Helium Fraction and Standardized Gases

[Kevrumbo]

Is your argument based on anything besides a warning in a computer program? Real experience? Or just something a computer made up?

Genuinely curious here. Despite being called a myriad of names and slurs.

Again, the simple logical means to an end --if you're trying to off-gas Nitrogen loading from your bottom mix, why are you switching to a intermediate "standardized deco gas" with significantly more Nitrogen than your bottom mix??? (i.g. 10/70 bottom mix has fN2 of 20%, or 12/60 with fN2 of 28%; and you switch to a 21/35 intermediate deco mix that has a significantly greater fN2 of 44%???)

Another article below goes on to explain and implies why this is a bad practice. The simple physical principle behind it IMHO is valid & applicable, and again begs the above question:

Decompression from an N2-based dive is longer with N2 containing deco mixes because some N2 is continuously diffusing into tissue during deco. Decompression from a He-based dive can be longer with N2 containing deco mixes because N2 is diffusing into tissue as He is diffusing out of tissue. The decompression obligation of a tissue compartment is based on the sum of gas partial pressures in the compartment. This means that if a tissue is loaded with N2 as He is being removed, its tissue has a greater decompression obligation than when no N2 is added to tissue during He off-gassing. . . The gas partial pressure gradient for movement from tissue into blood is not controlled by ambient pressure; it is controlled by the gas partial pressure in the tissue and in arterial blood. As long as the arterial [inert, non-metabolic] gas partial pressure is zero, the gradient for [inert, non-metabolic] gas removal from tissue is maximal . . .It should be intrinsically obvious that removal of a gas from tissue can be speeded by elimination of the gas from the inspired mixture. If the arterial partial pressure of a gas is zero, then no gas will diffuse into tissue while the gas is diffusing out of the tissue. . .Gas Exchange, Partial Pressure Gradients and the Oxygen Window, p.12, J.E. Brian M.D.

 

[Tortuga68]

I think it is wrong to say everyone will be bent, or that there is any such thing as an "undeserved hit"....Each of us has a profile we can stay within, given our normal physical fitness and health/hydration, etc., where we will not be bent if we stay in this... I have since 1972 of personal history saying this..not that this means much statistically--but statistics means squat to YOU as an individual.

Where were you when this thread was raging??

 

[JKPAO]

People make decisions.....computer do calculations! IJS
See you topside! John

 

[PfcAJ]

My only thought on why this works (and it clearly does, btw) is because you aren't really decompressing from the nitrogen at this stage so much that you're decompressing from the helium.

If this was bad practice, why don't we see people getting bent doing it?

 

[boulderjohn]

My only thought on why this works (and it clearly does, btw) is because you aren't really decompressing from the nitrogen at this stage so much that you're decompressing from the helium.

This was the answer AG gave to pretty much the same question during a Ratio Deco class for UTD.

 

[MXGratefulDiver]

People make decisions.....computer do calculations! IJS
See you topside! John

... and there is a significant difference between knowledge and understanding ...

... Bob (Grateful Diver)

 

[lamont]

Again, the simple logical means to an end --if you're trying to off-gas Nitrogen loading from your bottom mix, why are you switching to a intermediate "standardized deco gas" with significantly more Nitrogen than your bottom mix??? (i.g. 10/70 bottom mix has fN2 of 20%, or 12/60 with fN2 of 28%; and you switch to a 21/35 intermediate deco mix that has a significantly greater fN2 of 44%???)

Another article below goes on to explain and implies why this is a bad practice. The simple physical principle behind it IMHO is valid & applicable, and again begs the above question:

Uhm, you are stating that a rule of thumb from 1999 or earlier is valid without any proof, actually.

Run dives through Doolette's multicompartment model of the inner ear (or find an updated model more current than 2003) and show that you get significant overpressurization in 21/35, and/or work with a chamber that is treating inner ear DCS cases and work with divers to get actual model validation against the real world. If IBCD from 21/35 is so bad, you should get quite a few real world datapoints showing up in chambers.

Oh yes, but you don't have any of that, you've just got an article written in 1999...

 

[limeyx]

you win kevin, JJ promotes "best mix", GUE is a fraud, you've uncovered the big secret... game over...

Now you're just being a syllogistically logistical malingering marauder!

 

[Kevrumbo]

My only thought on why this works (and it clearly does, btw) is because you aren't really decompressing from the nitrogen at this stage so much that you're decompressing from the helium.

If this was bad practice, why don't we see people getting bent doing it?

Uhm, you are stating that a rule of thumb from 1999 or earlier is valid without any proof, actually.

Run dives through Doolette's multicompartment model of the inner ear (or find an updated model more current than 2003) and show that you get significant overpressurization in 21/35, and/or work with a chamber that is treating inner ear DCS cases and work with divers to get actual model validation against the real world. If IBCD from 21/35 is so bad, you should get quite a few real world datapoints showing up in chambers.

Oh yes, but you don't have any of that, you've just got an article written in 1999...

My only thought on why this works (and it clearly does, btw) is because you aren't really decompressing from the nitrogen at this stage so much that you're decompressing from the helium.

If this was bad practice, why don't we see people getting bent doing it?

This was the answer AG gave to pretty much the same question during a Ratio Deco class for UTD.

The fact remains that counter diffusion of Nitrogen from this "bad practice" still exists and may --or may not elicit signs & symptoms of DCS; and there is no reasonable justification for blindly following such a practice just because "we don't see people regularly getting bent from it".

Again, the simple logical means to an end --if you're trying to off-gas Nitrogen loading from your bottom mix, why are you switching to a intermediate "standardized deco gas" with significantly more Nitrogen than your bottom mix???

Intuitively, if you can eliminate possible factors that can preclude a DCS hit (even rare but always seriously acute Inner Ear DCS) wouldn't you sensibly do so?

----

Decompression from an N2-based dive is longer with N2 containing deco mixes because some N2 is continuously diffusing into tissue during deco. Decompression from a He-based dive can be longer with N2 containing deco mixes because N2 is diffusing into tissue as He is diffusing out of tissue. The decompression obligation of a tissue compartment is based on the sum of gas partial pressures in the compartment. This means that if a tissue is loaded with N2 as He is being removed, its tissue has a greater decompression obligation than when no N2 is added to tissue during He off-gassing. . . The gas partial pressure gradient for movement from tissue into blood is not controlled by ambient pressure; it is controlled by the gas partial pressure in the tissue and in arterial blood. As long as the arterial [inert, non-metabolic] gas partial pressure is zero, the gradient for [inert, non-metabolic] gas removal from tissue is maximal . . .It should be intrinsically obvious that removal of a gas from tissue can be speeded by elimination of the gas from the inspired mixture. If the arterial partial pressure of a gas is zero, then no gas will diffuse into tissue while the gas is diffusing out of the tissue. . .Gas Exchange, Partial Pressure Gradients and the Oxygen Window, p.12, J.E. Brian M.D.

 

[vjanelle]

Because you're not going to be on it long enough to significantly load nitrogen into your own compartments, while the helium is going to offgas a great deal faster than the nitrogen is loading? You've quoted two people that have said as much.