Riding GF99 instead of mandatory/safety stops

[old frogman]

Except your whole post is inaccurate. Gasses are independent of each other. The metabolism of O2 does not "make space" for anything else to offgas. Nor does the solubility of CO2 help or hinder the offgassing of nitrogen.

As I said, human physiology is a complex process.

 

[boulderjohn]

As I said, human physiology is a complex process.

I have no idea what this means.

 

[Nick_Radov]

As I said, human physiology is a complex process.

OK? But this is more like basic physics. Is there some reason to think that gas laws don't apply in divers' bodies?

Regardless of the theoretical basis (or lack thereof) for the "oxygen window", is there any empirical or experimental evidence that applying it produces better decompression results? I confess that when I started tech diving 20+ years ago I followed the guidance of modifying deco profiles to be more S-shaped and sit on gas switches for extra time beyond what the Bühlmann decompression model would indicate. But that doesn't seem to matter in practice.

 

[old frogman]

I have no idea what this means.

And there is your challenge. Why is it so?

If you believe that the gas tension, diffusion, solubility and perfusion in blood is not related to decompression and DCS then you are going to have a problem explaining the phenomenon of isobaric counter diffusion (ICD).

This is where divers sometimes get bent when switching from an He/N2/O2 mix to high N2/O2 mix during a stop on ascent from a deep dive. DCS results without a change in ambient pressure (it occurs during a stop).

 

[inquis]

DCS results without a change in ambient pressure (it occurs during a stop).

Not hard to explain at all. IBCD is caused by... wait for it ... the partial pressure gradients of the inert gases. Whether you add more gas or drop the level of tolerated tension matters not. Excessive super saturation is going to ruin your day.

 

[old frogman]

Not hard to explain at all. IBCD is caused by... wait for it ... the partial pressure gradients of the inert gases. Whether you add more gas or drop the level of tolerated tension matters not. Excessive super saturation is going to ruin your day

 

[old frogman]

And what causes the pressure gradient?

 

[inquis]

And what causes the pressure gradient?

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?

 

[halocline]

That is not enough. It is the pressure in the individual gases that counts, otherwise no graident for that gas to do any movement. You can drop the O2 or the CO2 or whatever as much as you want, but unless the N2 pressure in the tissues/blood drops, there is no pressure gradient to move N2.

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.

 

[inquis]

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?

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.)