unacceptable CO2 levels

A long time ago (~25 years) branches used to teach the art of breathing from an ABLJ: crack open the emergency cylinder, breath, exhale back into the bag, breath again - up to 4 cycles before exhaling to water and refreshing the bag with new air from the cylinder. I can see that the ppO2 in the bag stays above a hypoxic threshold of 0.1 bar at any depth below about 2m (assuming 0.21 bar initial pressure and consuming 0.04 bar of oxygen per cycle) but I can't find any material that describes the impact of the build up of CO2 during this exercise, especially at sample depths. Any information or explanations appreciated...

Thanks in anticipation,

Ariel

A long time ago (~25 years) branches used to teach the art of breathing from an ABLJ: crack open the emergency cylinder, breath, exhale back into the bag, breath again - up to 4 cycles before exhaling to water and refreshing the bag with new air from the cylinder.

but I can't find any material that describes the impact of the build up of CO2 during this exercise

Actually it works just the same way as O2. You exhale a constant ppCO2 (about 48mBar per breath) which builds up in the bag. I hope they taught you to use 'minimum loop volume' so there wasn't much left in the bag from cycle to cycle. This is a semi-closed rebreather with a dud scrubber and the fallback for a CCR if you think the scrubber has quit.

I don't have any agreed figures for max ppCO2 but the three or four breath rule is still used.

Thanks Nigel,

For O2, assuming a 0.04bar reduction in each cycle, at the surface you'd get approx. 21->17, 17->13, 13->9, 9->5%. Hypoxia threshold around 0.1bar oxygen makes the 4th breath interesting...I guess I'd switch to nice surface air...
At a mere 2m depth the calculation changes: 0.25bar ->.21, .21 -> .17, .17 -> .13 and .13 -> .09bar oxygen. In this case depth assists the exercise (keeps the O2 partial pressure above 0.1bar), but provides a small risk of shallow water blackout as the diver surfaces.
I'd like to understand the impact of the CO2 cycle as the depth changes (I'm an open circuit diver, so if this is CCR 101 please forgive me). For surface exercise the CO2 buildup (using 48mb increase per cycle and 4mb initial, gives:

mbar of CO2 in "rebreathed" air
Cycle 1 2 3 4
In 0.000 0.048 0.096 0.144
Out 0.048 0.096 0.144 0.192

These partial pressures are pretty steady regardless of depth (since the initial 4mbar is insignificant even at say, 50m)so I guess that depth is not really a factor. I'd missed that the ppCO2 doesn't change much with depth, whereas the ppO2 does, due to the initial mix of air. (As opposed to contaminated air containing excessive CO2, where the ppCO2 increases with depth, as in the BSAC manual example - which of course is illustrating a rather different issue...)

For O2, assuming a 0.04bar reduction in each cycle, at the surface you'd get approx. 21->17, 17->13, 13->9, 9->5%. Hypoxia threshold around 0.1bar oxygen makes the 4th breath interesting...I guess I'd switch to nice surface air...

Humm... I prefer the one surface litre per minute of O2 model rather than x bar per breath. One litre is generous so on a 5L tidal volume starting at 21% you have about one litre and you can breath about half of it and still keep your average O2 level above 0.15bar which is what I thought the limit is for keeping some mental control. So 30 seconds top wack at the surface and more deeper.

We would have the same effect on CO2. You are producing it at a fixed rate dependant on work load and we would need numbers for time rather than the usual 'exhaled breath' values to get a propper view of the rate the ppCO2 would rise and I don't have those numbers to hand.

Breathing the jacket may no longer be taught and that may be a good thing. If you are going to carry spare gas a pony wins over a tiny bottle any day. Exercises that might work in the pool look perilious when combined with an ascent so I assume that the real response to an OOA was to blow the jacket and have a gasp off it on the way up.

You are bringing back memories that are lodged deep in the back of my brain but I was taught to squirt a bit of air (from the 400ml bottle) into the bag, inhale it, exhale out into the water, then squirt a bit more into the bag. CO2 was never an issue. It was great fun, a great test of co-ordination, and a test of resistance to lung infections.
Thank goodness those days are gone! I was also taught (by my German instructor) to do a free ascent from 30m and fin one km with only one fin!

So long as you dont have a scrubber in circuit, your normal sense of being out of breath will work fine when breathing off your buddy jacket as it's keyed to CO2 levels. It's only gets scuppered by things like hyperventilating or scrubbers.

John.

> So long as you dont have a scrubber in circuit, your normal
> sense of being out of breath will work fine when breathing off
> your buddy jacket as it's keyed to CO2 levels.

That's how the breathing rate is controlled - but "will work fine" is not appropriate here IMHO. Breathing in CO2 is not a Good Thing(tm). A number of us who have experienced hypercapnia at depth have reported a feeling of terror; this is not good for sorting out an ascent.

HTH

Vic.

> So long as you dont have a scrubber in circuit, your normal
> sense of being out of breath will work fine when breathing off
> your buddy jacket as it's keyed to CO2 levels.

That's how the breathing rate is controlled - but "will work fine" is not appropriate here IMHO. Breathing in CO2 is not a Good Thing(tm). A number of us who have experienced hypercapnia at depth have reported a feeling of terror; this is not good for sorting out an ascent.

HTH

Vic.

I can see your point about abject terror not being good for a controlled ascent ... but as far as body feedback to the brain goes ... I'd say abject terror was probably the correct message needed for when you are underwater and building elevated levels of CO2 in your lungs .... even if it's not ideal .. its beter than the situation with a scrubber where your first inkling that there is a problem .. is tunnel vision followed quickly by blackout, hey but thats just down to personal preferences.

My own personal preference is not to be unfortunate enough to encounter either situation :-)

John.

I'd say abject terror was probably the correct message needed for when you are underwater and building elevated levels of CO2 in your lungs ....

Sorry, I'm with Vic here. I'd much rather be in a position to deal with a problem in a calm and rational way rather than in a panicked state.

Stop Breathe Think Act.

Janos

>>I'd say abject terror was probably the correct message needed for when you are underwater and building elevated levels of CO2 in your lungs ....

>Sorry, I'm with Vic here. I'd much rather be in a position to deal with a problem in a calm and rational way rather than in a panicked state.

>Stop Breathe Think Act.

I've done elevated CO2 and it wasn't abject terror it was just I MUST breath faster. MUST! NO ALTERNATIVE!

I was very cold and rational and I had a reg with nice known gas in my hand but taking the mouthpiece out of my mouth for long enough to do the swap was not an option. I'm glad the brain was on line because I was looking for a solution that didn't read like blow the wing and ascend like a cork and I found one.

That was at about 20m. Deeper, I hear, it is like an oxygen hit and your brain just takes a time out but I'm glad that is not on my CV to discuss.