Why 2 gradient factors ?

[L13]

@lowwall is right about the definitions of GF Low and GF High in Baker's paper, and in the implementation by most dive computers and planners. But, @inquisit is right that the basic mathematical model proposed by Baker is perfectly able to handle a GF Deep higher than GF Surface definition of 2 GFs, though the High and Low names no longer make sense. The model can even handle GFs greater than 100.

GF Low << GF High was proposed By Baker based on the idea (from bubble model/deep stop hypothesis) that preventing bubbles in fast tissues early in deco was more important.
EDIT: This choice of GFs is no longer considered consistent with modern evidence, but the core model is still considered the best so far.

The modern evidence seems to indicate that GF Low ~= 0.83 GF High is probably a better choice.

If we hypothesized that preventing ongassing by slow tissues was more important than preventing bubbles in fast tissues, then GF Deep > GF Surface would make sense.
EDIT: I know of no evidence based reason to pick this choice.

Here is (my, worth half what you paid for it) summary of gradient factors based on modern research, in order from more confident to fairly uncertain but based on expert (not me) judgement.

• A lower GF High is more conservative than a higher GF High (but too low is impractical without a chamber)
• If GF High > 60%, GF Low > 50% is more conservative than GF Low < 50%.
• GF Low < or = GF High is probably more conservative than GF Deep > GF Surface
• GF Low ~= 0.83 GF High might be most conservative for a given GF High

 

[inquis]

Going "over GF High" could in theory give you the first stop above the last stop,

It would require tissue loading to be between GFhigh*M0 and GFlow*M0. Edit: forgot about subtracting off the ambient pressure. A simpler way to view it is the issue arises when GFhigh < surfGF < GFlow, where "surfGF" is the normalized tissue loading at the surface (surfGF = 1 if you're at M0 and 0 if you're at ambient).

ETA: Any implementation accepting GFLow > GFHigh would want to deal with this. An easy way is to "round up" the tissue tension, resulting in a small ceiling. Practically speaking, a 10' stop until all tissues are below GFHigh.

 

[inquis]

a substantial portion of your initial ascent and some time around every stop you will have tissue saturations greater than you feel is safe at the surface. Why would you want to do that?

Fast tissues can tolerate greater stress than slower tissues. A GFlow > GFhigh is leveraging that for a faster ascent. Effectively, less cushion for the faster tissues.

I personally wouldn't do it. Heck, I don't even run equal factors!

ETA: I agree Baker's original intent was for GFLow to not exceed GFHigh. I mean, it's implicit in their names.

 

[dmaziuk]

It would require tissue loading to be between GFhigh*M0 and GFlow*M0.

It would require GFlow * M-current to be <= GFhigh * M0. Since M's at depth are greater than M0's, you'd need GFlow < 1. Your tissue loading at depth is guaranteed to be greater than that after a couple of minutes.

 

[Cio]

My take from reading Deco for Divers is that the lower CF at depth lets you off gas while minimizing microbubble formation. microbubbles at depth could turn problematic (large pressure drop to the surface.) but this is less of an issue when shallow ( low pressure drop to the surface). So, basically you don't want any microbubble at depth but some when shallower are not as concerning so, in theory you should be less aggressive at depth and more when shallow. I say in theory since the recommendation for increasing GF low seems to be the new thing.

 

[inquis]

in theory you should be less aggressive at depth and more when shallow

The problem is what is "aggressive" when different tissues are considered? "Protecting" the fast tissues (i.e., at depth) is less aggressive -- for them -- but is actually more aggressive for slower tissues. I believe researchers feel there is be a "sweet spot" that is overall best for all tissues, but an evidence-based assessment of precisely where that is lacking thus far. All we have are the broader indications: Doolette uses GFLow = 0.83*GFHigh (yielding the same degree of supersaturation for all tissues). [1] Mitchell has condoned a GFLow range of 55-75. [2] Baker uses a GFLow between 50 and 60 [sorry, no specific reference].

[1] Gradient Factors in a Post-Deep Stops World, 5/29/2019 (retrieved 4/21/2022)
[2] What is optimal decompression, www.youtube.com/watch?v=nIO9qI5XODw, 2020

 

[L13]

Made some quick edits to my last post.

A (grossly over)simplified explanation of Baker's GF algorithm:

• Start with the Buhlmann model (empirically derived from theory and testing by Workman and Haldane).
• define GF of 100% for each tissue as the Buhlman inert gas supersaturation limit for that tissue.
• Define GF of 0% as saturated tissue.
• Now for any combination of target depth(pressure) and inert gas loading, we can represent it with a percentage, GF Now, for each tissue on that scale. define GF Now(max) as the GF Now of the tissue with the highest GF Now for current inert gas loading and target depth.
• For the rest of this explanation ascent rate is ignored for simplification.
• define NDL as the time when GF Now(max) for the surface = GF High (GF Surface) limit.
• If time > NDL (Deco dive):
  • For each deco stop depth increment calculate the GF Now(max) for that depth. The shallowest depth where GF Now(max) < GF Low(GF Deep). This depth is the first stop.
  • For each depth increment from First Stop to surface, linearly interpolate a value GF Limit between GF Low(GF Deep) and GF High(GF Surface).
    • Proceed to the next stop depth when, due to outgassing, GF Now(max) at that depth has dropped to be at or below the GF Limit for that depth.
    • repeat until the surface is reached with GF Now(max) < or = GF High(GF Surface).

Notes:

• Baker explicitly limits GF Low < or = GF High, but the algorithm works without this limit if we change the names.
• GF High/GF Low of 100/100 is basically Buhlmann's original empirically determined limits.
• GF < 100% is staying below those limits.
• Fast tissues dominate GF Now(max) at early stops (deeper stops). Slow tissues dominate GF Now(max) at later stops (if the dive was long/deep enough to accumulate inert gas in those tissues).
• A lower GF Low implies greater concern with faster tissues, or early bubble formation.
• a higher GF low implies greater concern with ingassing of slower tissues if we stop deeper to allow outgassing of faster tissues before proceeding shallower.

I'm sure there are details I have left out, don't be too pedantic since it is supposed to be simplified. However, if I have something fundamentally wrong, please correct me!

 

[dmaziuk]

One thing to keep in mind is that all the micro-bubble-formation stuff is mostly largely theoretical. Doppler ultrasound was new and exciting back when bubble models started, but by now we know there are always bubbles and that when there are a lot of them, that diver is likely bent. As for the rest of it... it's mostly maybes.

 

[L13]

My take from reading Deco for Divers is that the lower CF at depth lets you off gas while minimizing microbubble formation. microbubbles at depth could turn problematic (large pressure drop to the surface.) but this is less of an issue when shallow ( low pressure drop to the surface). So, basically you don't want any microbubble at depth but some when shallower are not as concerning so, in theory you should be less aggressive at depth and more when shallow. I say in theory since the recommendation for increasing GF low seems to be the new thing.

Recent empirical evidence suggests that ingassing of slower tissues at deep stops is a bigger threat than microbubble formation if GF Low is "too low." However, there are good theoretical reasons to believe that microbubble formation would be a bigger threat if GF Low was "too high." To date, there isn't as much empirical evidence of what "too high" would be. The community seems to be settling on GF Low ~=0.83 GF High as a guesstimate for the sweet spot for now.

 

[Tracy]

I say in theory since the recommendation for increasing GF low seems to be the new thing.

I don't know if it is the new thing, theory has been constantly evolving. When deco diving on a computer started to become mainstream, many were still padding their last stops by 10-20 minutes because they couldn't believe they were actually clear already.
My first tech class, my instructor was a die hard VPM guy, we were running 10/70 to match his VPM profiles. I actually dove that way for about a year and assumed you were just supposed to be exhausted after a deco dive. Since then, like others, I have just played with settings to see how I feel after different dives at different settings.
I know how tired I am on some settings, I know where I tend to get skin bends on other settings. I have now gotten to where I use numbers that I know work for me. The settings are adjustable for a reason.