In this, the latest installment of our series on how to get started and progress through technical diving, we introduce rebreather diving. Simply put, rebreathers recycle the diver’s breathing gas, allowing them to reuse a large part of it. In the case of closed-circuit rebreathers (CCR), they also provide breathing gas at a set partial-pressure This allows for shorter decompression obligations.
Before delving any further into the subject, full disclosure: I am a rebreather diver, not a rebreather instructor. This article is simply intended to explain basic principles of rebreather diving and how to get into it.
How does it work?
In French, the word for rebreathers is recycleurs – recyclers – and that’s exactly what they allow divers do. Rather than exhaling all the gas you just breathed, you’ll reuse a large part for your next breath. Two small cylinders, usually two or three liters in volume rather than the standard 11 or 12 liters, deliver oxygen and a diluent. Diluent is the gas we use to dilute the oxygen. This can be air on dives to 130 feet (40 m) or trimix if divers are going deeper.
As divers exhale, absorbent soda lime filters out the carbon dioxide. The CCR tops up the oxygen to keep a steady partial pressure at a set point for closed-circuit rebreathers or a percentage of oxygen for semi-closed-circuit units. The unit also adds diluent, especially during descent as the air in the breathing loop compresses.
Depending on whether divers choose a manual or an electronic rebreather, they can perform these adjustments manually. They can also opt for the system take over much of the work.
Rebreather courses are unit-specific, so do your research before signing up for one. Units differ in both size and suitability for different types of diving. Potential CCR divers should also consider factors such as access to maintenance and spare parts. Then there is the question of back-mount versus side-mount. For simplicity’s sake, we’ll focus on closed-circuit rebreathers for this article and won’t compare the different brands just now.
Who is rebreather diving best for?
There are many reasons to start diving rebreather and not all of them have to do with technical diving. Photographers, for example, enjoy newfound proximity to marine life not scared away by noisy bubbles.
Air junkies will find that they are no longer the first ones out of the water, because although they might be taking big breaths, these are being recycled.
Having said that, most rebreather divers are definitely techies. If you cast your mind back to the most recent post in this series, you may remember reading about the cost of trimix. As rebreathers need so much less gas, the amount of helium required for a specific trimix is much smaller. This makes dives below 197 feet (60 m) suddenly much more cost-effective. With more gas available and comparatively less decompression time, the benefits all come together.
Wait, why is there less decompression time? At the beginning of this post we mentioned diving at a constant partial-pressure of oxygen. In general, during the bottom part of the dive, this is set to 1.3 bar. In basic terms, this means the diver is breathing ideal nitrox throughout his dive and therefore absorbs less nitrogen. Less nitrogen absorbed means less decompression time.
How to get started
Many rebreather instructors offer trial sessions in a pool or in shallow water. These allow you to test a unit first and decide whether this type of rebreather suits your style of diving. The biggest change most divers notice initially is that they no longer control their buoyancy with their lungs.
Once you’ve chosen a unit, you are looking at the initial level of qualification, often referred to as ‘Mod 1’, module 1. Typically, this means being qualified to dive on a rebreather using air as a diluent. Those already qualified for decompression diving can immediately qualify for rebreather decompression dives. Individual training agencies limit depth to 130 to 148 feet (40 or 45 m) respectively.
TDI, for example, offers unit-specific air-diluent courses, as well as decompression air-diluent courses for those already qualified in decompression diving. Somewhat differently from the more technically oriented dive-training agencies, PADI offers two levels of recreational rebreather diver course, qualifying students to a maximum depth of 59 feet (18 m) and 130 feet (40 m) respectively.
To dive deeper and utilize trimix generally requires a ‘Mod 2’ qualification which, for most training agencies, means limited use of trimix, allowing divers to reach depths in the 197- to 230-foot (60 to 70 m) range.
Finally, there is ‘Mod 3’, which qualifies divers to take their rebreather to 328 feet (100 m), similar to the open-circuit advanced trimix qualification. Moving on between levels requires divers to build up experience on their rebreather in terms of dives done, hours dived and depths reached.
Typically, students must have 50 hours and a set number of dives to move on to Mod 2. They’ll need another 50 to move on to Mod 3. Note that these are absolute minimums — rebreathers are complicated pieces of equipment. Divers must be both comfortable and competent in their use. For some this might happen quickly; for others, it takes longer.
Is rebreather diving safe?
Accidents happen and, in general, technical diving carries greater risks than recreational diving. The safety record for rebreathers is generally good. But, just as we hear about plane crashes in more detail than car accidents, rebreather accidents tend to get more press.
Diving rebreathers offers numerous options if a problematic situation occurs, many more than there would be on a comparable open-circuit dive. The best-known strategy if problems arise is to bail out and switch to the separate open-circuit tank that CCR divers carry. Depending on the depth and complexity of the dive, rebreather divers may have more than one bailout. Trained rebreather divers have plenty of options before bailing out, but there is a lot of truth in the saying, “if in doubt, bail out.”
This, paired with tough training courses as well as regular skills practice makes for safe and enjoyable rebreather dives.