Rebreather World - View Single Post

jaap · 17th August 2006, 09:06

Hi

I have a vague memory of seeing some documentation years back (maybe an old patent) about a SCUBA that recycled <50% of the breathed volume. It did not have a scrubber and was intended to do the same thing you are after.

I think the "problem" is that the true dead space is not very large (~0,15l) and that our lungs would be a very inefficient construction if most of the CO2 would be found only in the end of an exhaled breath. But it would be interesting to get detailed info on CO2-concentration profiles.
A quick search yielded this:
http://jap.physiology.org/cgi/content/full/85/2/642/F2
http://jap.physiology.org/cgi/content/full/85/2/642/F3

I'm convinced this is a well documented thing.

I think that this sort of (questionable) device would work more on the fact that an moderate increase in the steady state level of CO2 and decrease in O2 could be tolerabel if the breathing pattern is correct and the depth is limited.

A fast approximation of the depth dependent O2 levels (CO2 indirectly in the non scrubbing case) of a Rebreather recyling ~50% can be made by using the equations for an DC55/RB80 style RB. At least I think so...

The equations can be found here: http://www.teknosofen.com/dc55_tech.htm

So if I use this (eq K) for air with a dump ratio (Kratio) of 0,5 and an extraction ratio of 20 it gives me the following.

An O2 conc in the mix of ~13.5%...at 1 atm (surface). At 10m (2bar) this would be ~17% O2. Of course going deeper its getting better and better.

I only ran the numbers once and have not been careful so don't take it for any sort of fact...

So its not looking good for this sort of device, at least not near the surface. And remember that the increase in CO2 would be proportional to the decrease of O2 if no scrubber is used.... So the CO2 loading would be enormeous close to the surface...

EDIT:
I just realised I was going too quick with the calculations. A dump ration of 0,5 does not correspond to 50% recyling. The Kratio should be 1 for 50% dumping.

So running the same numbers but with a Kratio of 1 instead give more forgiving numbers: About 17% O2 at the surface, ~19% at 10m and ~19,6% at 20m.

So this type of device could work, especially for the deeper part of say a trimixdive... At depth the fresh gas would be very close to the loop mix and the CO2-% would be low. But the PCO2 could still be dangerous, although less and less the deeper you go.

But is a say 50% improvement in gas economy worth the trouble of bringing such a device? The step from this sort of thing to an RB80-style Rebreather is not that far. Simply add a scrubber and you allow better dump ratios getting alot better gas economy. But then the step is not so far to a CCR either...

17th August 2006, 09:06	#3 (permalink)
jaap Normal people worry me Current Rebreather/s: Classic Kiss Other CCR RB80 / Clone Ray Other SCR Home Build Other Rebreather/s: Other CCR RB80 / Clone Ray Other SCR Home Build Join Date: Jun 2005 Location: Stockholm Sweden Posts: 445	Re: Semi Rebreather Hi I have a vague memory of seeing some documentation years back (maybe an old patent) about a SCUBA that recycled <50% of the breathed volume. It did not have a scrubber and was intended to do the same thing you are after. I think the "problem" is that the true dead space is not very large (~0,15l) and that our lungs would be a very inefficient construction if most of the CO2 would be found only in the end of an exhaled breath. But it would be interesting to get detailed info on CO2-concentration profiles. A quick search yielded this: http://jap.physiology.org/cgi/content/full/85/2/642/F2 http://jap.physiology.org/cgi/content/full/85/2/642/F3 I'm convinced this is a well documented thing. I think that this sort of (questionable) device would work more on the fact that an moderate increase in the steady state level of CO2 and decrease in O2 could be tolerabel if the breathing pattern is correct and the depth is limited. A fast approximation of the depth dependent O2 levels (CO2 indirectly in the non scrubbing case) of a Rebreather recyling ~50% can be made by using the equations for an DC55/RB80 style RB. At least I think so... The equations can be found here: http://www.teknosofen.com/dc55_tech.htm So if I use this (eq K) for air with a dump ratio (Kratio) of 0,5 and an extraction ratio of 20 it gives me the following. An O2 conc in the mix of ~13.5%...at 1 atm (surface). At 10m (2bar) this would be ~17% O2. Of course going deeper its getting better and better. I only ran the numbers once and have not been careful so don't take it for any sort of fact... So its not looking good for this sort of device, at least not near the surface. And remember that the increase in CO2 would be proportional to the decrease of O2 if no scrubber is used.... So the CO2 loading would be enormeous close to the surface... EDIT: I just realised I was going too quick with the calculations. A dump ration of 0,5 does not correspond to 50% recyling. The Kratio should be 1 for 50% dumping. So running the same numbers but with a Kratio of 1 instead give more forgiving numbers: About 17% O2 at the surface, ~19% at 10m and ~19,6% at 20m. So this type of device could work, especially for the deeper part of say a trimixdive... At depth the fresh gas would be very close to the loop mix and the CO2-% would be low. But the PCO2 could still be dangerous, although less and less the deeper you go. But is a say 50% improvement in gas economy worth the trouble of bringing such a device? The step from this sort of thing to an RB80-style Rebreather is not that far. Simply add a scrubber and you allow better dump ratios getting alot better gas economy. But then the step is not so far to a CCR either... Last edited by jaap : 17th August 2006 at 11:52.
(Offline)