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Thread: Rebreather Questions

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    Rebreather Questions

    I am currently working on a school project in which I chose to research rebreathers. I am familiar with the science of how they work, but I need some clarification on a couple of things. First, in an oxygen rebreather, is the breathing loop flushed with oxygen before to dive? Second, does the oxygen concentration (relatively) stay the same during the duration of the dive if the rebreather has a demand valve in the counter lung?

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    RBW Member abscuba is an unknown quantity at this point abscuba's Avatar
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    Re: Rebreather Questions

    If you are using an O2 rebreather then the O2 is always 100% which is why they can only be used at very shallow depths. They have limited application and are normally only used by military. Their benefit is there is not nitrogen to obsorbe into the system so if you are a combat swimming working very hard you are not concerned with getting bent. You also have to worry about O2 toxicity as you are breathing 100% O2. For the demand valve if it is an O2 rebreather i will again always be pure O2 as you descend the pressure will go up so you will go from 1 atmosphere to a higher as depth increases.

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    Re: Rebreather Questions

    Thanks for the quick reply. Has there been anything to show that there is any harm to breathing pure O2 for extended periods of time?

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    RBW Member manni-yunk is an unknown quantity at this point manni-yunk's Avatar
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    Re: Rebreather Questions

    Quote Originally Posted by hnobles12  View Original Post
    Thanks for the quick reply. Has there been anything to show that there is any harm to breathing pure O2 for extended periods of time?

    Very well documented. Research Oxygen Toxicity. Both from an acute event and from time and exposure (at depth)
    Last edited by manni-yunk; 30th September 2015 at 18:25.

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    Re: Rebreather Questions

    I will thanks again

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    Re: Rebreather Questions

    Quote Originally Posted by hnobles12  View Original Post
    Thanks for the quick reply. Has there been anything to show that there is any harm to breathing pure O2 for extended periods of time?
    pulmonary oxygen toxicity

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    Re: Rebreather Questions

    Quote Originally Posted by hnobles12  View Original Post
    I am currently working on a school project in which I chose to research rebreathers. I am familiar with the science of how they work, but I need some clarification on a couple of things. First, in an oxygen rebreather, is the breathing loop flushed with oxygen before to dive? Second, does the oxygen concentration (relatively) stay the same during the duration of the dive if the rebreather has a demand valve in the counter lung?
    1. Yes. In fact, not only is the loop flushed with O2 but the body is too (by breathing on the pure O2 loop and flushing for a few minutes before the dive). Otherwise the nitrogen dissolved in the body comes out and fills the loop, on a long dive with small enough loop it can become dangerously LOW on O2 as a result!

    2. Yes, apart from the N2 offgassing from the body, because on an O2 rebreather the demand valve (if there is one) is adding O2 to replace the O2 metabolised from the loop.

    The biggest advantage of an O2 rebreather, if you're not staying down for more than a couple of hours and not going too deep, is the compact simplicity of the things.

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    RBW Member tkfx2000 is an unknown quantity at this point tkfx2000's Avatar
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    Re: Rebreather Questions

    For the second question, the actual concentration of oxygen will change, and increase substantially with depth. The increase in surrounding pressure causes an increase in the concentration of the oxygen.

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    Re: Rebreather Questions

    Quote Originally Posted by tkfx2000  View Original Post
    For the second question, the actual concentration of oxygen will change, and increase substantially with depth. The increase in surrounding pressure causes an increase in the concentration of the oxygen.
    Concentration, FO2, will remain at 100%.

    Partial pressure, PPO2, will increase from 1.0 at the surface by 1.0 bar every 10m.

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    RBW Member tkfx2000 is an unknown quantity at this point tkfx2000's Avatar
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    Re: Rebreather Questions

    Captain, Not exactly: I think you have it backwards.

    FO2 is a fraction, a percentage. Air is 21% o2, regardless of depth. But that is not a concentration. However, the partial pressure is direct measure of concentration.

    Concentration :the relative amount of a given substance contained within a solution or in a particular volume of space; the amount of solute per unit volume of solution.


    so, the number of oxygen molecules, for a fixed volume, will increase with depth. The concentration at 2ATA (~33ft) will double compared to the same volume at the surface.

    The volume in the rebreather is not fixed, the counter lungs act as a volume adjustment mechanism. Assuming no gas is added the volume will be half at 2ATA compared to the surface.

    So for that same volume, with no gas added, there will be twice the number of O2 molecules at 2ATA compared to 1ATA. Hence, a doubling of the concentration of 02.

    at 2ATA, the FO2 is 100%, the concentration of 02 is doubled, so the partial pressure of the 02, the is 2.0 ata. (twice the concentration at the surface).

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