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Learning the Hollis Prism 2 Rebreather

I just completed my CCR Air Diluent Deco Certification on the Hollis Prism 2 Rebreather! Diving a rebreather has been on my bucket list for the past 10 years, I finally had the opportunity, so I jumped on it, and jumped in!

I went into the course believing rebreather diving was very complicated, and required a lot of calculations and complex math formulas. I was relieved to learn that they are much simpler than I anticipated.

Rebreathers work by simply 'scrubbing' carbon dioxide from your exhalation, and adding oxygen to the breathing loop, depending on the set point of PPO2. When we breath air, it’s generally 21% O2 and 79% Nitrogen. Upon exhale we release about 18% Oxygen, meaning we are quite wasteful in terms of O2. Rebreathers are designed to recycle our exhaled oxygen which make it an incredibly efficient tool for gas consumption.

One of the things I was quite nervous about was buoyancy. When diving open circuit scuba we rely on inhalation and exhalation for buoyancy control, but diving rebreathers there is no change in buoyancy since our exhalation stays in the counter lungs. Needless to say my first ocean dive was a bit of a struggle, learning to control my buoyancy with my drysuit, wing and counter lungs. It took a few dives until I was completely comfortable.

One of the things I noticed during the first dive was the silence. There is almost no noise produced from the Prism 2, only the solenoid and ABV valve. When diving with open circuit you have the noise of bubbles on every exhalation. This silent world also allowed me to notice that marine life was less spooked, I even swam under a school of rockfish with them barely noticing me, this is going to be hugely significant when I begin to shoot photos whilst using my rebreather.

Another benefit was more no-decompression time. Because the rebreather maintains a set point of PPO2, you have a significant increase in bottom time. It was pretty incredible to be down at

70ft and see an NDL on my dive computer of 99!

Now that I’m certified on the Prism 2 my goal is to get as many hours on the unit as possible, aiming to get out for at least one or two dives per week. It’s going to be a game changer for underwater photography and videography, with its efficient gas use, NDL limits, warm gas, and no bubbles!

See more about the Prism 2 Rebreather at:

Image by Joe Platko


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