Are you right for CCR?
Closed-circuit rebreathers remain controversial, but John Bantin believes that they are hard to beat - so long as the user is prepared to learn an entirely new language
AP Valves' Buddy Inspiration is the only closed-circuit rebreather (CCR) to bear a CE mark and so to be marketable in Europe. When it was introduced commercially to the UK a little over three years ago, Diver hailed it as possibly the greatest advance in scuba-diving since Jacques Cousteau introduced the aqualung.
Since then, a number of people have suffered fatal accidents while diving with rebreathers, including the Inspiration, the hi-tech CIS Lunar and even relatively simple semi-closed circuit rebreathers. While we know of no accident that has been attributed directly to the equipment itself - as opposed to way in which it was used - its presence does raise a serious question: "Are you right for it?"
The CCR is not merely for a small minority who want to go deep. The greatest advantages are to be seen in the normal European recreational diving range of 20-50m.
The device varies and delivers to the diver the optimum mix of nitrox at any given depth. This means that the diver absorbs the minimum amount of nitrogen and breathes reduced amounts of the gas on ascent. This reduces dramatically the decompression requirements over normal open-circuit (OC) scuba or semi-closed circuit rebreathers (SCR).
No-stop time at 18m for a diver using a CCR (with an oxygen pressure set constant at typically 1.3 bar) is more than three hours. No-stop time at 18m for an OC diver using air is 51 minutes, according to the BSAC 88 tables.
At 52m, the CCR diver breathes the equivalent of air, but this mix progressively richens as he comes shallower, speeding decompression.
OC scuba divers sometimes have trouble getting their minds around the fact that CCR divers use less oxygen as they go deeper. In fact gas supplies cease to be a problem. The limiting factor is CNS oxygen toxicity and CO2 scrubber life - effectively around three hours.
Those who use OC scuba can have useful dives to 50m if they take enough breathing gas with them, and can shorten decompression penalties by breathing progressively richer mixes as they ascend. For example, they might take nitrox 21 (air) for use at the greatest depths, plus additional tanks of, say, nitrox 32 for use between 18m and 33m and nitrox 50 to use shallower than 18m.
This, however, means taking extra cylinders and different regulators from which to breathe. There is always the risk that if a regulator is not clearly distinguishable, the wrong one might be used at too great a depth - with possibly fatal results.
And there is always the possibility of running out of gas, something that should never happen with a CCR.
The Inspiration user's supply at 30m is equivalent to that of an OC diver with more than twenty 12 litre cylinders. So if he starts with 200 bar of O2 in his 3 litre supply, he has more than enough for a weekend's diving!
SCRs, such as the DrŠger units, use a constant flow of a fixed nitrox mix into the breathing loop. They offer a slight saving in gas requirements over an OC rig (reflected in the smaller nitrox cylinder needed) but offer no decompression advantage.
The flow-rate is the same regardless of work-rate and excess gas is allowed to bubble off, although in reduced amounts compared to OC scuba.
Some would say that SCRs offer no advantage over a suitably large OC twinset filled with the same nitrox mix. And it is simpler to put the right OC regulator in your mouth at the right time than to grasp the theory needed to cope with the dangers of rebreathing CO2-scrubbed gas.
With OC scuba, the diver inhales breathing gas and exhales it into the water. Rebreathing is like being locked in an airtight trunk, relying on the technology within it to keep you from asphyxiating.
More specifically it's like breathing from a plastic bag, except that the scrubber unit removes the waste carbon dioxide, the build-up of which normally signals to our brains that the air supply is inadequate, and makes us breathe harder.
Without this signal, we can continue to breathe normally until there is inadequate oxygen left to support life. The first sign of hypoxia is unconsciousness, quickly followed by death if the diver is under water.
The CCR automatically tops up the breathing-loop or "polythene bag" with the required amount of oxygen. The diver uses his instruments to tell him what he is breathing , like a pilot who has to rely on his control panel to gauge his height.
An SCR tops up constantly with pre-mixed nitrox whether it is needed or not, as long as there is no obstruction to the flow, so it can be regarded as a good first step on the way to using a CCR.
The advantages of bubble-free operation are often emphasised as an argument for CCR. This is exaggerated; CCR divers are not invisible, and wildlife is still nervous of any big animal that suddenly appears with a box on its back. What CCR does give you is time for creatures to get used to you before you need to head back to the surface.
Rigged for diving to 52m, a CCR such as the Inspiration costs nearly £4000. For open-circuit scuba, three cylinders, three high-performance regulators and a technical BC with redundant bladder costs around £3000. Many advanced OC divers feel that four cylinders are essential.
The simplicity of open circuit seems attractive until you address the problem of entering the water while wearing a twinset on your back and possibly two other cylinders hooked on to your BC harness. There is the weightbelt needed to get it all under water (especially when using aluminium cylinders) and all the plumbing.
Divers with multiple cylinders have jumped into the water before now without their tanks turned on and, unable to inflate their BCs, disappeared to great depth under the weight of them, drowning on the way.
The CCR diver is far less encumbered, but has to pay even more attention to equipment maintenance and essential pre-dive checks. Jumping in without doing these could be equally fatal.
If an OC set-up doesn't work properly, you don't need to be a rocket-scientist to spot it. But if your CCR is not working properly or, indeed, if it has not been switched on, you can still breathe - until the point at which you become hypoxic and pass out. There are no symptoms, so it's the perfect way to commit suicide.
You must rely on instrumentation and your ability to correctly read it.
SCRs are not immune from this effect, either. Their simplicity is seductive but the flow-rate of nitrox into them must be rigorously checked, and it seems hazardous to use one without some method of constantly monitoring the oxygen partial pressure in the counter-lung, although many users have no such device.
Alas, mistakes are made. A CCR diver died from hypoxia while breathing from it swimming head-down on the surface. It is said that he did not have it turned on. The extra complication and unfamiliarity can be distracting. Two CCR divers have died from embolisms connected with making very rapid ascents; one, it appears, when the direct-feed to his suit jammed on.
If you choose to take the CCR route, remember this: you might jump into your car and drive off without making any checks, but you wouldn't do that with an aircraft. In the same way, you can never afford to be cavalier with a CCR. But prepare to put your open-circuit training aside and be ready to start again from scratch.
Appeared in DIVER - October 2000
