By Mark Powell
A rebreather is a fantastic tool that can be used to extend the range of what is possible in terms of exploratory diving. For deeper and longer dives, rebreathers can significantly reduce concerns over gas usage. That is, unless there is a problem. If you are unable to use the rebreather for whatever reason and need to bail out then the rebreather becomes a very expensive BCD. Not only that, but you are right back to all the limitations of open circuit gas volumes as you need enough bailout gas to get you safely back to the surface.
For dives without any mandatory decompression stops, this is relatively straightforward. As there is no decompression obligation we simply need enough gas to be able to ascend to the surface. A stage cylinder can provide plenty of air for the ascent. However, as we progress into decompression diving, the amount of gas required becomes more of an issue. The advantages of a rebreather include the fact that you can do much longer dives due to avoiding the limits of open circuit gas volumes. Together with the optimized mix that the rebreather gives you, this means that many divers will do longer dives than an equivalent open circuit diver. However, in the case of a bailout, you need to carry enough open circuit bailout to ascend and decompress safely. This means that even though you may not be planning on using those stage cylinders, you still need to carry them, just in case.
When performing any gas calculating, including calculating the amount of bailout gas you need to carry, the divers breathing rate is a key element of the calculation. Whatever your average breathing during a dive, you can guarantee that if you bailout your actual breathing rate will be much higher. There is always a reason why you have bailed out, even if it is only an imagined reason; whatever the reason, it is bound to increase your stress levels and hence your breathing rate. One of the situations where you would definitely bailout, and not return to the loop, is where you have a carbon dioxide breakthrough. In this case you would definitely not return to the loop and would need to do the entire ascent on bailout, but more importantly, you would have an even higher breathing rate. For these reasons TDI recommends using a breathing rate of at least 45 l/min (1.0 cf/min) until the first deco stop when calculating bailout gas requirements. In reality, even this rate may be exceeded for the first few minutes of a carbon dioxide breakthrough.
For deeper dives the volume of bailout gas to be carried can quickly become very significant. For dives below 80m (260 ft) this can become problematic, and the additional cylinders can introduce problems with drag and can, in themselves, become an issue due to the effort required to swim with them. TDI believes that each diver should carry enough bailout gas to be able to get them to the surface. An alternative approach is team bailout where the divers between them carry enough bailout gas to get one team member to the surface. This is not a technique to be used without specific training and unless you are diving within a well-practiced team. For most rebreather dives this means that the limiting factor for the dive is not scrubber duration, exposure to CNS or anything else related to the rebreather itself but rather the amount of bailout that can be carried.
The gas selected for the bailout cylinders requires some thought. As they will hopefully not be required, rebreather divers tend to use a standard set of bailout cylinders and gasses that will be used for repeated dives. The first bailout bottle needs to be breathable at the maximum depth as the diver may well have to bailout on the bottom. It is common to use a first bailout that has a higher partial pressure of oxygen than the setpoint being used on the rebreather. This is to optimize the decompression ascent. Subsequent bailout gases are chosen by balancing out the decompression requirements and the gas planning requirements.
Another technique that is sometimes used, and is starting to appear in some planning tools, is to modify the decompression model so that a different approach is used for a bailout ascent rather than a normal ascent. When ascending normally the diver may well want to use deep stops or some form of bubble model approach. However, during a bailout the diver may want to get shallower slightly faster than they would otherwise have liked in order to reduce gas management problems.
Until rebreathers with built-in redundancy of all features or bailout rebreathers become common place, we will still need to plan for bailout. Until that point, rebreather divers will still ultimately be limited by the restrictions of open circuit gas calculations and the ability to carry sufficient bailout.
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