No Bubbles, Only Troubles: Our Journey to Build Safety Into Dual-Rebreather Operations

By Daniel Lee. Images courtesy of the author unless noted.

A Wake-Up Call From The Deep

I was about 130 m/427 ft down when I realised I had to bail. Fortunately, I was in warm, open water; unfortunately, there was such a strong current on the way up that I found myself rotating bailout stages with one hand and hanging on for dear life to rocks with the other to keep from being swept off the reef we had been exploring. I had plenty of time during decompression  to breathe dry gas and watch the helium in my tanks disappear, wishing this dive hadn’t ended up so interesting and fantasizing about being back on a rebreather. What if instead of all those stages I’d had a second rebreather as bailout?

These weren’t my first musings on the topic. Being fond of deep, long dives, as well as long cave penetrations, carrying sufficient bailout was often the limiting factor. And, on challenging dives, it’s very easy to end up in a situation where open circuit bailout isn’t sensible unless you use support divers or complete multiple setup dives—neither of which my team makes a habit of doing. As Covington and friends noted in a recent article in Diving and Hyperbaric Medicine, for some dives that are reasonable for today’s cave exploration community, even ten standard stage cylinders would be insufficient for open-circuit bailout. Are you really going to carry all that?

The truth of the matter was that, if my team wanted in-water autonomy, the solution was a bailout rebreather—based on our interests as a group and our available resources. And for remote locations, transporting a bailout rebreather was just as cumbersome as transporting enough open-circuit regulators (not to mention the tons of in-water advantages). We didn’t know it, but we’d embarked on the path to becoming COBRA divers: a nerdy acronym for “Cave Operations Bailout Redundancy Association.” But, of course, the path to get there wasn’t so simple.

Do you really want to manage two rebreathers?

My team is not an echo chamber; we cultivate a culture of openness, respectfully challenging each other’s ideas and learning from each other. This psychological safety and willingness to spar meant that the idea of a bailout rebreather was initially the subject of tons of skepticism.

First, a sanity check. Andrew Fock has laid out very good reasons to consider CCR diving to be a risky activity as compared to open-circuit diving, and others have noted that the risks of bailout CCRs in particular are not well-understood yet. So would a second CCR really limit risk in light of the potential for mechanical failures and, probably more importantly, with the greater potential for procedural error?

Unfortunately for the team, I am a huge nerd with a background in environmental modeling, with most of my experience in the space industry where I’ve become accustomed to structured risk management. The result was a tsunami of spreadsheets and modelling. From a mechanical standpoint, it was clear: Due to the serial nature of open-circuit bailout, for the dives we were doing, an equipment failure was less likely if using a CCR.

Of course, this analysis ignores other factors that are harder to quantify and apply in different degrees to open-circuit and closed-circuit bailout:  for instance, task loading, procedural complexity, in-water performance characteristics. However, the numbers told a compelling story: If we could get the procedural aspects right, bailout rebreathers would give us a more robust system with greater range.

That was just the beginning, though. As Tim Blömeke has noted in this magazine, “There is no broad consensus yet on best practices, no SOP Manual, no standardized configuration, no published training standards for dual rebreather diving by any training agency.” We were going to have to figure out a lot of things for ourselves—and do so very cautiously—because our lives were literally on the line.

Baby Steps First

To start, we needed to select bailout rebreathers. We set up a requirements matrix and scoured the market for feasible models, then put those into a weighted features matrix to help make a good decision. After shortlisting, we dived the candidate models and added in an (again, weighted) subjective “awesomeness” score. In short, we wanted a rebreather that was, at minimum:

• Fully independent of the first rebreather
• Streamlined enough for diving in heavy currents and restricted environments
• Modular enough to easily transport and kit up at caveside or on boats
• Similar to our existing configuration to minimise cognitive “switch costs” when going back and forth between rebreathers
• Great on work of breathing, since we liked diving deep
• Well supported by the manufacturer so that spares and maintenance wouldn’t be a problem

Of course quality of life features on top of that were welcome, but that’s what a rebreather needed to be on our shortlist.

Our criteria quickly ruled out dual backmount CCRs. Initially, I loved the idea of a chestmount CCR but, after trying it, found that it cluttered critical mounting space on the chest more than I liked. In the end, after trying a few models we settled on the T-Reb as our sidemount unit. Several features caught our eye: the great work of breathing, easy-to-manage loop volume, and dimensions that are basically the same as an 80 cuft stage tank, to name a few. Plus, one of our team members was already diving that model; so, we already had a source of experience available.

Next, the new T-Reb acolytes in the team needed to gain proficiency with the new CCR. That meant a lot of time in the water before diving both rebreathers at the same time. I drafted one of my notorious multi-page training plans to increase the likelihood that we’d anticipate the necessary skills and failure scenarios, and we got to work. We started from the simplest configuration we could imagine—two rebreathers and a stage that was used to plumb in diluent to the T-Reb—and layered on top of that only when we could handle all situations we could foresee blind. The scenarios and skills we wanted to master stemmed from obvious issues, such as how to:

• Comfortably mount the second device
• Route the offboard diluent hose so it was easy to attach and detach and didn’t snag
• Stow the unused loop and switch between them smoothly (the fact that we normally dive with helmets made this a bit more annoying than it would be in open water)
• Manage counterlung volume and content in both devices

Stacked on top of this were several scenarios and procedures that were derived from the experience of the team as a whole, a formal failure mode and effects analysis (FMEA) combined with corresponding recovery procedures, lots of discussion and debate, and some trial and error executed in safe conditions.

When trying new things or extending our performance envelope, we always started out in the easiest possible conditions where it was safer for things to go wrong before moving them into increasingly harsh environments: our preferred environments that are cold, dark, and deep. Even after hitting those stygian depths, we still tend to consider every dive a training dive.

Where We’re at Now

Since getting comfortable with the new setup and developing procedures that have served us well so far, our team’s gotten back to what we love most: diving, diving, and diving. We’ve had multiple dives below 100 m/330 ft and over 6 hours and the odd dive with just one rebreather now feels decidedly odd.

One of the ways we do this is by using checklists before we hit the water. No amount of condescension can separate me from my beloved checklists; as a former pilot and current space professional, it’s hard enough for me not to use a checklist when brushing my teeth. Our pre-dive checklists are designed to be dead simple and generic enough to work with any rebreather. This means that you need to be an expert on the rebreather you’re using to execute them because the checklist doesn’t micromanage you, but they’re also simple enough that even I can remember all 9 steps in total. Some steps can be executed during assembly, whereas others are executed just before diving; the point is that we can objectively assess that we’ve done everything and make ourselves accountable for their completion.

Underwater, checklists aren’t that practical, and thus we’ve got a repertoire of procedures that we use in different situations. We’ve even gone so far as to codify a version of them in a model-independent form. As I stated earlier, we’ve always seen the greatest challenge in managing the human factors of diving with two rebreathers. To that end, we took Andrew Fock’s advice to heart:

1. Minimize perceptual confusion
2. Make the execution of action and response of the system visible
3. Use constraints to lock out the possible causes of errors
4. Avoid multimodal systems

We could go into endless detail in how these ideas are reflected in our procedures—and, if you ask them, my team will roll their eyes and inform you that I have, to excess—but let’s just look at a few examples here.

We minimise perceptual confusion by using Shearwater electronics on all of our rebreathers. This means that we don’t need to deal with differing presentations of information or controls when switching between rebreathers.

We make the execution of action and response of the system visible using a set of signals that we’ve developed as a team in order to be able to declare what actions we are taking and why. Of course, we’re still able to shout into our loops but dedicated hand signals are less ambiguous.

We use constraints to lock out the possible causes of errors by trying to make our equipment as predictable as possible: for example, by choosing a low setpoint in our backup rebreathers that keeps the loop breathable but prevents the rebreather from injecting oxygen unexpectedly. Another way that we do this is by not using constant mass flow (CMF) valves; in the meantime, multiple people in the team dive rEvos, which we have modified to be pure eCCRs rather than hCCRs for situations in which we’re not breathing the rEvo’s loop. As wonderful as CMFs are, if we were to use one in that situation, the loop oxygen would constantly rise, so we’ve removed what in our configuration had become a possible error source.

One way that we avoid a multimodal system is by managing the gas in both loops actively, rather than using a pure bailout mode. This reduces the variability of the system as a whole and forces us to understand both loops as gas mixes that we could potentially be breathing at any time, and which are therefore always kept breathable. If we didn’t do this, one of the loops might contain an unbreathable gas, so that we could not safely bail to it as quickly as we’d like.

In a nutshell, all of our procedures are geared toward making things as straightforward as possible. By using the T-Reb as our sidemount CCR, we’ve got a device with similar dimensions and handling to a stage tank but, unlike a stage tank, it has variable volume and contents. Our procedures aim to compensate for this by minimising the volume, and thus buoyancy, changes that take place without our involvement, and by keeping the contents in an acceptable, breathable range. Thus the rebreather is as close to a traditional open-circuit bailout as we can get it, without sacrificing the advantages of a closed-circuit system. We’ve already mitigated mechanical error sources by choosing a rebreather rather than a series of open-circuit systems, so it’s really important that the procedural error sources don’t eliminate the gains we’ve made in system resilience.

Where Do We Go from Here?

After spending a lot of time and energy in becoming competent, we feel more confident diving with two CCRs than with an open-circuit bailout. Finishing any project means it’s time to inspect and adapt, and we did just that after completing our initial setup. This has resulted in some significant changes, and I’ll walk you through some of them.

When we started out, we were offboarding our diluent into our sidemount CCRs. We did this for simplicity and, for simple dives, it’s fine. For more serious dives, we wanted the increased redundancy of using onboard diluent on our sidemount CCRs (I calculate that this reduces the total risk of mechanical failure by about a third). When we decided to start using onboard diluent, we used our typical approach of checking plausibility at minimal investment before refining. Stated plainly, we used cambands to strap on a spare inflation system to plumb into the T-Reb to check how it worked. Mounting was a pain, and it messed with the T-Reb’s buoyancy. After proving that we wanted to keep it, we switched to a sleeker bottle mount that lets us strap on the bottle at the waterside, a carbon bottle to keep the whole system neutral and provide more gas volume, and a better regulator. An added advantage is that we can jump faster because the unit is fully self-contained, so it doesn’t need a lot of adjustment before the descent.

Each of us has now figured out how we like mounting our loops. Everyone stows their backmount loop under our chin when it’s not in use, but where does the sidemount loop go? In the meantime, we’ve tried using a bungee necklace, a bolt snap to clip it to a chest D-ring, and various other things. Most of us are now using magnets to attach the loop either to the left chest D-ring or a bungee necklace so that it sits securely when not in use, even when scootering, and can be tugged free immediately if needed.

In refining the dual CCR system that we developed, I also ended up switching to a different backmount rebreather—a rEvo customised as an eCCR—in order to harmonise equipment in the team and optimise my overall equipment to conform better to the design principles we’d worked out for dual CCR diving. We remain a heterogeneous team and that’s not expected to change, but my own setup and thus the overall setup in the team is more optimal this way for the dives that we execute together.

I wish that I could claim that we’d reached an optimal setup; but, alas, that will probably never be the case. There are other ways of doing it and situations where our approach wouldn’t fit as well. The next optimization for me will be adding a HUD to the T-Reb mouthpiece, so that each mouthpiece has a readout of the loop it’s connected to—another way of “minimizing perceptual confusion.” Maybe the system will be perfect when that’s done… But I doubt it.

Whatever may be the case, we continue to refine our equipment, build out our experience in new environments, and enjoy some fantastic dives—all with greater confidence and safety because of the time, work, and money we’ve invested in a great overall system. We’ve got plans to use this system for some big dives. If you want to know more about what we’re up to or would like to reach out, follow us on Instagram. See you in the water!

DIVE DEEPER

InDEPTH: Our Journey Into Bailout Rebreathers By Bob Beckner and Matt Vinzant. 

InDEPTH: When Easy Doesn’t Do It: Dual Rebreathers in Extended-Range Cave Diving by Tim Blömeke.

Divesoft Talks: Three part CCR Bailout Talk Series with Joseph Bosquez

aquaCORPS: Designing a Redundant Life Support System by William C. Stone (1995) 

Dr. Daniel Lee is a serial entrepreneur who’s worked on three continents in the renewable energy, robotics, weather prediction, and space industries. In these contexts, he’s worked in research and development, operations, and all the other bits and ends that needed taking care of. The lessons he’s learned in these varied environments transfer well to his other passion: diving on mesophotic reefs, in caves and mines, in murky lakes, or anywhere else there’s water. He is a founding member of the COBRA divers and finds particular joy in putting his diving skills to use for science.