matt
Well-Known Member
Hi Simon,
The chart is drawn in the same style as Erik C Bakers with the horizontal axis ambient pressure and the vertical compartment pressure. Does that make a difference?
Matt.
What's your Low Gradient Factor Selection
- Thread starter Tmccar1
- Start date
Hi Simon,
The chart is drawn in the same style as Erik C Bakers with the horizontal axis ambient pressure and the vertical compartment pressure. Does that make a difference?
Matt.
Mark Chase
ATB=AlwaysTalkingBollocks
How about this offering
|Mind you in hind sight the 0m value would never make the surface
ATB
Mark
|Mind you in hind sight the 0m value would never make the surface
ATB
Mark
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Actually, you would. Eventually.
At 0 GF, ambient pressure = gas pressure at the alveoli = tissue (inerts) tension. Since alveoli includes oxygen (and H2O, and possibly CO2), the inerts pressure is less than the tissue tension, so you're offgasing. Very safe. Very slow.
Cheers,
Matthieu
Simon Mitchell
Well-Known Member
Hi Matt,
That is what I mean. The m value is the allowable supersaturation at a particular depth. Therefore, a fraction (or "gradient" - a misnomer really) of the m value should appear as a vertical line above the specific depth (just as you have correctly represented for your surfacing "gradient").
Simon
Mark Chase
ATB=AlwaysTalkingBollocks
Sounds like the oxygen window myth. Why would the partial pressure of O2 affect the Partial pressure of inert gas? Solubilities of one gas are not effected by solubilities of others. Either you are saturated at a particular partial pressure of inert gas (and hence tissue tension) or you are not - the only way an increase in oxygen would affect this is if the partial pressure of inserts drops - ie gas switch.
matt
Well-Known Member
Would that be your buddy, Mark?
Right. AFAIK. Considering a single compartment and nitrox to keep it simple.
In your tissue, you have dissolved N2. One way to measure that is concentration. Another is to go with henry's law and use the partial pressure it would be in equilibrium with. Now if the partial pressure of a particular gas in a tissue is higher than the partial pressure of that same gas in the lungs, it's above equilibrium and the molecules will diffuse out of the tissue and, via the blood, into the lungs (offgasing)(it's actually not what's happening, molecules go both ways, but the overall effect is as described). The total partial pressure, tension, of inerts is what's used by buhlmann and GF to determine the ceiling. Oxygen and ambient pressure is irrelevant to that. pInerts(tissue) = pN2(tissue)
In your lungs, on the other hand, the total pressure is the same as ambient, so we have p(ambient) = pO2(lungs)+pN2(lungs)+pH2O. Usually it's assumed that the lungs and loop are saturated with water vapor, which has partial pressure pH2O. Sometimes there's CO2 as well, but not in Buhlmann. pO2 will be the setpoint, pN2 will be the rest.
What GF=0 means is the stop is at p(ambient) = pInerts(tissue)
No a/b, no supersaturation, no nothing.
Therefore pO2(lungs)+pN2(lungs)+pH2O = pN2(tissue)
pO2 and pH2O are greater than 0, so pN2(lungs) < pN2(tissue)
The equilibrium would be with a higher pN2 in the lungs, or a lower one in the tissue, so N2 will migrate to the lungs. So you're offgasing.
With several compartments it gets more complicated, p(ambient) will be the same as the pInerts(tissue) of the compartment that has the highest. So there may be offgasing there and ongasing in others, but the diver will still be ascending.
With trimix and (stupid) switches it's possible to get into situations where He is ongased (faster) while N2 is offgased, so actually the diver needs to go down at first, to equilibrate with the new gas, but then he will ascend.
So the point stands, at GF 0, you will, eventually, reach the surface. A bit academic, tho
AFAIK.
Cheers,
Matthieu
If that were true high PO2 setpoints or deco gas would not work - as you might just be wrong you might wanna spare the condescending academic line too
Err, I did start and end with "as far as I know" and "academic" was merely a joke on the fact that nobody actually dives GF 0, that's it, no offense meant.
I don't follow you. If pO2(lungs)+pN2(lungs)+pH2O <> pN2(tissue), all it means is that the compartment is not at GF 0. If you do high oxygen you'll easily get to a situation where the fast compartments have an inert pressure that's less than ambient, so the effective GF at this point in time for that compartment is negative. I don't see the problem.
Cheers,
Matthieu
Simon Mitchell
Well-Known Member
Any better?
Matt.
Perfect!
If Tissue tension of Nitrogen was = to the sum of all the partial pressures found in the lungs (as you put pO2(lungs)+pN2(lungs)+pH2O) then tissue tensions of inerts would just be proportional to ambient pressure - thus there would be no point in nitrox or high PO2 deco gas.
It's not true because physiology makes it so, it's true because that's the deco plan.
A GF 0/0 ascent is an ascent where the tissue tension in the compartment that has the highest equals the ambient pressure.
If the diver switches to O2, he's going to get rid of the inerts in the tissues faster, so he ascends faster. Otherwise he's not diving 0/0.
Cheers,
Matthieu
Igor P
New Member
Hi Bobjr,
emmbee has it right in his answers. Your are confusing things a bit. Mabe it would be good for you to read this pdf to refresh a bit.
There are some other on that same site http://www.ddplan.com/reference/ that can help too.
Best regards,
Igor P
emmbee has it right in his answers. Your are confusing things a bit. Mabe it would be good for you to read this pdf to refresh a bit.
There are some other on that same site http://www.ddplan.com/reference/ that can help too.
Best regards,
Igor P
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Capt Morgan
Member
You don't argue with emmbee, he's normally right and can drive a tank
Simon Mitchell
Well-Known Member
Hello,
I don't think it is correct to call it a myth though it depends to some extent on how you interpret the term. I have placed a paper that discusses the "oxygen window" (page 191 onward) and some of the related matters debated here in my drop box:
https://dl.dropboxusercontent.com/u/22145620/Doolette and Mitchell 2011.pdf
RichardC
New Member
Igor P, thank you as well for shearing info.
The must important contribution to the forum are when we shear information base on scientific studies and is superb if we can access to the original sources.
Thanks to all of you, this is a wonderful community.
Sent from my iPad using Tapatalk
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Igor P
New Member
Completely wrong, the driver for off gassing is differential between partial pressure of N2 in lungs and partial pressure of N2 in tissue.
If pN2(lungs) < pN2(tissue) than off gassing is proceeding. By switching to nitrox or oxygen you rise pO2(lungs) and at the same time lower pN2(lungs) so the difference between pN2(lungs) and pN2(tissue) is even bigger and off gassing is faster.
This is writen as much as possible simplified...
here is some more reading for you that could help:
Oxygen_Window.pdf
Introductory-Deco-Lessons.pdf
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