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Cave Diving Rebreathers

Cave Rebreather – Is It Worth It?

By Natalie Gibb

Rebreathers themselves aren’t any different in a cave environment, but many aspects of cave diving, from gas planning to problem management, change subtly. Photo: Natalie Gibb

I cave dive every day. Flooded caves are one of the most difficult dive environments on the planet, and my job requires monitoring a variety of factors all at once: navigation, control, my buddy, and host of factors such as dive time, depth, time to surface, no-deco limit, gas—the list goes on. Yet here I was, on my first rebreather dive after MOD1, working hard to keep it together on a rebreather in the shallow open water. 

 “Did you have any idea where I was at all?” I asked my buddy after we had swum around for five minutes or so, “I completely lost track of you.” He admitted the same. Keep in mind that we are both cave diving instructors and it is our job to watch other divers on a daily basis.

Rebreather diving is the single most task-loading activity I have ever tried.  

Experience only goes so far

I have been working professionally in cave diving since 2009 and have amassed at least 5000 cave dives, ranging from short teaching dives to 8-hour exploration marathons in harsh environments. In the caves, I dive sidemount, backmount, multistage, decompression, mixed gas, and double DPVs. I have had my share of gear failures, zero viz, and other situations, and dealt with them all without stress. I am more comfortable in caves than in open water or any other dive environment. Cave diving is my thing. 

Still, I don’t know everything about cave diving, and I never will. That’s part of what makes the activity so addictive—there’s always room for improvement and something new to discover or learn. You’d think that after gaining the requisite hours on my rebreather, I would be happy to grab the machine and go for a cave dive, but that wasn’t the case. 

Most safe cave divers are risk adverse. Cave diving isn’t an adrenaline sport. Good cave divers are always calm and in control, and this attitude extends to problem management. In cave diving, it’s not about solving a problem and barely getting out, but about coming up with the best solution, one that will not snowball into unexpected issues. I don’t want to come up with this through trial and error in the cave if I don’t have to—not if someone else has already gone through the painful learning process and condensed their years of suffering into a 5-day course. I want simplicity and elegance in my problem solving, and if possible, I want it from the beginning. 

The rebreather itself isn’t any different in a cave environment, but many aspects of cave diving, from gas planning to problem management, change slightly and subtly. 

Bailout scenarios

Cave rebreather divers should carry redundant bailout bottles; I sidemount two Al80’s just like I do when I dive open circuit. This should make my bailout calculations easy…I thought. I can swim three times as far as I can swim on open circuit, and still get out if my rebreather fails. Wrong! I learned that it’s not enough to calculate your bailout on your normal, or even slightly elevated breathing rate—not because you are going to stress out and hyperventilate in fear, but that whatever causes you to bailout will make your breathing rate increase. 

On my machine, the only reason to bailout would be CO2 breakthrough or a horribly flooded breathing loop on the inhalation side. I would be bailing out because I inhaled a great deal of CO2, which will physiologically increase my breathing rate no matter how mentally calm I manage to remain, or because I partially drowned myself by inhaling a bunch of water. On open circuit, I plan with the assumption that in the worst-case scenario my breathing rate will still be similar to what it usually is—and I have proven this true over many incidents. With a machine that is custom-blending potentially hazardous breathing gases on my back, those rules go out the window. I plan double my normal breathing rate for a worst-case emergency exit on a rebreather.

Diluent management

Diluent management changes in the cave as well. On an open water rebreather dive (descend, swim, come up), diluent use is not much of a concern. It’s important to track your diluent pressure to make sure you don’t unexpectedly run out, but diluent is usually not going to be a limiting factor. In my shallow, Mexican caves, diluent becomes a serious consideration. 

On a rebreather, diluent is added to the diver’s breathing loop/counter lung upon descent, to keep it more or less equal to the diver’s lung volume. On many rebreathers, diluent is also used to inflate the diver’s wing and drysuit. Unfortunately, cave profiles are rarely flat. Where I dive, the profile of most caves forces a diver to descend and ascend almost constantly throughout the dive. As the diver ascends, he must vent his breathing loop, wing, and drysuit, then he must inflate them again on the next descent. This happens ten or twenty times on a normal, easy cave dive. A rebreather diver can blow through diluent fast in these conditions. There are ways to work with this, such as using an inflation bottle for the suit or wing. However, no matter what precautions a diver takes, diluent management and volume should be a major consideration in rebreather cave diving. 

Consider what this means for emergencies: if a diver bails out onto his open circuit tanks and begins to exit, he has to manage all three air spaces as he exits over multiple ascents and descents. It’s not enough to just dump all the gas out of the breathing loop and then work with the wing and drysuit—because if a diver descends too far without re-inflating the breathing loop, he can fully flood his rebreather, making the situation much worse: now he’s breathing a limited gas supply on open circuit while hauling a heavy, flooded rebreather out of the cave. In a zero visibility bailout scenario, a diver has to manage three gas spaces, while maintaining continuous touch contact with the line and his buddy. Not rocket science, but worth practicing while someone who knows what mistakes to look for can stop you from flooding your rebreather during practice. 

Efficient reactions

So many little details! I could figure them out eventually, but knowing I have tried all these things under the supervision of an instructor will give me a calmer, more deliberate, and more efficient reaction in a real gear failure scenario. 

The ability to react in an elegant and effective manner in an extreme environment will almost certainly save my life one day. For now, a better understanding of my machine and a framework for problem solving is worth the price of training. No diver, no matter how experienced, is ever so good that they can’t learn something new—not even in an environment with which they are intimately familiar. It was fun to be the student and make silly mistakes under the eye of an experienced instructor. Even now, after the cave rebreather course, I am still learning on every dive. 

Natalie Gibb Is a full cave instructor, explorer, and researcher based in Mexico’s Riviera Maya. She is a published author, international speaker, and cave conservationist. Visit: Under The Jungle

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