We all know that a rebreather is an amazing tool for deep dives, wreck, and cave exploration. Its efficiency in respiratory gas consumption is immense and promises long times underwater even at great depths. Anyone who sees a comparison of open circuit and closed circuit consumption at 100 meters depth (220 litres of gas versus 1.5 litres of oxygen per minute) will begin to imagine how amazing and simple it is to sink to such depth with a rebreather. After all, I can just take the box on my back and I don’t need to do anything else. However, when we start getting more into the detail of the dive we come across a crucial limitation.
We always have to carry enough back-up gas in case of a catastrophic rebreather failure such as a flood or a broken scrubber. In such a situation, we have to bailout to an open circuit. In the case of the above-mentioned dive of up to 100 m with a bottom time of 20 minutes (including descent), a sufficient supply for the ascent and decompression would require four Al80 bottles.
At the same time, four bailout bottles are on the limit of comfortable diving without a scooter. So, it is nice that the rebreather will allow us to be 6 hours deep underwater, but without a bunch of bottles, we cannot do more difficult long exposure dives anyway. The reality is divers often dive deep or far into caves with a totally inadequate reserve gas supply set aside.
Team bailout is gambling
There was, of course, a tendency to solve this situation with a team bailout, with each diver having one depth mix and the rest of the gases for one diver being distributed among all team members. I consider this to be dangerous because only one diver is considered to be rescued, but if another team member is in trouble, the whole group would be at risk. In this case, the diver is never self-sufficient, and if for any reason he breaks away from the group, he loses the chance of survival in the event of a critical situation.
The solution is a bailout rebreather
In order to really exploit the potential of closed circuit, we need a completely equivalent backup. So, a second rebreather. People like Olivier Isler or Krzysztof Starnawski have solved this problem with a double rebreather in the back configuration. But, two rebreathers on the back take up a lot of space and are harder to control. Nowadays, when there is a trend of sidemount rebreathers that can be relatively easily attached to an existing device, the situation is much simpler and more accessible to more people than just a few individuals. Nevertheless, this approach is still in the early stages and at the time of writing this, the bailout rebreather diving course is still in its infancy. So, how to safely dive with two rebreathers?
Preparation is a paramount
I tell my students during CCR courses that detailed and thorough preparation and testing of the device before the dive is 80 percent of the success of rebreather diving. In the case of a doubled device, this applies twice. Bailout rebreather must be prepared as carefully as the primary device, including all tests and pre-breathing. I do this in the classic CCR mode before switching to CCR bailout mode. Only then can we rely on the backup system.
A bailout rebreather alone is not enough
I have already participated in many discussions about bailout rebreathers, whether private or public, and there is still a rather dangerous view that bailout rebreather is an omnipresent solution that replaces everything. Unfortunately, this is false. In critical situations where it is necessary to switch to a bailout immediately, there is no time to check the loop of the bailout rebreather before breathing from it. As we will learn in the next paragraph, an untested loop could kill us instantly. It is, therefore, necessary to first switch to the open circuit, either through a BOV or a suitably placed second stage. This will give us time to prepare the loop properly and move to it only when we are sure that the loop has the same pressure and the gas is breathable.
This obliges us to always take a suitable bailout bottle with a mixture that is breathable in most dive profiles. For very deep dives, at least two should be taken, one covering the deep part of the dive and the other covering the shallow. For dives up to 100 meters I usually carry one 12 litre steel bottle with me, which at the same time compensates my lateral trim for the bailout rebreather. Off-board oxygen with bayonet quick connection is also good when I want to be really secured for all scenarios.
The pressure difference can kill us
The greatest danger of the secondary rebreather is that if we are not breathing from it, we have no idea whether the loop pressure is sufficiently compensated with the ambient pressure. This should be done by ADV, but if it becomes inoperable for some reason (closed diluent valve, shut-off valve, diluent bottle empty), the pressure cannot equalize during the descent, causing a relative vacuum in the loop. The counterlungs will collapse and the corrugated hoses will pull together. If the vacuum in the device increases further, there will certainly be damage and flooding to the device in someplace. To inhale from the device under vacuum would mean that the device sucks out all the gas out of our lungs in an instant, and we would start drowning. On the ascent, there is a reverse problem when the overpressure dump valve is blocked. Inhalation from the pressurized device can immediately cause lung or stomach barotrauma.
Regular test procedures
To avoid the disaster, it is necessary to constantly monitor the status of the backup rebreather. I set up the following validation procedure: I detach the loop of the bailout unit and look and ensure that the hoses or counterlungs are not collapsed. I inject a small dose of diluent to be sure. The Liberty CCR helps me a lot with its pressure sensors inside the loop. I can compare the depths on a primary and secondary rebreather and if they differ, it means that the bailout rebreather is either underpressurised or overpressurised. After that, I check the partial pressure of the oxygen in the bailout loop and, if it is breathable, make two careful breaths to see if the device is flooded. Then I go back to the primary loop and clip the bailout mouthpiece to my chest D-ring.
I do this procedure together with the bubble check and bailout check at the beginning of the dive at a depth of about 5 m. Then, about halfway down or even several times during the descent if the dive is really deep. I test again after reaching the bottom and in about 15 minutes intervals. I would test the device at least four times during the dive. This is surprisingly easy because when you switch from one loop to another, you do not release any gas into the water and thus do not change buoyancy. It is important to keep your attention when handling the mouthpieces so that you always close the one you are not breathing from. The correct position of the backup device is absolutely crucial to ensure usable work of breathing. During the ascent, it is necessary to proactively release expanding gas from all buoyancy devices including the secondary rebreather. It is probably the least comfortable part of the dive, which must be well rehearsed before a serious descent.
Bailout rebreather features
If we use a classic rebreather with a classic CCR mode for the secondary loop, we will probably be in trouble, because the solenoid will at any cost attempt to reach the setpoint and waste our oxygen while complicating our buoyancy compensation. In the development phase of the Liberty Sidemount, we discovered that in addition to using the unit for classic Sidemount cave profiles, it would be wonderful to be used as a bailout rebreather. For this reason, we have added to it a special CCR Bailout mode, in which the solenoids are put to sleep. Nevertheless, the device constantly monitors the oxygen partial pressure and displays the voltage of all oxygen sensors. So, the partial pressure differs from that in the primary rebreather. Real time decompression calculation is based on a fixed setpoint, so I have the same setpoints in both rebreathers that I switch at the same time. Thereby the backup rebreather has information about the saturation of the diver’s tissues at any stage of the dive, and when it comes to the diver having to switch to a bailout rebreather, it continues to calculate on-line decompression. If the diver is stressed due to the emergency and forgets to switch to the standard CCR mode then, after breathing from the unit for a while, the Liberty will automatically switch from BO mode to Dive mode.
The bailout rebreather should, of course, be placed appropriately so that it has the ideal work of breathing in the broadest possible range of positions. This is physically possible only if the counterlungs are in close proximity to the diver’s chest. Ideally, the device is completely self-contained and does not take gas from the bailout bottles, so it is advisable to have a bottle for oxygen and diluent on-board. A fully equipped instrument should ideally have neutral buoyancy and minimum dimensions to be easy to handle and not to increase the buoyancy compensator volume requirements.
Scrubber of bailout rebreather
Fortunately, myths about the necessity to start a scrubber reaction are now being rebutted. The absorbent works from the moment we begin to breathe from it and there is no need to worry about having to pre-breathe for a long time when switching to a bailout rebreather (initial pre-breathing is still required to check the overall functionality of the device). It is not even necessary to keep the scrubber warm by alternating breathing from one device to another. This would only result in the scrubber being partially exhausted and would reduce the time of use. The only exception is hypothetically diving in the polar seas, where the water temperature drops below -2 ° C (28 °F). In such an environment, the scrubber that has been pre-breathed would freeze and become completely inoperable. Therefore, it is preferable to use a classic OC bailout instead of CCR in such an environment.
If we do not use the bailout rebreather and only perform the above test procedures during the dive, the scrubber is virtually unaffected. Once the scrubber capacity in the primary unit is exhausted, it is advisable to switch the scrubber from the bailout unit to the primary one and fill the new one into the bailout unit. This will keep the scrubber in both devices constantly fresh and usable. Of course, this procedure is only applicable when both units are the same, albeit in a different configuration.
Sufficiency of practice and experience
It should be noted that diving with two rebreathers in any configuration is really one of the most demanding diving practices. Everything is necessary to watch in duplicate, it is more difficult to maintain buoyancy, especially during the ascent. The diver that begins to think about getting a bailout rebreather should be a very experienced rebreather diver and have mastered the handling of the device. Personally, I recommend at least 200 hours on the specific unit that the diver will be using as his primary unit. If the bailout rebreather is in a configuration other than the primary, or even a completely different device than the primary, you need to get enough experience underwater with the unit itself before using it as a bailout rebreather.
A touch of freedom
If we master the diving technique with two rebreathers, we get an incredible feeling of freedom. The feeling at depth, when we are not limited by the supply of breathing gas, nor by the amount of bailout and the knowledge that you can stay at 100 meters depth for tens of minutes without having to change the configuration. With just one bailout bottle and one bailout rebreather, we open up entirely new possibilities.
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