Riding GF99 instead of mandatory/safety stops

[tursiops]

IIRC, the PO2 required for meaningful vacancy is well in excess of modern safety guidelines. (I don't have my copy of Powell handy to check ATM.)

I think what you are referring to is on page 106 of Powell; namely, PPO2 needs to be 2.7 ATM.

 

[boulderjohn]

That book was written in 2008, when bubble models were more in vogue than they are now. That may have some relevance to this discussion.

Powell was a strong believer in bubble theory and VPM. When I was still teaching for TDI, the (then) new AN/DP course materials followed his lead and were all in on it. He eventually (much later than most) converted to the more current belief that bubble models tended to create first stops in the ascent profile that are too deep.

 

[dmaziuk]

More to the point, there have been lots more doppler studies now and it seems that there are always bubbles. That would suggests that "bubbles cause DCS, ergo: eliminate bubbles" assumption behind the bubble models is overly simplistic at best.

 

[old frogman]

I wanted to weigh in on this 'oxygen window' discussion. I am by no means an expert, but I have read the Mark Powell book, and here's my best understanding of what he says. That book was written in 2008, when bubble models were more in vogue than they are now. That may have some relevance to this discussion.

He says there are basically three definitions of the O2 window, and fairly late in the book he more-or-less says that the 1st and 3rd definitions are so closely related that they can be considered one. So that leaves two unrelated uses of the term:
1. Drop in total gas tension in the venous side due to the consumption of O2 and only partial replacement with C02.
2. Increased inert gas gradient due to lower inspired inert gas partial pressure.

In my ANDP class, the instructor basically referred to the 2nd definition, and I suspect that most deco divers consider this the 'important' use of the term, because clearly its the lower (or totally absent) partial pressure N2 in high FO2 deco gases that increases the off gassing of N2. The presence of large quantities of oxygen is irrelevant. This is pretty simple to understand, even for me!

With regards to the first definition, Powell clearly states that the overall lower pressure in the venous side "doesn't help us to off-gas any quicker" and "the rate of off gassing is dependent only on the individual inert gas gradient." (Those are direct quotes)

But, he does say that the lower total supersaturation pressure in the venous side does help to prevent bubble formation, because bubble formation is a product of the difference between total gas pressure and ambient pressure. "Bubble formation is different to off-gassing in that we must consider all of the gas pressures together when calculating the supersaturation ratio." (another direct quote)

Unless that 2nd definition has been refuted in more recent research, it does appear that there is some benefit to the drop in total gas pressure in the venous side of our circulatory system. It theoretically would allow us to tolerate a lower ambient pressure a bit more without exceeding M values. How much lower, and how much benefit? Beats me.

A well-educated explanation. I agree.

 

[EFX]

More to the point, there have been lots more doppler studies now and it seems that there are always bubbles. That would suggests that "bubbles cause DCS, ergo: eliminate bubbles" assumption behind the bubble models is overly simplistic at best.

Actually the VPM bubble models were designed not to eliminate bubbles (see the bolded text in your post) but to keep those ever present bubbles from exceeding a critical size.

 

[old frogman]

The gas switch, obviously.

Also, why have you moved the goal posts talking about IBCD, a concern for a tiny fraction of technical divers making 150+ m dives? Can you please stick to a scenario that is relevant to the vast majority of the populace?

The gas switch is executed at the stop. Isobaric counter diffusion occurs while under constant ambient pressure. There is no pressure gradient. The variable factor is not pressure but the different gas solubility of He and N2.

N2 is more soluble and is less susceptible to coming out of solution to form bubbles.

He is less soluble and is more susceptible to coming out of solution to form bubbles.

During ascent from the bottom the fast tissue will be off gassing. He which is the lesser soluble gas in the trimix will come out of solution. It will probably form some asymptomatic bubbles. This is due to the pressure difference in the tissues and the decreasing ambient pressure during ascent.

At the gas switch stop, if much a higher N2 trimix is introduced, the N2, due to its higher solubility, will dissolve into the blood and tissues very rapidly. The combined He and N2 gas tension will increase due to the increased solubility of N2 (as there is no pressure change at the stop).

The increase in gas tension in the tissue and blood will cause the He (due to its lower solubility) to come out of solution to form bubbles.

It is currently believed that the symptomatic bubbles travel to the cerebellum (located in the brain stem). The cerebellum is the body's computer which controls bodily functions. The function that appears to be upset by the bubbles is the balance system in the inner ear. The symptoms are vertigo and associated vomiting.

 

[old frogman]

The gas switch, obviously.

Also, why have you moved the goal posts talking about IBCD, a concern for a tiny fraction of technical divers making 150+ m dives? Can you please stick to a scenario that is relevant to the vast majority of the populace?

Isobaric counter diffusion (ICD) may occur in dives shallower than 100 meters. Depth is not the issue. Switching from helium-based gas to nitrogen-based gas. If the percentage of nitrogen is too high ICD may occur resulting in a vestibular DCS hit.

ICD is a phenomenon where two different gases diffuse in opposite directions under, wait for it, constant ambient pressure.

 

[halocline]

A well-educated explanation. I agree.

You do understand that I was not attempting to validate the efficacy of the drop in venous gas pressure as a decompression benefit, I was only explaining what Powell said in his book in 2008.

 

[rjack321]

Isobaric counter diffusion (ICD) may occur in dives shallower than 100 meters. Depth is not the issue. Switching from helium-based gas to nitrogen-based gas. If the percentage of nitrogen is too high ICD may occur resulting in a vestibular DCS hit.

ICD is a phenomenon where two different gases diffuse in opposite directions under, wait for it, constant ambient pressure.

Are you actually doing 100m dives with multiple gas switches? Because this doesn't happen with modern gas choices and reasonable ENDs. For all practical purposes IBCD is limited to diving bells where the diver is immersed in a helium mix - despite what multideco might "warn" you about.