Hypothetical question
- Thread starter boulderjohn
- Start date
Please register or login
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
- Ability to post and comment on topics and discussions.
- A Free photo gallery to share your dive photos with the world.
- You can make this box go away
Storker, that is not true in absolute form. Else you would subject yourself to a huge deco stress when switching to deco gas. Gradient in partial pressure is the main driver behind off-/ongassing, which is neither identical with decompression stress, nor bubble formation.
Besides this, for the point we are discussing here, this question is not really relevant. The diver will have to ascend on EAN 60 from 10 meters to 0.1 meters (or 0.0001 meters). During this, both N2 partial pressure and ambient pressure will change.
Besides this, for the point we are discussing here, this question is not really relevant. The diver will have to ascend on EAN 60 from 10 meters to 0.1 meters (or 0.0001 meters). During this, both N2 partial pressure and ambient pressure will change.
@doctormike, let me preface my response with what you said "I'm (also) not a deco expert.." so If I'm wrong here, I hope someone corrects me!
I don't think pressure itself is really the issue. Gases move into and out of tissues by diffusion. The bigger the concentration gradient, the faster they move. What pressure does is just change the concentration gradient. You on gas at depth because you are inspiring inert gas at a higher concentration than what exists in your tissues.
So in your two thought experiments, I think the answer would be no, you would not get bubble formation. Let's make a caveat for scenario number 1. I'm assuming there is a layer of gas above the liquid and that layer of gas has a PN2 of 0.79. Now in both scenarios, even though you are changing pressures, you haven't changed the concentration gradient. So yes, you can change the pressure, but now, there is no concentration gradient for the gases to follow.
Again, if I've really missed something here, someone correct it!
I think that the persistent issue in this thread is the confusion between decompression stress (risk of bubble formation) and offgassing. Even though they are both processes related to shifts of inert gas between a state where it is in solution and a free gas, they are different processes (see the diagram that I posted).
Offgassing means that the gas is in equilibrium between tissue and blood, and then flows along a concentration (PPN2) gradient across the pulmonary alveoli and is cleared through exhaled gas. That gradient is dependent on the PPN2 of the inspired gas, so if you switch to a gas with less N2, it will happen faster.
Bubble formation results when a tissue with dissolved gas is subject to a drop in ambient pressure. Those bubbles may then be cleared by the lungs. But switching to a gas with less N2 does NOT increase this risk, otherwise it would not be safe to switch to 100% O2 for deco.
One more point. We are talking about physiological systems in human divers. I don’t know if it is possible in a lab to create such a huge partial pressure gradient that off gassing itself would cause nucleation and bubbling...
OP
Further question's for thought....
How does the off-gassing in the different compartments on this ascent compare/contrast to off-gassing on an ascent after breathing air?
How does the ascent in this dive compare to the ascent of a free diver?
How does the off-gassing in the different compartments on this ascent compare/contrast to off-gassing on an ascent after breathing air?
How does the ascent in this dive compare to the ascent of a free diver?
EFX
Contributor
In off/ongassing the tissue/blood and/or blood/lungs are NOT in equilibrium. A change in pressure across those barriers will cause the gas to flow. Gas that is flowing is not in equilibrium because the particular tissues on either side of the barrier are not static, they are increasing or decreasing in pressure. Tissues in saturation are in equilibrium even though there may be local areas where there is a constant switching in flow of gasses across barriers. Over time these average out to a constant pressure. The gradient is dependent on the PPN2 of the inspired gas and the ppN2 of the inspired gas is dependent on the ambient pressure.
This came up in the other thread. Perhaps I didn't phrase this well, equilibrium was the wrong term.
The tissues are not in contact with the alveolar membranes and gas space, but with the blood. So there will be a gradient between tissue and blood, and a gradient between blood and alveolar gas, and that gradient drives offgassing. While the slow compartments may take a while to offgas, the speed of offgassing seems to be based at least to some degree on perfusion at the tissue level. On the other hand, by putting a richer mix in the alveoli (less N2), you increase the gradient between the blood and the inspired gas and increase offgassing efficiency.
The PPN2 of the inspired gas is dependent on both the ambient pressure AND the FiO2, which is why you can increase the gradient without changing ambient pressure.
That one's easy: in a freediver on-gassing is limited by the mount of gas in their lungs, so their max. loading should be negligible in terms of decompression models. I think.
Similar threads
