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Rebreather

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The week before last, I spent a day in the pool and three days at sea completing my rebreather training with Grant. We were diving off of Jeff Jonas’s boat. (Thanks Jeff!) We were training off of Catalina where I was doing my Divemaster training in December 2010 when I took my first phone call with Nicholas Negroponte about the idea of becoming the director of the Media Lab. Seems like so long ago!

Now living in Boston, I’m doing most of my diving off of Cape Ann, Gloucester with Matt and Marissa. Marissa works with me at the Media Lab and both Matt and Marissa are rebreather divers so it’s always my bottom time that ends the dives.

I’ve been really interested in rebreathers ever since I learned about them. Probably the most exciting thing is that they are so gas efficient that your dives times aren’t limited by the amount of gas you can carry. Also, since all of the air stays “in the loop” you don’t blow any bubbles and it turns out that most undersea wildlife barely notice you’re there if you don’t blow bubbles. Lastly, since you dynamically change the gas ratios, you can optimize the decompression times substantially.

Rebreathers until recently were mostly used in the military. They are useful in the military because the lack of bubbles adds to stealth and the super-long bottom times increases range and reduces the difficulty of dealing with carrying loads of gasses around. However, this magic is achieved through sophisticated technology that adds to the complexity which increases the risks.

As computers, sensors and the other elements of rebreathers have advanced, so have the design of rebreathers and the adoption of rebreathers by recreational divers. Rebreathers are still a high risk form of diving with a variety of new ways to die, which makes the training so important.

Rebreathers are very similar to space suits or space ships. They use some sort of CO2 “scrubber” to remove CO2 from the air and adds O2 to the air as you metabolize it. The rebreather has a “loop” where one “counterlung” takes your exhaled breath and sends it through a canister that has the CO2 scrubber. The material in the scrubber reacts with the CO2 and absorbs it. There are electronics that measure the partial pressure of O2 in the air and if it is lower than the partial pressure of O2 that you have set as your “setpoint” it will fire a solenoid that injects a tiny burst of O2. The air that has been “scrubbed” and mixed with any new O2 gets pushed to the counterlung on the other side which the diver then inhales.

Although most closed-circuit rebreathers are roughly similar, there are a variety of designs. I bought and trained on a Prism 2 rebreather by Hollis. My rebreather, like many other rebreathers, has three oxygen sensors in the main “head”. Although the sensors are quite good, humidity, lower temperatures and aging can cause the sensors to fail so the three sensors serve as redundancy and a kind of “voting” mechanism to figure out the amount of O2 in your breathing loop.

This is very important because two general ways you can die involves O2 - hypoxia, not enough O2, and hyperoxia, a less commonly known way of dying. There are two types of hyperoxia that we worry about. One is damage that comes from exposure to high levels of O2 over a long period of time, which we track, but the more dangerous one is the CNS oxygen toxicity that happens when the partial pressure of O2 exceeds 1.4 or 1.6 partial pressure of O2. An O2 CNS hit causes convulsions and other symptoms, which when experienced underwater, usually causes death.

Therefore, one of the most important things that the rebreather does is track the amount of O2 and regulate the amount of O2 in your loop so that the partial pressure is always within a breathable range. A lot of the training involves what to do in case of various types of failures in this system.

The other important function is the CO2 scrubbing. Hypercapnia is what happens when there is too much CO2 in your breathing loop. Hypercapnia is what you feel when you’re out of breath and is actually the main trigger of feeling out of breath - your body is trying to eliminate CO2 in the blood stream and it is a common feeling that we all know.

The problem is that if your CO2 scrubbing system fails, it will initially feel like you’re just out of breath. CO2 sensors are still not as reliable or suitable for a diving system as O2 so most units, including mine, do not have CO2 sensors. There are various ways to try to diagnose a failure in the CO2 system, but the key is to pack the scrubbing canister well and not dive beyond the limits of the material - it is the main “perishable” material that the rebreather uses other than the gasses.

When the rebreather is working properly, you can almost forget about it, but the key is to learn how to notice and diagnose problems and have all of the recovery techniques perfected. Also, the maintenance, preparation and the checklists - much like an airplane - are super-important to make sure it is always working properly.

Another major difference from traditional scuba is that the air you exhale just goes back into the loop. This means that breathing doesn’t affect your buoyancy - which for anyone who has done traditional open-circuit diving is very strange. In open-circuit scuba, you use your breathing as the primary way to fine-tune your buoyancy and it becomes second nature allowing you to dive or ascend naturally like a fish.

In a rebreather, you have to manually change the air in your loop through adding air or venting in addition to the air in your drysuit or your BCD to change your buoyancy. Physics dictates that as you go deeper, the air pockets compress and your buoyancy becomes negative. So on a rebreather, the main way that you waste gas is when you change depth a lot requiring addition and venting of gasses - often you use your gasses less for breathing and more for adjusting buoyancy.

So why go to all of this trouble and risk? It’s really amazing. Suddenly the sound of scuba diving isn’t the sound of your bubbles - it’s quiet. You can approach fish and other wildlife and they don’t run away and swirl around you as if you weren’t there. And you can dive forever. The duration of your dives are limited, if you’re in a drysuit, by your bladder and a need to pee, and in the case of a wetsuit, usually thermal - getting too cold.

It’s an amazing piece of technology and changes everything. It’s going to take me a lot more diving on it before I feel comfortable, but I’m excited to log some more hours learning and then really using it for underwater photography, foraging and other things that suddenly become a whole new experience.

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