Deco Stop offgas feedback by watching PP02

[Bobjr]

Hum. Thinking through this myself... And making some heroic assumptions.

1) The body isn't saturated at 6m - certainly the slow tissues aren't. However I'm going to make a simplifying assumption that the body is mainly fast tissues that are saturated. I know this is wrong!

2) The safe degree of over-saturation at the surface is 1.6atm (Bulhmann's M0 value)

3) The safe degree of oversaturation at 6m is roughly 1.6 x ambient, = 2.56 bar, call it 2.6 bar between friends.

4) Hence difference in tissue saturation between arriving at 6m stop, and being ok to surface (ie leaving 6m stop) is 1.0 bar.

5) Solubility of Helium in blood is: 0.008 ml / ml-atm [1] or 8ml per litre of blood in sheep.

6) If a 100kg person is 50% blood and other fast tissues (brain etc) then that would be 0.4 litres of He off-gassed if breathing pure He.

7) Hence 0.2 litres off-gassed if breathing 50% He before reaching 6m.

8) Given a tidal volume of 2 litres, that's about 10% so you'd expect to see the ppO2 drop from 1.6 to 1.44 over the course of deco.

Janos


[1] - Source: snip

Was thinking about this also today.

Assuming total saturation and a 1.6 M value that means the worse the body can arriving at 6m would be 1.6 x 1.6 = 2.6 (between friends LOL)

The figure from my earlier post of 1.5mg per L per atm

Then take your 100kg diver - assuming we are mostly water (I think its about 60%) we get 100L

So worse case total breathing pure Helium would be 2.6 *1.5*100 = 390mg gas or .39g

Assuming that we could surface at when tissues reach 1.6 that means we'd leave 6m after we'd off gassed (1.6/2.56*0.39) 0.24g of Helium.

.24g of Helium has a volume of (.24/4*22.4) = 1.34L at surface and at 6m 0.84L (which interestingly is the same figure as your example since you say 50% heliium and 50% body composition water and fast tissues giving .21L)

Since worse worse case you have 0.2 (Janos) - 0.8 (me) L offgassing into a lung and loop total volume of something like 10L (much more than your tidal volume example) giving us 2-8%. So PO2 drop of .12 over entire deco.

I would say we are probably more conservative by between 2 and 10x so likely off gassed Helium is much less important than water vapour and any other factor affecting readout of PO2.

 

[PeterN]

So, its water vapor? That explains the massive amount of lung butter.

Peter

 

[Iain3]

I think a big factor in the drop in PPO2 is caused by metabolisation.

You don't add O2 to the loop because ppO2 rarely drops low enough, but the loop slowly collapses as a result of your body extraxting O2 and the scrubber extracting Co2. The only gas not being depleted and therefore making up a higher and higher proportion of the mix is your innert gasses.

You could test for this do a deep dive, then O2 flush at 6m, wait for 30 minutes and measure the drop in PP02. Then do the same without the deep dive first, climb in, swim down to 6m, do an O2 flush and sit for 30 minutes and compare the results. The difference should mostly be a result of offgassing.

 

[TR300]

Just started this new thread after a comment I put on the GF selection thread about using the loop as a pure 02 loop at the 6m (20 foot) stop:

Our practice is to isolate the ADV and then do a really good loop flush wth 02 to drive PP02's up after we reach 6M. We generally can obtain, oh... 1.4 with a good flush. Nothing is perfect.

The fun then starts as we watch the PP02 drop as we offgas helium into the loop. Starting at 1.4, you can actually see the offgassing take place as the loop PP02 drops lower and lower as gas enters the counterlung across the aveolar tissues. It's interesting to watch. We re-flush at 1.2, and then the second period of PP02 decay takes longer... do it a third time and it's happening lots longer. Just an interesting feedback loop.

I'm wondering if a more scientific method might exist to use this sort of feedback loop to make ascent to next stop decisions... scratching head.


Dave


.

Hi Dave,

I agree, same effect seen on my loop, but I think it works only on mCCR, where I can manage my setpoint, eCCR is to tricky. I see this also on deeper stop and my strategie is, if my decoplan (computer, slaves) is missing, I add one more minute after the setpoint is stable. It's not rock solid but better then nothing and give, when, only weak DCS problems

Holger

 

[Mr fish]

i seem to remember a guy working on this some years back

think he had something in the loop that indicated the
off gasing of helium into said loop ,
think martin parker had a look at it ,,

cant remember much more ,,

 

[GKAM]

...Our practice is to isolate the ADV and then do a really good loop flush wth 02 to drive PP02's up after we reach 6M. We generally can obtain, oh... 1.4 with a good flush. Nothing is perfect.

The fun then starts as we watch the PP02 drop as we offgas helium into the loop. Starting at 1.4, you can actually see the offgassing take place as the loop PP02 drops lower and lower as gas enters the counterlung across the aveolar tissues. It's interesting to watch. We re-flush at 1.2, and then the second period of PP02 decay takes longer... do it a third time and it's happening lots longer. Just an interesting feedback loop.

I'm wondering if a more scientific method might exist to use this sort of feedback loop to make ascent to next stop decisions... scratching head...

Interesting - a recent thread on RBW led to similar discussion to which I participated hence could not resist sharing some thoughts on here as well.

Based on literature on this subject I understand two main factors affect the FiO2 in the loop: breathing circuit saturation with water vapor and the efficiency of the purge procedure. To make things even more interesting, O2 sensors have an error of +/- 2% FS plus in this environment their face can be covered by humidity at the later stages of a dive and that introduces additional error (lower readings).

As a rule of thumb, max FiO2 in O2 CCR mode can seldom go higher than 0.95. Any value above 0.90 is great, but quite often actual fraction can be as low as 0.75. There seem to be some common misconception about why FiO2 never reaches 100% in the loop. A human body off gassing N2 or He while on the Rebreather loop is in no way capable of affecting the O2 concentration significantly. A good starting point to read on the subject is the following report from the Naval Submarine Medical Research Lab (NSMRL) h t t p ://archive.rubicon-foundation.org/3611.

In short the idea to derive any kind of deco decisions based on the ppO2 drop in the loop does not seem very sound at this point. A few years back there was an interesting online debate with someone who wanted to gauge scrubber health by weighing the stack before and after a dive and claiming the difference was (only) CO2 bound to the granules...

/GKAM

 

[rossh]

I seem to recall from the old VPM list that calcs had been done that suggested that the amount of gas absorbed and released was measured in litres. Aren't there anecdotal tales of pure O2 units going hypoxic from not doing regular flushes?

This situation is similar to breathing 100% at the end of a dive. Note that we are all pre-saturated at 79% N2 from the surface.

The O2 rebreather diver, will exhale some of that stored N2 over the course of his dive. i.e. the inspired N2 gas pressure vs the tissues stored N2 gas pressure, is what drives the off gas here.

The normal trimix OC diver or CCR with diluent, while on 100% O2 at the end, may also reduce the absorbed N2 tissue levels to less than the 79% value. This would occur after using a trimix, and the helium component was large and limiting / controlling the ascent. That allows time for the N2 to reduce below surface values. Also, staying for extended shallow stop periods on O2, could also lower the N2 tissue value to less than 79%.

After all of these situations above, an on-gassing of N2 occurs at the surface, from the air to get the tissues back to 79% N2.

Regards rossh