circuit diver usually discovers the problem late, sometimes too late. Frequently a problem is underestimated or ignored, which then becomes compounded when further malfunctions occur. But in CCR these problems are indicated by the instrumentation or easily recognizable changes in the system’s breathing characteristics. The MKVI is the world’s most advanced rebreather interface, constantly checking itself for 100% correct performance, and warning the diver and those around him by comprehensive visual, audio, and tactile alarm signals when a problem occurs, including ABORT and DO NOT DIVE alerts on the primary electronics display. As a result, the diver and his buddies get immediate warning of a problem with sufficient time to deal with it. Equipment problems are one issue, but divers should never underestimate their capacity to do something really stupid. After logging thirty or forty hours, they often become over-confident and complacent. This can lead to acts of stupidity. Fortunately, the MKVI’s multiple alarm system prevents a stupid act from becoming a fatal one. Provided the diver is well trained and practiced in the normal emergency protocols and procedures, even casual stupidity is survivable. DAN’s “wishlist” alarm engineering is now available!
Although, all divers are trained in out-of-air procedures such as ascending on an alternative air source provided by a buddy, a controlled emergency swimming ascent (CESA), and buoyant ascents, these often break down in a real emergency. In open-circuit, and even in previous generations of CCR, to be able to bail out safely, divers have had to be equipped with a separate “pony” cylinder which is turned off during normal diving to reduce the risk of gas loss. This obviously increases the weight and complexity of all the equipment carried. However, even this redundant system has its problems, because the diver is required to locate the cylinder valve, turn it on, locate the regulator, detach it from its storage device, get it in the mouth, blow it clear with whatever breath is left or purge it, before being able to breathe. This has often led to divers fumbling for the separate bail out device with tragic circumstances. All these problems have been removed in the MKVI CCR by a significant new design of switchable mouthpiece which enables the high-performance regulator to be changed from closed-circuit to open-circuit at the flick of a switch. Prompted when to switch by the unit’s alarms, the most serious of malfunctions can be dealt with by the built-in bailout. Even a complete system flood only requires that the diver switch to open-circuit bailout.
Lighter, Balanced and Compact
Open-circuit dive time will be very limited on any cylinder less than 12 litres, and even a 15 litre single or twinset with additional cylinders clipped on either side will not come close to the gas capacity of a CCR. Such systems are heavy, and the weight is not only off-putting for the majority of divers, particularly ladies and juniors, but carrying such heavy weights or diving with smaller cylinders and insufficient air increases the risk of serious injury. Modern CCR’s are lighter and extremely compact. The MKVI is one of the world’s smallest rebreathers, weighing only 15kg “ready to dive”, and the compact layout of the two, 3 litre cylinders and CO2 cartridge housing mean the unit sits snugly on the diver’s back without the side to side “wobble” associated with large single cylinders. And at only 8kg “travel weight” (without cylinders) it goes as carry-on baggage when flying – again safer to be diving on your own equipment than having to hire resort equipment with an unknown servicing history.
Modern closed-circuit diving is much safer
than diving on standard, open-circuit scuba