My understanding is that ambient pressure is total pressure for you inspired gas. If you have not ascended yet, your GF will be somewhere < 0%. GF0 is where tissue inert GAS = ambient. Assuming you didn't switch gas, if your GF = 0 then your tissue inert gas pressure must be > inspired inert gas pressure. Either that or you don't have any O2 in you mix.
I’m pretty sure that you don’t have a gradient or offgas at GF=0, by definition. Let me go over what I think is happening here…
While I think that I understand decompression fairly well, the commonly cited graph that we are discussing here really needs some clarification for me, even after reading Baker’s paper on
Understanding M-Values. I’m coming to the conclusion that there may be a graphic design issue here.
I think that the problem is confusing labeling. In order to have the “ambient pressure line” have a slope of 1.0, you need to make the Y-axis the tissue PPN2, (tPPN2, which is how it is usually labeled) and the X-axis to be the PPN2 in the inspired gas (gPPN2). But the X-axis is commonly labeled “ambient pressure”. If it was really the total ambient pressure in your breathing gas (which is what the label implies), then the slope would vary dependent on the mix, and would only have a slope of 1.0 if you were breathing 100% N2. Yes, I know that it’s really the total partial pressure of inert gas, including helium or anything else, I am just using PPN2 for clarity.
So the choice would be to have a separate chart or line for every breathing gas, or to just normalize it by labeling the x-axis as the actual relevant variable - gPPN2. When tPPN2 = gPPN2 for the controlling compartment, that compartment is fully saturated according to Henry’s law. When you ascend, gPPN2 drops instantly, temporarily becoming lower than tPPN2, resulting in supersaturation of the controlling tissue compartment (GF>0), an inert gas gradient, and off gassing.
If you are on the ambient pressure line, that means that your controlling compartment tissue PPN2 is equal to the PPN2 in your breathing gas (i.e. saturation), assuming that the x-axis is correctly labeled. If you haven't ascended, and if your leading compartment is fully saturated, then your GF=0 (GF is defined as a percentage of the distance between saturation and the M-value for that depth). If your GF>0 you will be offfgassing (supersaturation). If your GF<0 (have never heard that term, don't know if anyone uses the concept of negative gradient factors but I see what you mean), then you would still be ongassing.
The ambient pressure line is clearly GF=0, since it is labeled as such at both ends (it intersects the GFLo and GFHi bars at 0%). On that diagram, decompression and offgassing only occurs above the ambient pressure line. Excerpted from the
DiveRite article:
"...ascent and decompression occurs between the M-value line and Ambient Pressure line....The Gradient Factor defines the amount of inert gas supersaturation in leading tissue compartment. Thus, GF 0% means that there is no supersaturation occurring and inert gas partial pressure equals ambient pressure in leading compartment"
And thinking about it, that last sentence could be a little clearer. Maybe ".. inert gas partial pressure in the leading compartment equals inert gas partial pressure in the inspired gas"..?
Or maybe I'm missing something...