Why plan decompression with a Gf (lo)?

[Nirvana]

I am interested in knowing what are the theoretical and practical bases for planning decompression using gradient factors Low and High. That is, the reason behind choosing a Gf Lo and a different Gf Hi. Why ZHL-16C 40/70 and not ZHL-16C 70/70, for instance.

The reason for introducing Gf (lo) and Gf (hi) to Bühlmann's ZHL-16 algorithm, as I recall, was to try and reduce the amount and volume of bubbles formed during decompression. It was, in a sense, a departure from its dissolved gas content foundation. It was taken in a time when it became clear that even DCS free dives produced some detectable bubbles, and people started directing their effort to try to control bubble growth during ascent. If I am not mistaken, the adoption of gradient factors was more or less concurrent with the gain in popularity of Pyle stops and VPM.

More recent research, however, has downplayed the need to control the bubbling that occurs early in the ascent. From my understanding of the discussions of the past few years, it is now thought that the most important metric in determining the probability of DCS is the integral of supersaturation. It appears, then, that it is more important to emphasise the reduction of supersaturation of slow and medium compartments.

Given all that, my question is: what is the reason for continuing to employ a pair of gradient factors, instead of scraping the Gf Lo altogether (or, put it another way, set it the same as Gf Hi)? Has any study been done that shows that using any Gf (lo) produces less DCS cases, or at least less intravenous bubbles, than not using it? Has it been shown that using a Gf (lo) reduces the calculated integral of supersaturation?


edited for clarity

 

[Diver0001]

Given all that, my question is: what is the reason for continuing to employ a pair of gradient factors, instead of scraping the Gf (lo) altogether? Has any study been done that shows that using any Gf (lo) produces less DCS cases, or at least less intravenous bubbles, than not using it? Has it been shown that using a Gf (lo) reduces the calculated integral of supersaturation?

Deep stops don't appear to be helpful to optimizing the efficiency of decompression in terms of total run times. This would seem to argue in favour of setting the GFlo higher. However, deep stops DO protect faster tissues during the early parts of the ascent, so it's not like they are doing nothing.

On the whole a deeper ascent line should lead to longer ascents in most models. If you don't mind this in exchange for protecting faster tissues early on in the ascent then go ahead. Just be aware that this strategy won't get you out of the water faster or cleaner (both things that have -- incorrectly -- been suggested in relation to certain models in the past).

In certain models these deep ascent lines clearly lead to higher tissue loading in intermediate tissues and therefore a higher risk of DCS -- particularly on more aggressive dives -- unless those intermediate tissues are adequately off-gassed with more time than is apparently calculated. This appears to indicate a possible calibration problem in some models that, by design, favour protecting faster tissues at the expense of slower tissues. What we seem to be learning lately is that protecting fast tissues, while all well and good, doesn't seem to offer the benefits that are promised in terms of protecting the diver from DCS risk in the big picture.

I don't think anyone really knows what an "optimal" model looks like but we have convincing evidence that deep ascent lines do not lead to faster or cleaner decompression than shallower lines.

So where does that leave us? I'm actually not sure. 30/70 used to be the norm. Recent research has a lot of divers using 50/80 or 50/85 but we're still feeling our way through this. I don't think anyone really has a rock hard answer right now.

R..

 

[rsingler]

Edit

 

[Diver0001]

I think he's asking why you would set GFlo to anything other than the same value you picked for GFhi.

That's what I assumed when I answered his question, anyway.

R..

 

[Nirvana]

I'm not sure I understand this question. GF Lo and Hi, together, create the points that determine the slope of the line that forces a stop based upon the saturation of the leading compartment (I think).
So I'm not sure what you mean when you propose eliminating one of those points. What will the slope of the line be?
Are you essentially proposing using Buhlmann/Workman's 100% point for one end of that line (e.g., GF 100/xx)? You surely can't mean GF 0/xx. Or perhaps you mean GF xx/xx?

Can you pose that question a little differently?

I'm sorry I was not clear enough. I meant, as @Diver0001 stated above, why not set Gf Lo the same value as Gf Hi.

 

[Nirvana]

Deep stops don't appear to be helpful to optimizing the efficiency of decompression in terms of total run times. This would seem to argue in favour of setting the GFlo higher. However, deep stops DO protect faster tissues during the early parts of the ascent, so it's not like they are doing nothing.
R..

Thanks for your reply.

Protecting the faster tissues certainly was one of the reasons for introducing deep stops. But do they, in fact, protect those tissues? I believe that neurological DCS is not necessarily produced by bubbles generated locally, but more often than not by bubbles generated throughout the body that travel to the brain. It doesn't seem that making deeper stops (setting a lower Gf Lo) correlates directly with reducing the incidence of DCS in faster tissues.

 

[rsingler]

I'm sorry I was not clear enough. I meant, as @Diver0001 stated above, why not set Gf Lo the same value as Gf Hi.

My assumption for the rationale in splitting Lo from Hi is this:
Most will accept that diving 100/100 poses a risk. Either from fast tissues that bubble, or from a total deco time that is insufficient to offgas to an extent that will keep overall bubbling (which we know occurs anyway) to a reasonable level.

So if you pick a GF that is below 100, at some point you need to split Lo from Hi to have enough gas to complete the dive.

Diving 85/85 would work from a carried gas standpoint. Diving 30/30 might not. It's as straightforward as that. So at some point, your GF #1 must become a GF #2.

Now I will yield the floor to those who feel more strongly about a given number.

If your question was really, "Why not dive 70/70", I don't think I'll go there.

 

[KenGordon]

When GF was introduced it was faking up bubble model profiles and stuff like 30/80 seemed a good idea. Now we (think!) we know better and the low lo value is not appropriate. However, nobody can say for sure what is ideal and changing slowly seems sensible. Thus values like 50/80 rather than 80/80.

Also, it doesn’t make much difference to the run time for the sort of dives 95% of deco divers are doing.

 

[Kevrumbo]

Going back to Erik C. Baker's excellent article explaining the trade-offs of utilizing Deepstops:

This is what GF Lo/Hi of 20/75 would look like for a 90msw depth, 20min BT (and by the implications of the NEDU Deepstop Study would still claim to be too deep):

Conversely, this tissue compartment graph from Baker's article below would be a radical interpretation & application of the results of the NEDU Study with a much shallower first stop at 33msw (i.e. the first intermediate deco gas Nitrox36 switch). Note the huge over pressure gradient and supersaturation of the first five Fast Tissue Compartments.

(Anybody try this 90msw profile in figure 1 above yet?) If you believe that the Fast Tissues are perfused and "robust" enough to handle the peak & relative integral of supersaturation earlier in this deco profile, then go ahead and set a GF of something like 80/80 or 75/75.

The point is that in comparison, trying a GF Lo/Hi of 50/80 or 40/70 seems to be a better conservative starting value of gradient factors to strategically apply in the evolving de-emphasis of deepstops.

 

[DevonDiver]

There's a bunch of published research regards deep stops, micro-emboli considerations etc; much of it referenced and extensively discussed here on Scubaboard.

GF high and low offers divers the capacity to tweak their ascent curve according to their individual preferences and needs.

Mark Powell's 'Deco for Divers' is probably the best starting point for a layperson understanding of gradient factors.

Decent tech instructors will expand on that with supplementary lectures in their classes.

These lectures should link the theory with the practical application; there's a myriad of personal preferences and specific dive parameter needs that dictate how GF may be used to create a desired outcome.