Question Shearwater GF99 vs Baker's Gradient Factors

[stuartv]

For the sake of completeness, here is what Shearwater said. This was only after 2 preliminary rounds of email back and forth where their response seemed predicated on an assumption that I had OW-level understanding of deco.

The gf99 is not designed to simply show supersaturation, however, it is designed to show tissue inert supersaturation above ambient, total pressure as the risk of DCS only occurs when tissue tension is above ambient pressure. Therefore, the gradient is defined as 0% at ambient pressure, or approximately 1 ATA at sea level. What you are observing is the inert gas pressure fall further towards equilibrium (approximately 0.79). Current models have shown that tissue pressure is not relevant until above ambient pressure and is not represented by the GF99 value.

I didn't find that completely satisfactory because it makes it sound (to me) like they were just choosing to ignore the off-gassing between 0.79 and 1.0, because it doesn't pose a risk of DCS.

NOW I understand why it works like it does.

 

[inquis]

@stuartv, your posts got me wondering just how negative the GF values get. Based on this, it's little surprise that it takes a LOT for a slower tissue to become the controller. In case you're interested, here's the "clean" picture (at equilibrium at sea level) with all tissues at 0.79 atm:

 

[JMarc]

What does that mean?

Nothing essential, just a silly allusion to this cartoon.

 

[stuartv]

Nothing essential, just a silly allusion to this cartoon.

Subtle allusions to the New Yorker? In this crowd?!?

 

[atdotde]

Of course, it's up to you how you define things. But imagine breathing from an O2 cylinder rather than from ambient air. Should that increase your GF99? If you compare to breathing gas partial pressure (and not from total ambient pressure) it would. These are just words. But you can also look at it value to predict DCS: I would be very surprised to learn that somebody triggered a DCS hit by breathing O2 instead of air at the surface (I would expect rather the opposite). That supports measuring relative to ambient pressure and not partial pressure.

 

[dylanfromwinnipeg]

The M-Value calculations don't consider inspired gas partial pressures, only total ambient pressures. And since GF's are %'s of M-Value they also only consider total ambient pressure.

A little thought experiment I like to think about is if you had your computer set to GF 0/0, would you be able to ever safely ascend, or would you have to stay at depth forever. And the answer is you could ascend (just very very slowly). Because you can still be off-gassing but below a GF of 0 (when the tissue is above the inspired gas partial pressure but below total ambient).

 

[EFX]

The M-Value calculations don't consider inspired gas partial pressures, only total ambient pressures. And since GF's are %'s of M-Value they also only consider total ambient pressure.

Unless I'm misunderstanding you you seem to suggest something opposed to what Erik Baker PE said. In his paper "Understanding M-values" he said, "This included M-values which expressed a linear relationship between ambient pressure and tolerated inert gas pressure in the hypothetical tissue compartments." The tolerated inert gas pressure is a partial pressure of the total gas content. Can you elaborate further?

 

[Wibble]

Wonder what the SurfGF would be from a 60m/200ft dive some 4 hours later? The main challenge is it's not available post dive (or is it?!?).

Must have a look at this after surfacing.

 

[dylanfromwinnipeg]

Unless I'm misunderstanding you you seem to suggest something opposed to what Erik Baker PE said. In his paper "Understanding M-values" he said, "This included M-values which expressed a linear relationship between ambient pressure and tolerated inert gas pressure in the hypothetical tissue compartments." The tolerated inert gas pressure is a partial pressure of the total gas content. Can you elaborate further?

There's 3 numbers in this conversation:
1. Inert gas partial pressure in the tissue
2. Inert gas partial pressure in the lungs/breathing-gas (aka inspired gas partial pressure)
3. Total ambient pressure

For M-values (and GFs) you only use #1 and #3 to calculate it.
For rate of off-gassing/on-gassing you use #1 and #2

 

[EFX]

Wonder what the SurfGF would be from a 60m/200ft dive some 4 hours later? The main challenge is it's not available post dive (or is it?!?).

Must have a look at this after surfacing.

On the Perdix SurfGF is displayed post dive and counts down to zero as the leading TC off gasses. It might be displayed as GF99. I don't remember exactly. Whatever is displayed, SurfGF is equal to GF99 on the surface. I don't know what data and how long it gets logged once on the surface since I don't look at it.