Riding GF99 instead of mandatory/safety stops

[inquis]

The % of oxygen in the breathing gas is what drives the oxygen window washout of nitrogen from the tissues

No, this is flat out wrong. The authors supporting the O2 window specifically state inert gas transport is driven by the inspired inert gas partial pressure. That means the benefit gained is a FN2 of 60 (for Nx40) vs 79% -- a relative increase of 24% faster N2 reduction. Your choice to do that, but it's not without risk, as others have mentioned.

Further to the inert gas washout (using He as an example, but N2 acts identically), E. Brian (an O2 window proponent) says this in his 2007 article:

The reality is that at any given ambient pressure, regardless of the size of the oxygen window, as long as there is no inspired He, the rate of He off-gassing will be unchanged. [emphasis mine]

The O2 window does NOT impact off-gassing rate. It is partial pressure that dictates that. Brian even states this directly:

Gas in solution moves by diffusion from an area of higher partial pressure to an area of lower partial pressure.

I don't know how it could be any more clear.

 

[old frogman]

They only allow you to adjust the GFhi because once you've started the deco the GFlow is in the past.

And for the same reason they show you surfGF and GF99. So you can add time and surface with a lower GF. If you set your GFhi to 85 at the surface, have a very intense dive, you can edit it while you wait on your deco to GFhi 80 and surface more conservatively. Or looks at GF99 and wait until its below 80. Resetting it lower just allows you to see how much longer you need to hang

Thank you. I agree.

 

[old frogman]

Here is a thread from a decade ago on ScubaBoard. It has about 1,300 posts, including many from some of the top experts in the world. In it, you will find people making your same argument. Rather than deal with it myself here, I will leave it to the experts who refuted them then to refute it now.

D

Thread 'Deep Stops Increases DCS'

Dec 8, 2014

The study below from the Navy Experimental Diving Unit indicates that Deep Stops increases incidence of DCS. I've been taught Deep Stops in my Tec Training and use Min Deco on non Tec dives on air. Should we rethink the practice of deep stops? See link below:

http://archive.rubicon-foundation.o...23456789/10269/NEDU_TR_2011-06.pdf?sequence=1

• Divetech99
• Replies: 1,301
• Forum: Technical Diving

• 

Thanks for the thread. I will look out for the experts' comments.

While the NEDU experiment was for Navy application, what I take from it, is that both the Navy tables and VPM resulted in DCS cases (the disparity between the results does not phase me). What NEDU demonstrated to me, is that if tables and computers are not used conservatively, regardless of formula/algorithm on which they are based, decompression stress or DCS is likely to occur.

 

[Blackcrusader]

If you have access to 40% why dive 21%?

40% MOD 25m yes?

 

[old frogman]

No, this is flat out wrong. The authors supporting the O2 window specifically state inert gas transport is driven by the inspired inert gas partial pressure. That means the benefit gained is a FN2 of 60 (for Nx40) vs 79% -- a relative increase of 24% faster N2 reduction. Your choice to do that, but it's not without risk, as others have mentioned.

Further to the inert gas washout (using He as an example, but N2 acts identically), E. Brian (an O2 window proponent) says this in his 2007 article:

The O2 window does NOT impact off-gassing rate. It is partial pressure that dictates that. Brian even states this directly:

I don't know how it could be any more clear.

No one has actually explained the oxygen window concept. I will give it a crack based on John Lipmann and Simon Mitchell's book "Deeper into Diving". I will paraphrase from memory. When O2 in a mixture is metabolised in the tissues CO2 is produced. CO2 being about 20 times more soluble in venous blood than oxygen, the CO2 tension (pressure in Blood) will be much lower, essentially creating space for excess nitrogen during off-gassing. This contributes to the ability of the body to tolerate some excess nitrogen tension.

Simply put, as oxygen is metabolised space is created in the venous system to accommodate more nitrogen.

However, nitrogen off-gassing as stated by others is created by pressure differential between the ambient water pressure during ascent and the nitrogen gas tension in the tissues (which include blood).

Please correct me if I am wrong.

 

[grantmac]

40% MOD 25m yes?

Depends on what PPO2 you are targeting. But if you have 40% available then you can easily make 32%.

 

[Blackcrusader]

Depends on what PPO2 you are targeting. But if you have 40% available then you can easily make 32%.

Not assuming but I do not know anyone not using 1.4 Do some people use 1.6 yes

 

[boulderjohn]

No one has actually explained the oxygen window concept. I will give it a crack based on John Lipmann and Simon Mitchell's book "Deeper into Diving". I will paraphrase from memory. When O2 in a mixture is metabolised in the tissues CO2 is produced. CO2 being about 20 times more soluble in venous blood than oxygen, the CO2 tension (pressure in Blood) will be much lower, essentially creating space for excess nitrogen during off-gassing. This contributes to the ability of the body to tolerate some excess nitrogen tension.

Simply put, as oxygen is metabolised space is created in the venous system to accommodate more nitrogen.

However, nitrogen off-gassing as stated by others is created by pressure differential between the ambient water pressure during ascent and the nitrogen gas tension in the tissues (which include blood).

Please correct me if I am wrong.

This is exactly what I wrote about earlier. That theory is a violation of Dalton's Law, as Mark Powell wrote in Deco for Divers. The theory was the basis for the GUE and UTD S-curve ascent profiles. Both agencies later determined that the science behind it was not valid.

I read the paper on which it was based about 15 years ago. The theory comes out in one paragraph, with the claim unsupported in any way by the data as far as I could see. I was then with UTD, and it led to a continuing argument between me and UTD owner Andrew Georgitsis. I don't know if my arguments were instrumental, but Georgitsis did eventually agree that the science was not valid.

 

[tursiops]

This is exactly what I wrote about earlier. That theory is a violation of Dalton's Law, as Mark Powell wrote in Deco for Divers. The theory was the basis for the GUE and UTD S-curve ascent profiles. Both agencies later determined that the science behind it was not valid.

I read the paper on which it was based about 15 years ago. The theory comes out in one paragraph, with the claim unsupported in any way by the data as far as I could see. I was then with UTD, and it led to a continuing argument between me and UTD owner Andrew Georgitsis. I don't know if my arguments were instrumental, but Georgitsis did eventually agree that the science was not valid.

The theory argued that space was made available for more off-gassing by O2 metabolizing....but failed to provide a mechanism to fill that space -- which is Dalton's Law and is inapplicable without the N2 gradients. and if you have the N2 gradients, you don't need the extra "space."

 

[old frogman]

The theory argued that space was made available for more off-gassing by O2 metabolizing....but failed to provide a mechanism to fill that space -- which is Dalton's Law and is inapplicable without the N2 gradients. and if you have the N2 gradients, you don't need the extra "space."

My understanding of the oxygen window is that it is an isobaric phenomenon based on Henry's Law. which relates to the solubility of gases into liquids.

Isobaric phenomenon also explains isobaric counter diffusion (a different phenomenon) encountered in gas switch from HE mixes to N2 mixes during ascent from deep dives.

Dalton's Law relates to the partial pressure of gases in a mixture and only partly explain the O2 window. You need to view the two laws together plus Boyles Law which comes into effect when you ascend.