Semi Rebreather

PaulTG2

There are quite a few very smart and experienced people on this board.

I was wondering if anyone knew of a source of information concerning the concentration of CO2 in exhaled breath over time -- That is, during the course of an exhalation how does the concentration of CO2 change?

My thinking is this.... the first air exhaled comes from the mouth and throat then from the main lung channels and then from the sack which exchange gases with the blood. It would seem that there would be little or no CO2 in the first portions of the exhaled breather and the CO2 concentration would gradually increase as you exhale.

IF the first x% of the exhaled air contains little CO2 could it be captured in aand re-inhaled on the next breather in order to extend the duration on open circuit scuba?

Even if if there was some CO2 it might not amount to enough to cause any problems if combined with fresh gas from the cylinder. Perhaps capture the first 1/2 of the exhalation and then use it for the first half of the next inhalation to double the duration?

It seems this would be mechanically very simple, not prone to failure, and might result in a doubling of the duration for a cylinder.

Ok... so, where am I off track here?

--Paul

abowie

You're not especially off track.

The volume of gas you're referring to is called "dead space", and is around 150ml in an average adult. The regulator contains about another 30 to 50 ml of gas that also is not involved in gas exchange.

In anaesthesia dead space affects the design of anaesthetic circuits in a number of ways; most of the time we are attempting to minimise it.

As to "recycling" the unutilised exhaled dead space gas, who knows?

__________________
Andrew Bowie

Rebreather-friendly Buddy

jaap

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...

abowie

Tracing B on the first link bears a fair resemblance to what i see daily on anaesthetic machine capnographs.

__________________
Andrew Bowie

Rebreather-friendly Buddy

Marc T

Ive tried that several years ago ( before being interested by Rebreather ) by simply beathing in my BCD in a pool, using it like a pendulum Rebreather and flushing from time to time to eliminate CO2.
I think the greatest danger in shallow water is hypoxia.
Could be interesting to try that again, simply adding a p port and PO2 meter.

jaap

Why not add a small scrubber and some O2 too it? Unless you enjoy that heavy panting and anguish laden sensation of breathing a high CO2 mix ;-)

http://www.dykarna.nu/photos/origina...0814124451.jpg
http://www.dykarna.nu/photos/origina...0814124339.jpg

Duncan Price

The prototype for Rick Stanton's sidemount rebreather used an empty sweet (candy) container as the "scrubber" and a water bag C/L. The device used an ADV to feed pure O2 and could be dived for about 5 minutes before the loop needed purging of CO2. It was just a device to test out the chassis layout for a proper sidemount unit - i.e. CL at the top, scrubber at the base.

Duncan

Crazyduck

I really like the photos- that was inventive.
Looks like a Drager or Russian Kip basic fire fighting scrubber?

You know if you had two small scrubbers and a 4 liter counterlung- it would basically would be the Tiger Frog design.

Or this could be incorporated into a Tom Rose pendulum style unit.

Is there a limitation on the dead air space through the breathing hose?

Also Dave Sutton showed how some pendulum rebreathers have separate paths for exhaling and inhaling through the scrubber bed.

Andrew

__________________
Howdy Senor- What’s Happening!
Rob Davie April 2005-

Presently in a state of transition from Open Circuit to Closed Circuit.

"You will not be punished for your anger; you will be punished by it." - Buddha.

jaap

Yes thats version 3 of my attempts to make an ultracompact O2-Rebreather. The previous ones where chest mounted similar to Tom Rose pendelum or the original Fenzy. But this one is far slimmer. The wing is very oversized as a CL and the DSV on the pictures is rather improvised. But it really nice WOB with the wing _in front of the backplate_ and silent (no annoying sound from checkvalves)

Scrubber is from a Fenzy
http://www.nobubblediving.com/fenzy.htm

Only I ripped out the insulation on the inside of the scrubber to increases its capacity and eliminate the risk for channeling.

But yes, I also have the same setup with the slightly larger KIP-8 scrubber. Even have a crazy half finished shoulder mount conversion of an Exendair-kanister, but its a bit on the big side to fit on shoulder...

I try to keep the dead space as low as possible. The hose and scrubber on the picture has about 0.15l deadspace to the bed. Also since the O2 addition is just by the mouth that helps to ventilate the dead space.

Have dived this version 3 for a maximum of 45 min in ~20 C water and no CO2-problems. That is taking full breaths, if I deliberately take small breaths the not so nice CO2 comes creeping.

Next improvement will be an elbow with a small dumpvalve on the hose. As it is now there is no real possibility to get rid of water. Even if it works fine to go on and off the loop UW its very annoying when a small amount of water gets gurgling in the hose.

Sorry if I'm hijacking the thread with this description.

Duncan Price

A bit like my P1SS - I have a spare adapted mouthpiece if anyone wants one - this one in fact.


more details: http://www.sump4.com/rebreather/p1ss...tml#mouthpiece