Gradient Factors in Use

[Michael Guerrero]

OK, so if you feel some visceral reaction and an immediate need to tell me you aren't going to tell me what GFs you run, then please save your fingers the effort of the keystrokes and my time in reading your nonsense.



Now, for everyone else who is comfortable engaging in the discussion, what are your thoughts on GFs. I'm really having difficulty reconciling the NEDU results and other studies with a "low" GF setting. I recognize that Buhlmann's algorithm had a certain degree of acceptable DCS (2.5% I think for ZHL-16A, or maybe that's just the Navy tables), and that most computers run ZHL-16C which is supposed to be more conservative. I also recognize that this is a personal choice, and that we are all human guinea pigs experimenting with very much uncertain physiological reactions to decompression and bubbles.

Anyway, based on study results, what are you using for your GF Low? Right now I'm running 30, which is the default for the Petrel, but I'm really not convinced it needs to be that low. I'm thinking 40 or even 50 to get my butt off the bottom.

And before you recommend more reading, I've read all the way through the Rebreather World thread on it with Simon and Ross going back and forth. And I've watched the presentations from DAN's conference on the NEDU study as well as the subsequently released NEDU report. And of course, I've read a bunch of deco theory and other stuff.

So, just looking for your thoughts, not recommendations for further reading or lectures on why I should read more or anything else like that .

Anyway, sorry about such a negative start to the thread. Just trying to cut through to the meat of the discussion.

 

[Dr. Lecter]

I also followed the recent Deep Stops threads on RBW with interest. With 10/50 dil, I've been using 35/75 on CCR dives in the 200'-300' range lately and have been feeling good afterwards, and I'll probably go to 40/70 next since the setting I'd been happiest with on OC dives was VPM +3/70GFS. I think another one of the divers on these dives is on 10/60 dil, has used 30/70, and is saying he's feeling better than when he had something closer to a traditional 10-20 LO/90-100 HI GF for deep trimix dives. Then again, there's plenty of trimix divers who have done extremely low LO settings and 100-125 HI settings, and been fine afterwards.

Basically, for trimix dives in the 200'-300' range, the current thinking in our small group is that the deep stops have been overemphasized and the shallow stops underemphasized by prior GF setting choices, and something in the 30-40 LO range and 70-80 HI range is making us feel better. Which is not to say that what has worked for us lately won't bend someone else on the exact same dive, or us on a different dive, like a pretzel...so reader beware.

 

[Michael Guerrero]

Does that sentiment change when diving shallower air dives in the 160s? I've watched presentations discussing whether He is really a "faster" gas, which presumably is the argument for deeper stops on He.

 

[Dr. Lecter]

Does that sentiment change when diving shallower air dives in the 160s? I've watched presentations discussing whether He is really a "faster" gas, which presumably is the argument for deeper stops on He.

Just for me, sort of, but for a reason that doesn't directly relate to the issue of whether (or probably more accurately, by how much) HE is faster in/out than N2. Without getting into personal depths/times/settings, I don't think you're taking on the same DCS risk using a given setting for a marginal deco dive as you would be if you used the same setting for 45 minutes of BT at 300'. I personally needed a more conservative setting to feel good after bigger dives, whether they were on air or not, and took advantage of the shorter deco time of more aggressive settings on shallower air dives.

I think there is something to be learned about HE being faster from this, though it's entirely based on a limited amount of personal experience. When I first switched to Tx, I found I did not need the same HI GF setting to feel good after bigger decos that I'd settled on for air dives with bigger decos. Right now, I'm still moving the HI setting around, but while 70 was the only surefire setting for me on air, I may settle somewhere in the 75-80 range for 50% and greater Tx. That suggests to me that HE is somewhat faster. Then again, I haven't really changed my LO GF even for HE-rich mixtures and I'm not having problems...suggesting it's not that much faster than N2 (one could argue I'm now bending myself deep but mending enough shallow that I just don't notice).

Realistically, I think things like ascent speed, stop accuracy, proper hyrdration, and thermal protection are probably a much bigger deal than the shape of the curve...until, of course, you hit upon a setting that doesn't agree with your body.

 

[Michael Guerrero]

Yeah, all those other factors matter. So your thought is the dives I'm doing really aren't pushing the envelope enough to really be so concerned about tweaking the GFs that much. That makes a certain amount of sense. I admit my dives are pretty tame from a tech perspective. Makes me feel like all the more reason to not set the low too low and just make sure my shallow hangs are comfortable and sufficient for me and that 2 - 4 hrs after surfacing I still feel good.

I still do my Pyle stop though .

 

[Dr. Lecter]

Should have also added avoiding overexertion exiting/after exiting…learned that lesson after helping haul an anchor 15 minutes into a SI after a long, deep dive with a 75 surfacing GF.

I think any commonly acceptable GF settings, whether it's the current default settings or the old default settings, are fine assuming proper execution and they leave you feeling good. If not, slight tweaking may be worth trying. There are certainly dives where things like 'how many hours can I deco before I get too cold' and 'holy that's a high CNS even with air breaks!' start to intrude on how conservative your deco can get…but until you're there, no reason to rush out of the water IMO.

 

[ajduplessis]

45/95 above 230 ft, 45/90 down to 300ft and 45/85 down to 400ft. These are aggressive profiles that works for me, might not work for others. I will also use my Petrel to throttle the settings depending on dive conditions/workload and further adjust if my body is not in agreement.

Gradient Factors is just a method to calculate deco. Gas selection, where and how you use the gasses is a very serious consideration in addition to algorithm.

Many divers only look at settings without any further considerations or understanding of the total picture.

 

[Michael Guerrero]

Thanks DL. Unfortunately, exertion after a dive is somewhat unavoidable for me as I have to climb back up the hill to the car with my gear. I rest a bit in the water before doing this though, and rest after. I'm going to have to continue to rely on my physical conditioning to help me out there--whether it really does nor not. And I also agree I'm not in a rush to get out of the water. I dive with appropriate thermal protection to be able to do a much longer deco on air if needed, so I'm not cold on my hangs and I usually get out at a GF of mid - high 70s even though that's not what my computer is set to.


Thanks aj. Those do seem aggressive, but maybe because I've been conditioned with lower GFs . And I agree completely that deco gas selection makes a big difference.

 

[Dr. Lecter]

45/95 above 230 ft, 45/90 down to 300ft and 45/85 down to 400ft. These are aggressive profiles that works for me, might not work for others. I will also use my Petrel to throttle the settings depending on dive conditions/workload and further adjust if my body is not in agreement.

Gradient Factors is just a method to calculate deco. Gas selection, where and how you use the gasses is a very serious consideration in addition to algorithm.

Many divers only look at settings without any further considerations or understanding of the total picture.

Indeed…but I find it interesting you have the same LO GF for all three of the depths listed, though you must have increasing HE %s. I think it's also only the HI GF that one can edit on the dive itself with a SW, so here we have an example someone committed to a pretty aggressive LO GF regardless of how much HE is in there.

 

[Dsix36]

I run 80/90.

I am not a fan of deep stops at all and want to be shallow fast.

I will also lie about my helium content on some dives.

I can always pad my shallow stops to extend my deco if I feel the need due to exertion.