What are the risks of a pendulum rebreather?

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I know that pendulum rebreathers have a large dead space. Could this lead to a CO2 hit in real life? Or simply hypoxia?
Oxygen must be replenished manually to keep the counterlung volume constant. Riks of hypoxia?
Other issues?

- I am not planning on getting one -
 
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You are in the vintage scuba forum, so I'm assuming you are talking about the old-time pendulum rebreathers using pure oxygen. Hans Hass used these for many years, but they used them in shallow water. Cousteau experimented with them, but almost died of oxygen poisoning twice, gave them up and used the experience to figure out a air open circuit unit with Emile Gagnan that he named the "Aqualung." The first air unit by Northill was a pendulum unit, and it was rejected by the U.S. Navy because of the potential for CO2 buildup and difficulty clearing the unit of water; they subsequently invented the Northill Air-Lung (LaRocchelle, Ed, "The Northill Air-Lung Regulator, A Collector's Review," The Journal of Diving History, Third Quarter, 2014, Volume 22, Number 80, pages 23-26). So yes, there is a potential with pendulum rebreathers for hypercapnia (too much CO2), as well as hypoxia (if the unit is not purged of nitrogen) and oxygen poisoning (if one dives an pure oxygen rebreather too deep--greater than about 25 feet).

SeaRat
 
Thanks this review of history was interesting and informative. It also proved my fears true.
Thanks for the links.

I didn't know that there are modern pendulum rebreathers too.
 
You are in the vintage scuba forum, so I'm assuming you are talking about the old-time pendulum rebreathers using pure oxygen. Hans Hass used these for many years, but they used them in shallow water. Cousteau experimented with them, but almost died of oxygen poisoning twice, gave them up and used the experience to figure out a air open circuit unit with Emile Gagnan that he named the "Aqualung." The first air unit by Northill was a pendulum unit, and it was rejected by the U.S. Navy because of the potential for CO2 buildup and difficulty clearing the unit of water; they subsequently invented the Northill Air-Lung (LaRocchelle, Ed, "The Northill Air-Lung Regulator, A Collector's Review," The Journal of Diving History, Third Quarter, 2014, Volume 22, Number 80, pages 23-26). So yes, there is a potential with pendulum rebreathers for hypercapnia (too much CO2), as well as hypoxia (if the unit is not purged of nitrogen) and oxygen poisoning (if one dives an pure oxygen rebreather too deep--greater than about 25 feet).

SeaRat

Nice reply, I just want to elaborate the need tp purge nitrogen applies to all oxygen rebreathers not just the pendulum type but the dead air space leading to CO2 buildup is definitely a risk on the pendulums. I assume those diving them were taught to take long deep breaths to avoid massive headaches.
 

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