Nitrox course. What's the point?

[Angelo Farina]

Yes a timer would help. Or, set the computer for 100% O2, immerse it in 3 ft of water, then breath your O2. The computer will start a new dive but at least it will calculate new tissue loadings while you are on surface O2. The added pressure from the water shouldn't make any difference.

I do not know how the Shearwater models the real world...
In the real world, increasing the ambient pressure while breathing pure oxygen "pushes away" the Nitrogen form your tissues much faster. This is the principle of hyperbaric oxygen treatment for DCS...
So, if the Shearwater models reality correctly, keeping the computer at some depth will cause it evaluating faster offgassing than what is really happening (as in reality you are not at such ambient pressure, you are still at atmospheric pressure).

 

[Ryan Neely]

I love this thread. You guys are awesome.

 

[DiveClimbRide]

I do not know how the Shearwater models the real world...
In the real world, increasing the ambient pressure while breathing pure oxygen "pushes away" the Nitrogen form your tissues much faster. This is the principle of hyperbaric oxygen treatment for DCS...
So, if the Shearwater models reality correctly, keeping the computer at some depth will cause it evaluating faster offgassing than what is really happening (as in reality you are not at such ambient pressure, you are still at atmospheric pressure).

Correct me if I'm wrong but I'm pretty sure that increasing the ambient pressure while breathing oxygen does NOT increase the rate of nitrogen off gassing because what we care about is the nitrogen partial pressure and that is 0 while breathing oxygen regardless of depth. This is also why some people choose to do a 6m final stop on 100% instead of a 3m stop because the time is the same but you're further underwater and less affected by the swell.
Hyperbaric oxygen treatment increases the depth to crush or recompress any bubbles in your body.

 

[Angelo Farina]

Correct me if I'm wrong but I'm pretty sure that increasing the ambient pressure while breathing oxygen does NOT increase the rate of nitrogen off gassing because what we care about is the nitrogen partial pressure and that is 0 while breathing oxygen regardless of depth. This is also why some people choose to do a 6m final stop on 100% instead of a 3m stop because the time is the same but you're further underwater and less affected by the swell.
Hyperbaric oxygen treatment increases the depth to crush or recompress any bubbles in your body.

I have no idea how the computer evaluate what's happening, but I can ensure you that in the real world, where physics laws operate, the speed of nitrogen leaving a tissue is proportional to the difference between the partial pressure of the nitrogen trapped in the tissue and the partial pressure of nitrogen in your lungs (which is zero, if breathing 100% oxygen).
That's the Fick's law, the law describing diffusion of a gas through solids or stationary liquids.
Of course your blood is NOT a stationary liquid, so even the Fick's law is not a complete and exhaustive model of what's happening in your body.
The partial pressure of Nitrogen in the tissue is a percentage of the ambient pressure. If the tissue is saturated at, say 30%, it means that the p.p. of Nitrogen in that tissue will be 0.3 times the ambient pressure. The higher the ambient pressure, the higher the partial pressure of Nitrogen in the tissue, the faster it will move away from it.
That is the reason for which, when doing deco in pure oxygen, it is actually advantageous to stay at 6m for the whole time, as degassing is faster than at 3m.
Be aware that Nitrogen also moves from one tissue to another one, if they have different p.p.. This is another thing which the computer is not modelling...

 

[Pressurehead]

Correct me if I'm wrong but I'm pretty sure that increasing the ambient pressure while breathing oxygen does NOT increase the rate of nitrogen off gassing because what we care about is the nitrogen partial pressure and that is 0 while breathing oxygen regardless of depth. This is also why some people choose to do a 6m final stop on 100% instead of a 3m stop because the time is the same but you're further underwater and less affected by the swell.
Hyperbaric oxygen treatment increases the depth to crush or recompress any bubbles in your body.

Off topic, sorry, I was not going to post on this topic. but can't help myself.
I have been involved in a rescue of a diver with oxygen toxicity [took two of us] , he was using a
Dräger CCR unit and was only at 6m on 100% [used for tactical diving ] and well above the 10m , but after what happened to this diver [Malaysian Navy diver under training] I know and have seen oxygen toxicity first hand and treat it with more respect and caution, I have done stops at 3m on 100% no problem, and the times I was required to Dive at 6m 100% that vision of the convulsions that diver had come "front of mind", big wake up call for me [young and dumb and....]

 

[mc42]

Correct me if I'm wrong but I'm pretty sure that increasing the ambient pressure while breathing oxygen does NOT increase the rate of nitrogen off gassing because what we care about is the nitrogen partial pressure and that is 0 while breathing oxygen regardless of depth. This is also why some people choose to do a 6m final stop on 100% instead of a 3m stop because the time is the same but you're further underwater and less affected by the swell.
Hyperbaric oxygen treatment increases the depth to crush or recompress any bubbles in your body.

I believe you're correct there on both counts. The inert gas gradient is already maximized when breathing O2 so added pressure from depth doesn't benefit the offgassing much, other than helping reduce bubbles.

I have no idea how the computer evaluate what's happening, but I can ensure you that in the real world, where physics laws operate, the speed of nitrogen leaving a tissue is proportional to the difference between the partial pressure of the nitrogen trapped in the tissue and the partial pressure of nitrogen in your lungs (which is zero, if breathing 100% oxygen).
That's the Fick's law, the law describing diffusion of a gas through solids or stationary liquids.
Of course your blood is NOT a stationary liquid, so even the Fick's law is not a complete and exhaustive model of what's happening in your body.
The partial pressure of Nitrogen in the tissue is a percentage of the ambient pressure. If the tissue is saturated at, say 30%, it means that the p.p. of Nitrogen in that tissue will be 0.3 times the ambient pressure. The higher the ambient pressure, the higher the partial pressure of Nitrogen in the tissue, the faster it will move away from it.
That is the reason for which, when doing deco in pure oxygen, it is actually advantageous to stay at 6m for the whole time, as degassing is faster than at 3m.
Be aware that Nitrogen also moves from one tissue to another one, if they have different p.p.. This is another thing which the computer is not modelling...

Angelo - I learned that offgassing is proportional to the difference in the tissue inert gas tension and inspired inert gas tension. Your ambient pressure doesn't affect the tissue gas tension (i.e. you don't multiply by the pressure). The tissue gas tension is just a pressure value representing the actual pressure of gas absorbed inside that tissue - that is the basis of most decompression models.

You would only multiply by ambient pressure just to determine the partial pressures of a gas when you know its fraction (e.g. 79% N2 at 2 atm = 1.58 atm), but that is not needed in this case.

Because tissue tension is what it is, just a pressure value (e.g. 2.3 atm), then in the special case of breathing pure O2, where the inert gas tension is zero atm regardless of depth, then the rate of offgassing is the same regardless of depth.

There are other biological factors, such as elevated O2 PP being a vasoconstrictor, so the rate of offgassing could be in reality slower at 6m than at 3m.

 

[ginti]

I believe you're correct there on both counts. The inert gas gradient is already maximized when breathing O2 so added pressure from depth doesn't benefit the offgassing much, other than helping reduce bubbles.



Angelo - I learned that offgassing is proportional to the difference in the tissue inert gas tension and inspired inert gas tension. Your ambient pressure doesn't affect the tissue gas tension (i.e. you don't multiply by the pressure). The tissue gas tension is just a pressure value representing the actual pressure of gas absorbed inside that tissue - that is the basis of most decompression models.

You would only multiply by ambient pressure just to determine the partial pressures of a gas when you know its fraction (e.g. 79% N2 at 2 atm = 1.58 atm), but that is not needed in this case.

Because tissue tension is what it is, just a pressure value (e.g. 2.3 atm), then in the special case of breathing pure O2, where the inert gas tension is zero atm regardless of depth, then the rate of offgassing is the same regardless of depth.

There are other biological factors, such as elevated O2 PP being a vasoconstrictor, so the rate of offgassing could be in reality slower at 6m than at 3m.

I am interested in understanding a bit more about these physiological factors. Where did you study them? Can you suggest me a book or a reference? Thanks

 

[mc42]

I am interested in understanding a bit more about these physiological factors. Where did you study them? Can you suggest me a book or a reference? Thanks

Start with Mark Powell’s book, Deco for Divers. He provides understanding beyond beginner level without getting into the scientific paper level so it’s probably the most accessible book on just about everything you wanted to know about decompression.

There are various PDFs, dive computer code, scientific papers and presentations on the internet by people like Erik Baker, David Doolette, Simon Mitchell, among others that I’ve found useful to supplement my understanding.

 

[ginti]

Start with Mark Powell’s book, Deco for Divers. He provides understanding beyond beginner level without getting into the scientific paper level so it’s probably the most accessible book on just about everything you wanted to know about decompression.

There are various PDFs, dive computer code, scientific papers and presentations on the internet by people like Erik Baker, David Doolette, Simon Mitchell, among others that I’ve found useful to supplement my understanding.

I read already Deco for divers, and I am now interested in more details about physiology. I'll try to look for some pdf material, thanks!

 

[chillyinCanada]

I have said this numerous times through the thread.

In simplistic terms, you can either:

• dive Nitrox as if it where air and get a safety benefit

OR

• dive Nitrox as Nitrox and gain extra in water time. And get no safety benefit [1]

Addendum
[1] Technically option 2, is not strictly true. As was explained by the DDRC medical team. Having a bend on Nitrox, compared to a bend on Air is far better for you. The prognosis for a full recovery is far better if you where using Nitrox in preference to air. There was a huge row about this when the medical team stood on up and said this at a BSAC conference, because the BSAC, at the time, still band the use of Nitrox for recreational diving.

That must have been quite some time ago?