Technically Speaking: Bailing Out On A Closed Circuit Rebreather
Tuesday, 02 June 2009 01:21
No one subject is discussed as much by rebreather divers as ‘bailout’. Opinions vary greatly, from the most optimistic, or ‘alpinist’ diver, to the most pessimistic who may opt for double redundancy. In an effort to clear the decks a bit, this article intends to outline some bailout circumstances, requirements and techniques in order to winnow our options a little. There is also not enough space to discuss gas choices, which will be the subject of
another article. The following is by no means complete in any sense, but may make things a little clearer. What is bailout ‘Bailout’ is the term used when a rebreather diver stops using his rebreather loop and changes to an alternative breathing source. While this sounds very simple, there are enough different scenarios to make the choice of how to bailout a challenging endeavour. So in discussing bailing out, one needs to weigh up various facets, such as the situation(s) that may require bailing out, the method used to bailout and the exact equipment that one bails out to. Having weighed up all these considerations one may hopefully have devised an equipment configuration that works when required: hoping of course it is never required. Some situations that may lead to bailout A bailout situation occurs simply when a diver cannot continue to use his rebreather with confidence. It may be they either become aware the rebreather they are using is unable to continue life support, or they are in a situation that are uncomfortable with and are not sure what steps to take next. An unsure situation calls for a period of time to regain one’s composure, check the operation of the rebreather and decide what course of action to take. Often this is commonly called taking ‘sanity breaths’. But, if the diver knows their unit is un-functional, then it is time to consider a bailout. This may be due to:
- Hypercapnia – build-up of CO2 due to the scrubber being un-functional through exhaustion flood or gas bypass. Can lead to unconsciousness and drowning
- Hypoxcia – the PO2 has fallen to low levels and cannot be raised due to failure of O2 delivery system(s), regulator or valve failure or running out of O2. Can lead to unconsciousness and drowning
- Hyperoxia – the PO2 has risen past safe levels (generally considered to be 1.6 ATA) and cannot be lowered / lowered quickly. This can be due to O2 delivery system failure, or regulator failure. Can lead to unconsciousness and drowning
- Flood – water has made its way into the unit and has made breathing difficult and/or washed through the scrubber causing a ‘caustic cocktail’, where the diver cannot breathe through the contaminated mouthpiece.
- Electronics failure – the diver has no way of knowing the PO2 of the loop gas. Continuing to breathe from the loop raises the very real possibility of hyper/hypoxicity leading to unconsciousness and drowning.
How to make a choice
The most difficult part of bailout is making a sensible choice that will address the particular scenarios of the dive being made. As different people have different expectations, life circumstances and risk acceptance, they will make different choices on how best to bailout. Personally, I have tried to look at the worst case scenarios, and attempted to address them using multiple OC bailout gas sources, including diluents and O2 gas carried on the rebreather. My rebreather also has a bailout valve (BOV) which is a rebreather mouthpiece and OC second stage integrated into one. In case of the need for sanity breaths or urgent isolation of the breathing loop one only needs to turn a knob or lever to go from rebreather to OC gas. That said, many rebreather incidents happen early in the dive, where electronics fail (or are not turned on), gas sources turned off or inaccessible, gas sources wrong, the rebreather does not maintain gas integrity (leaks) or the CO2 scrubber fails. The following options are quite simply that: options. What one diver feels is an optimal solution to his/her circumstances will not satisfy another, and be overkill/unnecessary complex to yet another. The following is not complete, nor perfect, but it is where I start my planning for bailout. Bailout methods / configurations
Go to semi-closed mode This is where the diver is unsure of the PO2 of the gas he is breathing and rebreathes the a known gas through the loop for a set number of breaths. They then dump that gas (by nose usually) into the water and injecting more gas to rebreather. This will extend the amount of bailout gas available during ascent, allowing a safe return to the surface. Pros: Limits the amount of bailout gas required. Removes the need for the diver to remove his mouthpiece and change to something else. Cons: does not take into account some failure modes of a rebreather such as loop flood/caustic cocktail or hypercapnia.
- Close loop and go to OC regulator The diver closes the mouthpiece of the rebreather to prevent water flooding the rebreather and starts to breathe from a plain old scuba tank via a second stage. They then make their ascent and complete any decompression obligation. Pros: the simplest remedy, requires less equipment and can be seen as the ‘KISS’ approach (not to be confused with KISS rebreather products made by Jetsam Inc.) Cons: does not take into account some failure modes of a rebreather such as hypercapnia (diver finds it impossible to change mouthpieces due to loss of breathing control), needs a large amount of (at times multiple) gas(s) to be carried.
- Use a ‘bailout valve’ (BOV) or similar to go to OC gas As in #2, but the diver only has to turn a knob/lever to be able to close off the rebreather loop and access OC gas. The BOV can be a dedicated device such as those made by, Jetsam, AP Diving, Gollum Gear, JJ valves, etc. or an adaptation of something similar such as a full face mask. Pros: Isolates the breathing loop and maintains whatever integrity it has, is a fast solution negating the need to hold one’s breath while changing mouthpieces, and allows time to regain control of a situation leading to a fast return to breathing from the rebreather or an alternate gas source such as a OC regulator. Cons: a standard BOV uses the same mouthpiece as the rebreather and doesn’t allow for caustic cocktails, which can cause extreme distress. Divers vomit, are nauseous, and need to remove the contaminated mouthpiece ASAP. A full face mask with nasal OC gas path does not allow for uncontrolled breathing rates and may lead to an escalation of a hypercapnia event: despite bailing out, the diver will still lose consciousness.
- Use a full-face mask Similar to using a BOV, some divers have an OC regulator set into their full face mask nasal cavity, on needing to bailout, they simply close the rebreather loop and breathe through their nose. Pros: Isolates the breathing loop and maintains whatever integrity it has, is a fast solution negating the need to hold one’s breath while changing mouthpieces, relieves the chances of the diver drowning through loss of consciousness, provides two gas sources at all times via mouth and nose. Cons: does not take into account a catastrophic flood and caustic-cocktail; nasal breathing may be sufficient in cases of severe hypercapnia. (May not be – if a bov is plugged or not. Some have a clip to change mouthpiece)
- Change to another rebreather All the diver needs is to have another rebreather handy! Some divers have made up a complete double rebreather worn on the back, others carry a ‘bail-out ‘breather’ (BOB) like a sling tank. That there is no commercially made bailout rebreather currently available gives a hint as to the potential for extra complications to arise using this method. However, being able to minimise the amount of bailout gas needed is of significant importance in deep/long penetration dives, where a diver/ dive team may not be able to carry enough gas to bailout. Pros: Gas requirements are greatly decreased. Cons: The complexity of the dive is greatly increased. Does not take into account events such as hypercapnia and uncontrolled breathing unless a BOV is also used – some divers have designed and made a double-loop bailout valve that can go from OC to rebreather 1 to rebreather 2 (this system also has pros and cons attached to it) Will my precautions be enough to avoid a serious incident in the event of needing to bailout? I hope so. In the face of some 300 dives on a rebreather I have never needed to bailout from the loop and aborted one dive due to a failure. With that one dive, aborted due to a PO2 display failure, I was able to make a controlled ascent, and surfaced without incident. This of course, doesn’t mean anything as far as my next dive is concerned.