Oxygen rebreather

James Appel

There has been something that has been floating around in the back of my mind for a while now, and was wondering if someone could answer a question I had on using a CS rebreather as an oxygen rebreather.

Just why is it that I were not to remove all the nitrogen from the loop (i.e. left even a small amount of nitrogen in the loop) it could produce a GREAT risk of hypoxia? How can I suffer from hypoxia if the loop contains mostly oxygen with just a trace amount of nitrogen?

wedivebc

Whats a CS rebreather?

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Cheers,

Dave....

Man is the only animal burdened with the knowledge he will eventually die

steevke

Basically because you are offgassing nitrogen when you use 100% O2. The higher the initial % of nitrogen in your loop, the quicker your loop will run hypoxic. That is also the reason that you are advised to flush the loop while using the Rebreather.

Steven

James Appel

Sorry mistype... hehe should have been CC (Closed Circuit).. newby alert newby alert). Still havent gotten these acronyms down yet (and who says Americans are the only ones that use acronyms).

abowie

It's not so much that there is a GREAT risk of hypoxia, more that it can occur sooner if there's N2 in the loop.

In theory with a perfect O2 CCR the loop only ever contains O2, which gets replaced as you metabolise it so you are always breathing 100%. In practise of course some N2 will always wash out of you.

If you can (a) measure the PO2 in the loop and (b) add O2 as required then it's not a problem.

One approach that it is possible to take to get deeper with an O2 CCR is to dilute the 100% O2 in the loop with an exhaled breath prior to descending. None of this is overwhelmingly safe, of course.

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Andrew Bowie

Rebreather-friendly Buddy

nigelh

Well I don't bother. I like the loop on the IDA to be less than 100%. It means that the occasional digression to 7 meters doesn't look quite as marginal.

As I descend, even if the loop started with air, it is going to get richer as the gas compresses and it will stay rich as I ascend and bubble it off (I am loosing loop nitrogen). I have heard arguments about the disolved nitrogen from the atmosphere in your body off-gassing and reducing the loop ratio but I have never seen the ppO2 meter fall on a dive.

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nigelh

jaap

Which IDA are you using on 100% O2?

I have been doing some experimenting with the IDA 59, 72D and 76 using them on 100% O2 for shallow dives. Mostly using the IDA-76 since its the smallest and a nice little O2-Rebreather.

I normally flush the loop three times prior to diving: fill to the max, close ADV,inhale from loop & exhale to ambient until it bottoms out, repeated three times. With this process my single O2-meter that I sometimes use with it gives me around 85-90% O2 prior to descent.

So I would not recommend skipping the flushing, escpecially not if there is no sensor in the loop.

With no flush and a shallow dive with a very slow ascent the O2 could get dangerously low.

One must also think about making sure that the gas in the tank is very close to 100% O2. Several O2-Rebreather divers have gone to sleep just because they did not take care flushing the loop or making sure they feed 100% O2 into the Rb. For example, when I got one of my IDAs the O2-bottle held 90% O2. Not so safe.

About the offgassing from the body. If I recall correctly an standard male holds about 2l of dissolved nitrogen in the body at standard conditions.
My guess is that it would take quite a long O2-Rebreather dive to get all of that out into the loop. I have a paper about the amounts of dissolved gas in humans, could try to find it again and check the numbers.

Marc T

"About the offgassing from the body. If I recall correctly an standard male holds about 2l of dissolved nitrogen in the body at standard conditions."

just have to take care using a CCR O2 without Po2 display on deco after a long dive, here offgassing is really important and your po2 can be really very far from 1.6 at 6 m ( more often 1.3 )
Its important to flush from time to time and perhaps calculate your deco for 80 % O2 instead of 100%

nigelh

IDA64A

But we're talking oxygen rebreathers here. I don't think this is going to be a significant problem. The one thing we never ever have to do is deco.

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nigelh

Marc T

Except military and very shallow dives I just don't imagine any other use for a CCR O2 than deco, ethen after an OC dive.
Snorkeling is a good way too but not for military nor for deco