Gradient Factors Question

[lamont]

Just so that I am understanding this correctly... Since the GF_lo value is applied to your first stop depth that means that different dives trace out different allowed supersaturation gradients for the same parameter (e.g. 20/85).

So for a dive with a first stop of 100 fsw you would get a slope of 65 / 100 = 0.65 %/fsw and for a dive with a first stop of 150 fsw you would get a slope of 65 / 150 = 0.43 %/fsw. Or on your 70 fsw stop, you tolerate an oversaturation of 40% on the dive with the 100 fsw first stop, but on the dive with the 150 fsw first stop, you tolerate an oversaturation of 55% on the 70 fsw stop.

tabularly:

depth / GF with 100 fsw first stop / GF with 150 fsw first stop:
0 85 85
10 78 80
20 72 76
30 65 72
40 59 67
50 52 63
60 46 59
70 39 54
80 33 50
90 26 46
100 20 42
110 13 37
120 7 33
130 0 29
140 -6 24
150 -12 20

Correct? If so it seems like 20/85 generates a very different deco for Tech2 dives than it does for Tech1 dives. Coming up from a Tech2 dive you are claiming that at 70 feet you tolerate a larger overpressurization gradient at the 70 foot stop across all compartments than you do on a Tech 1 level dive. That strikes me as odd.

Generally it strikes me as odd that the formula for the overpressurization allowed in any compartment at any depth knows something about the profile that you followed to get there. That seems like it requires the inert gas in your tissues to not only care about the measure of the tissue tension, but also carries a 'memory' of *how* it got there -- and why would a helium or nitrogen molecule remember that it was dissolved at 150 fsw or 200 fsw?

 

[jjones9527]

I think the difference would be due to the fact that you are in different parts of curve for the same depth. During a tech 1 dive at 70ft you are very early in the deco, and closer to that 30% GF, however with a tech 2 dive at 70ft you are farther on in the deco, and starting to approach the 85% GF. I'm not 100% but thats how I'm seeing it.

Looking again it makes perfect sense. We are moving from 30% M value of the controlling compartment to 85% M value for controlling compartment through the deco. Starting the deco at 150ft spreads that curve out over a larger depth than starting at 100ft. As we ascend we are pushing the gradient closer and closer to the 85%. Coming from 150ft to 70ft we are more than half way along the curve, where as going from 100ft to 70ft, we are just over a quarter way along. Also at the same depth the controlling compartment for the 2 dives are probably quite different.

 

[rjack321]

If you look at the "curves" in decoplanner you'll see a different line is furthest from the eqipotential pressure line at a given depth (depending on how you got there).

But really who cares about gradient factors anyway??

 

[limeyx]

I think my head just exploded!

 

[Bismark]

Stuff is starting to bubble and starting to come out my ears so time to plug in guitar hero on the xbox again.........

 

[lamont]

If you look at the "curves" in decoplanner you'll see a different line is furthest from the eqipotential pressure line at a given depth (depending
on how you got there).

Sure, it'll be a different compartment, but why not explicitly design a model then where the controlling compartment for the "tech1" compartment at 70 feet tolerates 39% of the m-value while the "tech2" compartment at 70 feet tolerates 54% of the m-value? You can probably generate a longer dive at shallower depths where you're using that compartment to control the decompression again at 70 feet but since you're diving shallower you are allowed a much smaller overpressurization gradient than you would on the same compartment coming up from a tech2 dive...

 

[rjack321]

Sure, it'll be a different compartment, but why not explicitly design a model then where the controlling compartment for the "tech1" compartment at 70 feet tolerates 39% of the m-value while the "tech2" compartment at 70 feet tolerates 54% of the m-value? You can probably generate a longer dive at shallower depths where you're using that compartment to control the decompression again at 70 feet but since you're diving shallower you are allowed a much smaller overpressurization gradient than you would on the same compartment coming up from a tech2 dive...

But GF low is just a kludge forcing everything deeper. Its not that one dive is "allowing" a certain M-value and another is not. I haven't played with decoplanner in awhile but do you get the same issues when running 100/100 for the 2 different dives?

 

[lamont]

But GF low is just a kludge forcing everything deeper.

yup.

Its not that one dive is "allowing" a certain M-value and another is not.

but that is a side-effect of the way the math is done.

I haven't played with decoplanner in awhile but do you get the same issues when running 100/100 for the 2 different dives?

Well, I really need to bite it and buy decoplanner. I've just been writing my own zhl16b+GF dive analyzer so that i could track compartment loading on profiles i pull off a reefnet sensus. I don't actually have code that generates deco profiles (yet), so I'm just trying to clarify my understanding of GF.

I'll try to see if I can generate some profiles which show different GFs for identical limiting compartments at the same depth if I get some free time...

 

[jjones9527]

I'm not sure if we are allowed to link to outside sides, but this is a good article I found from a google search that you may get some info from

http://www.rebreatherworld.com/general-new-rebreather-articles/5037-gradient-factors-dummies.html

 

[rjack321]

I'm not sure if we are allowed to link to outside sides, but this is a good article I found from a google search that you may get some info from

Gradient Factors for Dummies - Rebreather World Forums

Actually that link explained it better than I ever could.

If you interpolate GF30 to GF85 the overpressured controlling compartment will change as you move from stop to stop. And say compartment 7 may have slightly different overpressures (assuming its flopping around and not actually controlling) depending on what you did to arrive at a given stop.