Considering a Bail-out Rebreather

Considering a Bail-out Rebreather
By Cedric Verdier

info@cedricverdier.com


It always seemed to me so great to dive with a rebreather and so stupid to rely on the old fashioned open circuit scuba equipment if time comes to bail-out. So I played around with the idea of a bail-out rebreather (or a BOB: Bail-out Breather, as it sounds much better!) since quite a long time, without real success. And it looks like I’m not the only one.


BOB - A Rationale

When do we need to bail out when we use a rebreather, especially a Closed-Circuit rebreather? There are actually two types of situations:

• A problem that can be fixed - For example it could be the case if the diver experiences a Hyperoxia or a Hypoxia event. Something doesn’t work properly in the oxygen injection but the diver can override it and fly the unit manually. A few minutes Open Circuit (sanity breaths) will give time to come back to a safe loop content and to get a clear head to properly assess the situation and fix the problem.

A similar procedure will be applied in case of electronics failure. Some sanity breaths may help before switching to semi-closed rebreather mode. Same story in case of mild hypercapnia but some divers testify that it takes a very long time to recover from a CO2 hit and it can only be done on OC.

• A problem that cannot be fixed - The two main problems are Total Loop Flooding and severe Hypercapnia. In both case, the loop content is compromised and more or less the only option available is to switch to another source of gas to safely ascent to the surface. That’s where the “Open Circuit only” option may require a huge amount of gas in case of long/ deep decompression dive.

So it looks like, even if we have a bail-out rebreather, we still need a small amount of open circuit gas at least to clear the head, rest and properly think, before being able to start the emergency procedure.


BOB - Some Requirements

The designer of a bail-out rebreather could follow different paths, depending on the type of unit they want to use:

• eCCR, mCCR or SCR
• Front-mounted, back-mounted or side-mounted
• Twin-rebreather or a different unit than the main rebreather

Trying different options and designs and speaking with other rebreather divers with the same needs (deep mixed-gas dives, long decompression obligation, relatively streamlined equipment), it seems that a side-mounted mCCR is the most convenient option for my needs:

• It is not larger than a sling tank. Therefore it can easily be carried through a restriction in a cave or a wreck (as opposed to a front-mounted rebreather like modified OMG Castoro C96 and others). A lateral mouthpiece makes the breathing hoses easier to use.
• It is completely independent. You can give it to an out-of-gas diver in case of emergency. You can also easily ditch it if needed in case of emergency (as opposed to a back-mounted rebreather).
• It is a full CCR so it gives a very long dive time with a very small amount of gas (as opposed to a SCR).
• It uses the same mix and the same oxygen content than the main rebreather so the decompression requirements stay more or less the same (as opposed to a SCR).
• It is hands-off during normal operation. It shouldn’t impair a normal dive and the diver’s buoyancy, as long as the rebreather is fitted with an ADV (to avoid a loop crush during the descent) and an OPV (to avoid loop expansion during the ascent).
• It’s has no independent oxygen injection. There is no solenoid injection to bother the diver during normal operation. A manual injector or a manually adjustable Oxygen Valve is closed during normal operation and is open in case of emergency.
• The electronics are minimal. Just a pO2 display (1, 2 or 3 O2 sensors) to manually fly the unit if needed. It makes the rebreather robust enough to be treated like a stage (the counterlungs are protected inside the unit).
• The setting up pre- and post-dive is minimal, as the main goal is to check that the loop is sealed.

BOB - The Procedures

When you don’t need it:

• A proper pre-dive check will insure that everything works fine in case the BOB is needed. As soon as the loop integrity has been checked, the only thing to do is to make sure that all the gas injection system works fine (ADV and OPV open, Oxygen injection closed).
• A proper pre-breath sequence can help the scrubber reaction to be slightly more efficient.
• During the dive, breathing the BOB from time to time (something like a few minutes every 30-40 minutes) gives you the opportunity to check that the unit works properly (and also helps the scrubber reaction). Make sure you do this while not working hard, as you don’t know the efficiency of the scrubber when it’s rather cold).
• During the ascent, make sure that the loop volume stays relatively constant, or activate the OPV to avoid excessive buoyancy.

When you need it:

• Switch to Open Circuit first, time to check that clear your head and think about the correct sequence of actions.
• If needed, switch to BOB. The first 3 minutes should be done on SCR mode. It allows the scrubber to heat up a little bit and gives you the time to:

• Adjust the Oxygen Valve to the proper injection rate or manually inject oxygen into the loop to have the proper pO2.
• Deploy the pO2 display that becomes the main one now.
• Fully open the OPV of your former main rebreather, as you don’t want to see the closed loop expanding during the ascent.
• Start the ascent while manually controlling the Oxygen injection.
• If needed, plug other mixes into the BOB. Make also sure that’s your main rebreather doesn’t inject oxygen in its loop (disconnect the solenoid or the KISS valve, or switch to a very low setpoint, depending on the type of unit).

So a proper bail-out rebreather could be an interesting alternative to the usual OC bail-out way. Rather than carrying a lot of tanks in case of deep/long rebreather dive, a BOB helps in keeping the tanks as small as possible. Some manufacturers start to explore this option but the liability/safety issue and the small size of the market don’t really help to speed up the designing process. So the initiative comes from the homebuilt rebreather manufacturers and some units are already sold (Decorider, EDO 04, Joki). The lateral mouthpiece is also available through Airtess.

Nevertheless don’t forget you still need some OC bail-out gas at the beginning of any emergency situation…


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