Deep Stops Increases DCS

[Slamfire]

The analogy is: If you play American Football long enough, you will eventually get a concussion. It used to be okay just so you don't get a lot of serious Grade III's . . .

Getting an eventual hit after many, many dives is NOT inescapable. Yes, I understand the analogy and I believe it is well intentioned when it is aimed at de-stigmatizing bends. Anybody on any algorithm can get a hit. But I do not look at it with approving eyes when it can be used as a blanket statement to excuse less than effective deco strategies.

I know some very experienced deco divers that have never had to take chamber rides or IWR to treat bends. I believe I read in somewhere that Steve Lewis (aka Doppler) is one of these cases. My tech instructor never took a chamber ride in spite of spending decades teaching TDI's deep air courses in the cold, dark waters of Alaska and the Pacific Northwest. He was also very regularly organizing and executing dives to Pacific Northwest wrecks like the Admiral Sampson (320fsw) and the Bunker Hill (280 fsw). I've never taken a chamber ride, and, knock on wood, I hope I never have to.

"The only way to guarantee that you will not get a DCS hit is to avoid diving altogether". While I accept this to be true in and of itself, I refuse to use it as an excuse.

 

[PfcAJ]

If you make a lot of deep stops, then the offgassing algorithm adds more time to the shallow stops.
Result: more conservative plan, longer decompression.

I have always wondered why people are in such a hurry to reach the surface.

---------- Post added December 18th, 2014 at 09:22 PM ----------



No. They are not. You just add an appropriate amount of shallow deco stops, and all is well.
If it takes too long and the water is too cold, then get a heated vest.

What do you want to minimize? Time to surface or risk of DCS. Choose one.

You missed the memo about efficiency from Doolette.

Why add time deep just to fix it by adding time shallow ? No benefit.

 

[Kevrumbo]

And if you're NOT doing stops on a deco gas we're back to square one.

Deep stops on backgas are bad. That's what that means. Kev, stopping at 80% of ATAs or 75% avg depth is exactly what we're talking about as for being too deep.

You missed the memo about efficiency from Doolette.

Why add time deep just to fix it by adding time shallow ? No benefit.

If it is a high Helium back gas then you gotta do the Deep Stops because of Helium's higher diffusivity: you don"t want He diffusing out of tissue into any potential bubble seeds/micro-nuclei. You need the higher ambient pressure of the Deep Stop to ideally crush the bubble, or at least keep it small & few in number.

But with a high FN2 like Air back gas, a long Deep Stop profile will unload the fast tissue compartments, but may continue to load the slow tissue compartments because of Nitrogen"s greater solubility.

The most conservative deco profile in either case would be to continue the Deep Stops, but do more time shallow at the 6m O2 deco stop to eliminate inert N2 from the slow tissues before finally surfacing. . .

(Note: The water temp in Truk is 30deg C and I've got a wetsuit heater to use as needed)

 

[Dr. Lecter]

If it is a high Helium back gas then you gotta do the Deep Stops because of Helium's higher diffusivity: you don"t want He diffusing out of tissue into any potential bubble seeds/micro-nuclei. You need the higher ambient pressure of the Deep Stop to ideally crush the bubble, or at least keep it small & few number.

But with a high FN2 like Air back gas, a long Deep Stop profile will unload the fast tissue compartments, but may continue to load the slow tissue compartments because of Nitrogen"s greater solubility.

The most conservative deco profile in either case would be to continue the Deep Stops, but do more time shallow at the 6m O2 deco stop to eliminate inert N2 from the slow tissues before finally surfacing. . .

(Note: The water temp in Truk is 30deg C and I've got a wetsuit heater to use as needed)

Or, you know...not as much as your model assumes. There's a reason those of us spending 30-45 minutes at 80-100m on 50-60% helium blends aren't getting bent (and are actually feeling better with higher GF highs than we could take when doing more time deeper) even with GF lows in the 50-80% range.

 

[Subcooled]

You missed the memo about efficiency from Doolette.

Why add time deep just to fix it by adding time shallow ? No benefit.

The relevant question is: does additional time really hurt, and do the asymptomatic bubbles cause any damage? I mean, salt, fat, alcohol... It doesn't need to hurt to do some damage.

 

[WhiteSands]

The surprising thing about this research is that it seems to go counter to modern bubble models like VPM & RGBM. Or am I not understanding this correctly?

---------- Post added December 19th, 2014 at 09:33 AM ----------

The relevant question is: does additional time really hurt, and do the asymptomatic bubbles cause any damage? I mean, salt, fat, alcohol... It doesn't need to hurt to do some damage.

Reading what Doolette wrote on RBworld, deep stops add no benefit & instead increases the occurence of DCS.

What puzzles me is that the deep stop idea came from pearl divers, and their empirical evidence seems to say deep stops are better. Granted their conusion came not from carefully controlled experiments but by gut feel where several variables might be at play at the same time. They were also diving on air. Howeverthe NEDU study finds the opposite. Very curious.

 

[pensacolaracer]

The relevant question is: does additional time really hurt, and do the asymptomatic bubbles cause any damage? I mean, salt, fat, alcohol... It doesn't need to hurt to do some damage.

My relevant answer is "yes it does". I dove 15/70, 15/75, 30/70.... Etc. I felt like **** after getting out. And this was not just an occasional dive. It was 2-3 per week, every week.

I have moved to 80/85 with quite a few deep dives now and feel great afterwards.

I won't add time deep during off gas just for ****s and giggles. I get shallow and stay there.

 

[Kevrumbo]

The surprising thing about this research is that it seems to go counter to modern bubble models like VPM & RGBM. Or am I not understanding this correctly?

There's is nothing in the NEDU Study that addresses the relative solubility & diffusivity of Helium vs Nitrogen and their effects on the dual phase tenets of VPM & RGBM:

From Bruce Wienke, Technical Diving in Depth, Reduced Gradient Bubble Model (RGBM) In Depth:Helium NDLs are actually shorter than nitrogen for shallow exposures . . . Reasons for this stem from kinetic versus solubility properties of helium and nitrogen, and go away as exposures extend beyond 150 fsw, and times extend beyond 40 min or so.Helium ingasses and outgasses 2.7 times faster than nitrogen, but nitrogen is 1.5 to 3.3 times more soluble in body aqueous and lipid tissue than helium. For short exposures (bounce and shallow), the faster diffusion rate of helium is more important in gas buildup than solubility, and shorter NDLs than nitrogen result. For long bottom times (deco and extended range), the lesser solubility of helium is a dominant factor in gas buildup, and helium outperforms nitrogen for staging. Thus, deep implies helium bottom and stage gas. Said another way, transient diving favors nitrogen while steady state diving favors helium as a breathing gas.

---------- Post added December 18th, 2014 at 04:49 PM ----------

Or, you know...not as much as your model assumes. There's a reason those of us spending 30-45 minutes at 80-100m on 50-60% helium blends aren't getting bent (and are actually feeling better with higher GF highs than we could take when doing more time deeper) even with GF lows in the 50-80% range.

My relevant answer is "yes it does". I dove 15/70, 15/75, 30/70.... Etc. I felt like **** after getting out. And this was not just an occasional dive. It was 2-3 per week, every week. I have moved to 80/85 with quite a few deep dives now and feel great afterwards. I won't add time deep during off gas just for ****s and giggles. I get shallow and stay there.

Well that's your model's (Buhlmann GF's) Kludge Factor; do what's best for you and based on your own experience.

 

[GrimSleeper]

The surprising thing about this research is that it seems to go counter to modern bubble models like VPM & RGBM. Or am I not understanding this correctly?

---------- Post added December 19th, 2014 at 09:33 AM ----------



Reading what Doolette wrote on RBworld, deep stops add no benefit & instead increases the occurence of DCS.

What puzzles me is that the deep stop idea came from pearl divers, and their empirical evidence seems to say deep stops are better. Granted their conusion came not from carefully controlled experiments but by gut feel where several variables might be at play at the same time. They were also diving on air. Howeverthe NEDU study finds the opposite. Very curious.

If you read what Bruce Wienke quoted about pearl divers as posted by Kev earlier in the thread, they dramatically cut their decompression times… And then got very, very good at doing IWR to counter the dramatic increase in bends. Ditto for Hawaiian fishermen. And this is what everyone touting bubble models/pseudo-bubble-voodoo models seems determined to ignore, even when they themselves are padding out the shallow part of their schedules. Sure, there's an argument that starting decompression deeper might off-gas fast tissues 'better', but the diver is still on-gassing in slower tissues at those deeper stops and that has to be 'paid for' - either with shallow stops or an hour at 30 feet on Oxygen waiting for the pain to go away.

 

[Dr. Lecter]

The interesting question is what kind of horrible type II, 'helium leaving fast tissue too quickly' hits are being flirted with by getting really shallow really fast. Unlike the slow tissue aches, pains, and chronic feeling of tyness that accompanies too much deep stop time even with a "successful" decompression, I worry that really high GF lows and relatively high helium mixes are basically binary: 99.X% of the time, nothing at all goes wrong, but that 0.Y% of the time something does go wrong, it's not the mild type of hit that occurs. Which is why I'm still at 50 or 60% for my GF low, and am closely observing Pensacolaracer as my guinea pig

Then again, going huge on the shallow O2 time to make up for way too much deep time isn't doing your odds of oxtoxing any favors, so you pays your money and you takes your chances.