Question Shearwater GF99 vs Baker's Gradient Factors

[stuartv]

I did a bunch of tech diving last week. After one 200' deco dive in the morning and a 4 hour surface interval, I was getting ready to dive again.

I looked at my Shearwater Petrel (CCR controller) and noticed that the Tissue Bar Graph showed that my 4 (or so) slowest tissue compartments were still higher than the Inspired Inert Gas line (i.e. the black line on the Tissue Bar Graph). But, they were all lower than the Green/Yellow boundary (the Ambient Pressure line).

This indicates that my slow compartments were still off-gassing, but with not much left to go - each of those slow compartments now having less than 1 ATA tissue tension of inert gas, but more than 0.79 ATA of N2 and/or more than 0 ATA of He.

Given that, I was very surprised to note that the Petrel showed my GF99 as 0.

My understanding, in the simplest terms I can express them, is that if you are off-gassing, your GF99 is greater than zero. That seems axiomatic, to me.

I emailed Shearwater to ask them about it, as what was shown seems incorrect, to me.

They gave me an explantion, which I will reserve for the moment.

What I realized during the course of this is that what is "correct" for GF99 is to calculate it the same way the Gradient Factor algorithm does.

What Shearwater is doing is calculating GF99 as the percentage of the way that you are from 1 ATA to the M-value.

I was expecting that GF99 would be calculated as the percentage of the way from surface inspired inert gas pressure to the M-value.

I observe that the difference, when you're talking about real-world deco dives and their associated pressures, is tiny. So maybe the point is moot in any practical sense.

Nevertheless, I could go consult Baker's original Fortran implementation of Gradient Factors and figure out how it is "supposed" to be done. But, I'm being lazy and asking you all instead.

How SHOULD it be calculated?

In the end, I keep coming back to my own fundamental assumption: If I am still off-gassing (in ANY compartment), then I would expect GF99 to be > 0.

Thoughts?

 

[steinbil]

I have nothing to add to this, but I'm curious to see the Shearwater response, and also what people with a better theoretical understanding than myself would have to say.

PS.
For what it's worth (not much), intuitively, without any in depth knowledge, your assumption that the GF99 would correspond to the partial pressure of inert gas and as such correspond to the state and severity of off-gassing makes perfect sense to me.

 

[Capt Jim Wyatt]

I observe that the difference, when you're talking about real-world deco dives and their associated pressures, is tiny. So maybe the point is moot in any practical sense.

It is an academic point of only academic interest IMO.

In real world deco diving I agree with you; it is moot.

 

[JMarc]

if you are off-gassing, your GF99 is greater than zero

So you consider that your G99 depends on the gaz you are breathing? (Think about what happens when breathing pure O2)

(I agree with Shearwater, I don't have time to dig into sources now, but ping me and I'll try to find something with more credentials that a random dog on Internet)

 

[Capt Jim Wyatt]

I'll try to find something with more credentials that a random dog on Internet)

What does that mean?

 

[inquis]

While gas diffusion is driven by the partial pressure difference (tissue - inspired inert), bubble formation (or lack thereof) is driven by the difference from total pressure. As such, aligning GF 0 and ambient pressure is the more useful comparison.

ETA: Baker calls this the "% M-Value Gradient" = (Inert - Ambient) / (MValue - Ambient). Shearwater calls this "GF99".

(Microbubbles may temporarily form when inert pressure is below ambient but quickly disappear. This is much like water below the boiling point: a few molecules turn to gas while others condense back into liquid.)

 

[inquis]

In the end, I keep coming back to my own fundamental assumption: If I am still off-gassing (in ANY compartment), then I would expect GF99 to be > 0.

On this in particular... Off gassing (gas movement) continues into negative GF values. Again, the analogy is room temperature water will evaporate but no bubbles.

 

[inquis]

if you are off-gassing, your GF99 is greater than zero. That seems axiomatic, to me.

Another way to look at things: if GF99 depended on the inspired gas, a switch to a deco gas would instantly increase it.

 

[stuartv]

So you consider that your G99 depends on the gaz you are breathing? (Think about what happens when breathing pure O2)

If you're breathing O2 at 20', then the inspired inert gas is 0. The black line on the Tissue Bar Graph would be all the way to the left side of the graph.

(On O2 at 20') Ambient pressure is 1.6 ATA. Inspired Inert gass pressure is 0. So, GF99 should be the percentage of the way from 0 to the M-value (my thought going into this)? Or the percentage of the way from 1.6 to the M-value (the way Shearwater calculates it)?

And maybe this is really what explains it.

If GF99 were based on the inspired inert gas, then changing from (for example) Air to O2 at 20' would change one of the parameters of the GF99 calculation (the inspired inert gas pressure), resulting in the GF99 going way up - implying that you just significantly increased your risk of DCS.

We certainly believe that that is not the case. Switching to O2 does not suddenly increase your risk of DCS. Or so I believe.

And I would not expect my GF99 to change just because I switched breathing gases. My GF99 is my GF99 - expected to not change as long as my depth doesn't change (other than gradually going up or down based on my on- or off-gassing).

Thank you for the thought-provoking question.

(I agree with Shearwater, I don't have time to dig into sources now, but ping me and I'll try to find something with more credentials that a random dog on Internet)

I have Baker's Fortran code. I have studied it several times in the past. I was just being lazy.

 

[stuartv]

Another way to look at things: if GF99 depended on the inspired gas, a switch to a deco gas would instantly increase it.

Yep. Got it.

Thank you!