Using GF99 and SurGF

[dmaziuk]

Thank you for your input, you've spent a lot more time staring at GF things than I have. If I understand the math correctly, it should be impossible for that case to happen. Because two different exponential functions can only cross at one point, and the faster tissues will always be ahead of the slower tissues. Of course, you'll be able to find many examples of the opposite, where first a faster tissue controls, and then a slower tissue controls.

At a stop a faster TC can off-gas "enough" while slower TC remains close to its M-value. Go up and your slower TC is controlling, until you get high enough to push the faster one closer to its M-value. I don't know what it'd take in practice for a given model, but by numbers it's perfectly fine for them to switch back and forth during ascent.

 

[Brett Hatch]

At a stop a faster TC can off-gas "enough" while slower TC remains close to its M-value. Go up and your slower TC is controlling, until you get high enough to push the faster one closer to its M-value. I don't know what it'd take in practice for a given model, but by numbers it's perfectly fine for them to switch back and forth during ascent.

I definitely agree that in the case of an ascending diver, which compartment controls will vary. In this example, though, the article is comparing the SurfGF at the beginning of the 5m safety stop, versus the SurfGF after 3 minutes at 5m. In that situation, do you agree that this should be mathematically impossible? If I'm wrong there, then I must misunderstand the model, and would be happy to hear what I have missed.

 

[L13]

Another note:

If you do a deco dive, GF99 will be ~= GF Low (probably slightly lower) as you arrive at your first deco stop. GF99 will then drop till the end of that stop, then go back up as you ascend to the next stop.

As you arrive at each successive deco stop after that GF99 ~=(but lower than) an interpolated value between GF Low and GF High.

 

[dmaziuk]

I'm not sure it's even possible on a no-stop dive, actually, -- safety stop or not -- since fast TC controls the first stop, and on a no-stop that's the surface.

 

[EFX]

I would like to offer some information that may help our understanding of GF99 and SurGF.

GF99 = (p_ctc - p_amb) / (p_mv - p_amb) x 100
SurGF = (p_ctc - sp) / (p_mv - sp) x 100

where:
GF99 = % of the m_value at the current depth as measured from ambient pressure.
SurGF = % of the m_value at the current depth as measured from the surface pressure.
p_ctc = pressure in the controlling tissue compartment.
p_amb = ambient pressure.
p_mv = m_value pressure at the current depth.
sp = surface pressure.

The definition as given by Shearwater and mentioned in the article sited at the beginning of this thread is that SurGF is the value of GF if you could instantaneously surface from your current depth. The key word here is instantaneously. Obviously, you can't surface instantaneously so the value you read at depth is not the GF you'll see when surfacing. As you ascend your tissues will be off gassing which will reduce your SurGF when you eventually get to the surface. So, it represents a worst case value of the controlling tissue compartment (CTC) at the depth and time you look at it. As you ascend it will slowly decrease because the pressure in the CTC is off gassing and therefore the numerator in the equation above is decreasing faster than the base (p_mv - sp). SurGF should always be higher than GF99 because the sp is always lower than the ambient pressure during the dive.

What I wrote in the above quote in bold is not exactly true. Unfortunately, I was going by memory. When I ran my spreadsheet and put in a dive profile, the result was SurGF was increasing but at a slower rate than GF99. Both GF99 and SurGF are converging as the diver ascends. At the surface p_amb in GF99 becomes the surface pressure and both GF99 and SurGF must give the same answer. You can review my original post #24 in this thread.