Deco Stop offgas feedback by watching PP02

Dave Sutton

Dave Sutton

Banned
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


.
 
Dave I have always been very sceptical about the amount of gas we actually "offgass into the loop" I asked around with some of the diving physicians and they seem to think it was just millilitres
If it were enough to impact the loop po2 would we not see this as a dill loss at uptake at the start of the dive? I mean once you hit bottom and get the loop volume set right it doesnt change as long as you dont go deeper or shallower right? my dill usage seems to be about the same whatever dive I do.

I am only guessing here, but a lot of people comment about the amount of gas we offgas, it would very interesting to find out exactly what that amount actually is!!

cheers

Dave
 
Dave Sutton said:
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


.
Click to expand...

I just became aware of this recently, and started doing the same thing. P02 decay does seem to happen faster, then slower, at my 20 and 10 foot stops. It's interesting to watch.
 
I seem to remember it was somewhere in the region of litres of Helium. I think NigelH did the calculations somewhen.

Janos
 
I have defo noticed this on deco. For the final stops 9m up I go manual injection for 02 and the last 20-30mins ill be on pure 02. I can see the need for injecting and flushing with 02 dropping and eventually for the last phase of deco holding loop volume becomes easier and the need for flushing to maintain 1.6ish drops off considerably

For this to be apparent you have to be running manually.

I have no factual data but I was under the impression that off gassing HE was in ltrs and Nitrogen was in Milliltres

My concern is that we cant see the loop drop for Nitrogen only for He so clearing He out of our system doesnt necessarily meen we have cleard deco, just the HE bit.

With HE all the way out I understand that HE and nitrogen are playing simila roles in the final phase of deco comitment, but with other plans such as switching to 50% from trimix on OC, we see the HE elemen6t cleared quickly and Nitrogen controling the final deco.

QED on week helium mix on CCR id guess this affect will be much harder to spot.

When running dives on 15/65 as i often do, the effect is much more aparent.

ATB

Mark
 
It would be interesting to put a helium analyser into the loop to see whats going on. The tests we did for moisture definitely shows loop po2 rises but cell readings drop if you are running manual
if you are running auto then I am guessing you wont see a po2 drop but the actual loop po2 may go up, of course unless you are already on 100% o2 loop AND running manual you will then see cells dropping

so is this because you are offgassing loads of helium or is it the sensor outputs dropping due to moisture, heat, bent from HE bubbles, chemical changes!!!!

I dont know!!
 
NigelH said:
When I first thought about deco theory I wondered how much gas is actually involved and search as I might I could not find anybody quoting it. <sigh> However a dig in the usual sources (The old Rubber book) turned up the molar fraction solubility for nitrogen in water that leads me believe that at 0.79bar ppN2 (on the surface) we have about 0.72 grams of dissolved nitrogen in a 70Kg person. So a good exposure to breathing air at 25 meters will give us 2.5gms of nitrogen which is 3.2 surface litres.
Click to expand...

The above quote from Nigel over on YD implies it's measured in litres - but I would have guessed that's at full saturation. But deeper would mean more dissolved gas.

I know that Helium ongasses quicker, but I don't know if that means it's more soluble. I think it is.

Janos
 
ccrdave said:
It would be interesting to put a helium analyser into the loop to see whats going on. The tests we did for moisture definitely shows loop po2 rises but cell readings drop if you are running manual
if you are running auto then I am guessing you wont see a po2 drop but the actual loop po2 may go up, of course unless you are already on 100% o2 loop AND running manual you will then see cells dropping

so is this because you are offgassing loads of helium or is it the sensor outputs dropping due to moisture, heat, bent from HE bubbles, chemical changes!!!!

I dont know!!
Click to expand...

Given the difficulties of getting a helium analyser to work at 6m, would an easier/expedient test be swapping to a "fresh" rebreather for the final stop? Obviously it won't tell you the gas mix but it would give you an idea whether or not the drop in displayed pO2 is down to cells on a long dive or a change in gas mix. If nothing else it would give an idea as to whether the hassle of fixing up some sort of analysis system (assuming it's not chamber dives) is worthwhile or not for some tests.

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?
 
Rebus said:
Given the difficulties of getting a helium analyser to work at 6m, would an easier/expedient test be swapping to a "fresh" rebreather for the final stop? Obviously it won't tell you the gas mix but it would give you an idea whether or not the drop in displayed pO2 is down to cells on a long dive or a change in gas mix. If nothing else it would give an idea as to whether the hassle of fixing up some sort of analysis system (assuming it's not chamber dives) is worthwhile or not for some tests.

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?
Click to expand...
On an o2 unit there is no aditional inert gas, you need to flush the unit at the start of the dive but once all the nitrogen is flushed from the loop and your lungs there is no more inert gas to uptake
 
ccrdave said:
On an o2 unit there is no aditional inert gas, you need to flush the unit at the start of the dive but once all the nitrogen is flushed from the loop and your lungs there is no more inert gas to uptake
Click to expand...

Yeah, I know but I seem to recall reading something anecdotal on the loop going hypoxic. Can't remember the circumstances other than it wasn't normal O2 rebreather use and a lack of flushing being the theory. May have been from one of the old homebuilders' forums as it was years ago.
 
ccrdave said:
On an o2 unit there is no aditional inert gas, you need to flush the unit at the start of the dive but once all the nitrogen is flushed from the loop and your lungs there is no more inert gas to uptake
Click to expand...

this could happen if a diver switched to a pure o2 CCR on ascent from a deep dive... then you could test... as there is no way of adding other gases except by expiriation..just a thought... as the guys using those white arrow sweet decos.. etc
 
Not scientific at all but when I started on mix you can really notice it.
and it appears to be greater with the richer the mix (of course your run times are also longer so it may be bullshit).
Thanks
kevin
 
Janos said:
The above quote from Nigel over on YD implies it's measured in litres - but I would have guessed that's at full saturation. But deeper would mean more dissolved gas.

I know that Helium ongasses quicker, but I don't know if that means it's more soluble. I think it is.

Janos
Click to expand...

Some random thoughts on this (assuming correctness of original post):

- Not sure on the calculations but 2.5g of N2 is roughly 2L at surface (1mol of gas having a volume of 22.4L STP) . At 6m this would be less (P=1.6bar).

- Further to this that would assume the body is saturated at diving depth but at deco (you are in trouble if that is the case).

-quick google shows only ~1.5mg of Helium dissolves in water 40oC at 1bar ( check out www dot engineeringtoolbox.com/gases-solubility-water-d_1148 dot html) Assuming we are mostly water that would be 70kg person fully saturated at 10bar in pure helium approx 1g of Helium or 5L at suface. This would be 3L at 6m assuming the tissues were still fully saturated at 10bar (might be painful LOL)

The numbers seem to support CCRdave ie volume of gas meaningless (much of your offgassing is done by 6m)
 
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: Perfusion–diffusion compartmental models describe cerebral helium kinetics at high and low cerebral blood flows in sheep
 
Janos said:
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: Perfusion–diffusion compartmental models describe cerebral helium kinetics at high and low cerebral blood flows in sheep
Click to expand...



Mate I couldent green you for this but I just want to say you are a very clever sod on the QT :D

ATB

Mark
 
Janos said:
I doubt it - I just think by talking / typing. There's usually at least one mistake in my calculations. Good thing I don't do anything important for a living.

Janos
Click to expand...


Dont worry.

Their other f ups are so monumental, yours would go unnoticed :D

ATB

Mark
 
Janos
You lost me at the 2 litre tidal volume, if you are offgassing 0.2l helium how will that dilute an 8l loop by 20%ish over a time of say 45mins? Are you offgassing that every tidal breath?

As an aside the late Dr Max Hann told me he thought all helium was gone by the time you got to 3m!!

Cheers
 
You must log in or register to reply here.
Back
Top