Deep Stops Increases DCS

[Divetech99]

_t

Would someone be so kid as to PM me the link to RB world, if its against TOS to post here?

Litehedded posted it earlier. Scan through the posts above.

 

[rjack321]

I know multiple days of at least one and usually two 200'-240' air dives (often longer than 50 minutes you mentioned) is nothing particularly impressive, Kev. You're hardly the only one in this thread with experience doing that, and the difference between you and me of a year or two ago is that you only did that stuff a couple weeks a year. The other guys in here may never have dove air past whatever magical limit DIR imposed on them, but that ain't me.

And what I'm telling you is deep stop profiles for such dives produced for me similar, though less objectively bendy, results as you're reporting. I had two choices: slap a ultra low surfacing GF on top of the deep stop model's output, or, try something different. Both "worked," but having tried each way I think less time (not zero time) on deep stops that aren't nearly as deep is the way to go. YMMV.

I ended up doing the opposite, but that's because my friends and semi regular deco buddies like the RD deep stops. While I talked them out of the 80% of ATAs (which is exactly how it was originally taught back when AG was actually with GUE - I was in his last class with GUE) and we now do 2 ATAs off the bottom. Pushing the next stops up or shortening them so they were closer to low GF 40 or 50 isn't going to happen although I would be more than willing to try shorter and shallower "deep" stops more in line with low GFs in the 35 to 45% range. Hence the add time shallow (@10ft on O2) kludge that was I able to compromise with them over.

Only a few things have changed about RD over the last 10 yrs, none of them based on actual data or medical research...

1) 80% of ATAs became 75% of max depth (still too damn deep based on current science)
2) the 220ft 1:2 setpoint on 18/45 became 200ft. (20mins at 200ft = 20 mins on EAN50 and 20 mins on O2). 220ft was insane, even AG was getting tweeks on that and he's one of the world's better offgassers
3) 25/25 was introduced as a standard. At first it was -20% EAD but rumor has it the original 12 UTD instructors balked at actually diving that aggressively and it was rapidly reduced to -10% EAD.

AG has in the past compared RD to VPM+2 and Buhlmann 30/85 times. Both of these schedules are known to have high-ish hit rates (rather obviously) compared to VPM+5 or 30/70. The NEDU study models had hit rates of 1% and 5% and while I only have anecdotes, I would guess the VPM+2 and GF 30/85 are in that range as well. RD has not gotten more conservative in a systematic way to address the fact that we now know its baseline (VPM+2 and GF 30/85) have moderate hit rates and people desire more conservatism than a 1-5% chance of getting bent. With 20 to 30 deco dives per year your likelihood of getting a hit would be roughly annual. While potentially acceptable to the Navy, that's pretty bad for a recreational pursuit.

---------- Post added December 17th, 2014 at 11:50 PM ----------

Anyway, that's similar to the prescription that I'm going to start using for these types of dives: Long O2 deco profiles on top of Ratio Deco, with a Petrel Computer ZHL-16C algorithm showing a surfacing Gradient Factor of 40% to 50% -and take a day-off after four consecutive days (or 8 consecutive Deep Air Deco Dives). . .

Or you could get up faster and skip some of those deep stops and consequent slow tissue ongassing. Less accumulated N2 in the deep stops = less O2 time later, lower CNS on that protracted deco, and lower OTUs over multiple days.

Given your history with RD, I don't see what you have to lose here using a petrel with 40/70 GFs and perhaps doing a few extra mins below the first mandatory stop if its too big a hurdle to accept that the deep time is doing you more harm than good.

 

[Kevrumbo]

I ended up doing the opposite, but that's because my friends and semi regular deco buddies like the RD deep stops. While I talked them out of the 80% of ATAs (which is exactly how it was originally taught back when AG was actually with GUE - I was in his last class with GUE) and we now do 2 ATAs off the bottom. Pushing the next stops up or shortening them so they were closer to low GF 40 or 50 isn't going to happen although I would be more than willing to try shorter and shallower "deep" stops more in line with low GFs in the 35 to 45% range. Hence the add time shallow (@10ft on O2) kludge that was I able to compromise with them over.

Only a few things have changed about RD over the last 10 yrs, none of them based on actual data or medical research...

1) 80% of ATAs became 75% of max depth (still too damn deep based on current science)
2) the 220ft 1:2 setpoint on 18/45 became 200ft. (20mins at 200ft = 20 mins on EAN50 and 20 mins on O2). 220ft was insane, even AG was getting tweeks on that and he's one of the world's better offgassers
3) 25/25 was introduced as a standard. At first it was -20% EAD but rumor has it the original 12 UTD instructors balked at actually diving that aggressively and it was rapidly reduced to -10% EAD.

AG has in the past compared RD to VPM+2 and Buhlmann 30/85 times. Both of these schedules are known to have high-ish hit rates (rather obviously) compared to VPM+5 or 30/70. The NEDU study models had hit rates of 1% and 5% and while I only have anecdotes, I would guess the VPM+2 and GF 30/85 are in that range as well. RD has not gotten more conservative in a systematic way to address the fact that we now know its baseline (VPM+2 and GF 30/85) have moderate hit rates and people desire more conservatism than a 1-5% chance of getting bent. With 20 to 30 deco dives per year your likelihood of getting a hit would be roughly annual. While potentially acceptable to the Navy, that's pretty bad for a recreational pursuit.

---------- Post added December 17th, 2014 at 11:50 PM ----------

Originally Posted by Kevrumbo
Anyway, that's similar to the prescription that I'm going to start using for these types of dives: Long O2 deco profiles on top of Ratio Deco, with a Petrel Computer ZHL-16C algorithm showing a surfacing Gradient Factor of 40% to 50% -and take a day-off after four consecutive days (or 8 consecutive Deep Air Deco Dives). . .

Or you could get up faster and skip some of those deep stops and consequent slow tissue ongassing. Less accumulated N2 in the deep stops = less O2 time later, lower CNS on that protracted deco, and lower OTUs over multiple days.

Given your history with RD, I don't see what you have to lose here using a petrel with 40/70 GFs and perhaps doing a few extra mins below the first mandatory stop if its too big a hurdle to accept that the deep time is doing you more harm than good.

Yeah Rich . . .I believe that's an option as well coming off Air Bottom Mix; the Truk Dive Guides even on Buhlmann GF's 30/85 are usually 9m to 12m above me already by the time I finish my first Ratio Deco defined Deep Stop. Because of the high FN2 of Air Bottom Mix and N2 high solubility and proportionally high ppN2 (by Henry's Law), you gotta push that ambient pressure gradient a little more to start off-gassing from fast tissue, while not lingering any deeper than you have to on a Deep Stop that would still be potentially loading up slow tissue compartments.

But for a dive on standard 18/45 trimix (which btw in Truk is $260 for He alone to blend in one set of AL80's!), I would obviously revert back to RD1:2 -but only after a long O2 deco clean-up of the slow tissues on the day before or even a full day-off. . .

 

[ajduplessis]

You started way back with "look at my helium bill and my fat wallet", now your talking deep air and being bent on most trips while complaining about $260 fill for 18/45. WTF happened?

Your diving changed, your body obviously changed after a couple of hits and you still think you have a winning formula going .........

 

[Kevrumbo]

You started way back with "look at my helium bill and my fat wallet", now your talking deep air and being bent on most trips while complaining about $260 fill for 18/45. WTF happened?

Your diving changed, your body obviously changed after a couple of hits and you still think you have a winning formula going .........

And wth are you talking about? Quit your trolling son. . .

I've been diving Truk mostly on Deep Air every year since 2007; I've sparingly used 20/20 trimix for running line in wreck penetrations at 45m to 60m depths; and used 18/45 trimix only three times in the past three years for the deepest least visited wreck in the Atoll (Katsuragisan Maru) at 67m.

And I'm not complaining about a $260 Helium fill: It's not my problem if that upsets you 'cause you can't afford it. . .

 

[ajduplessis]

You are a bigger stroke than I initially thought. My bad.....

 

[tomfcrist]

You are a bigger stroke than I initially thought. My bad.....

I was wondering how long it would take for Trey to reveal himself.....welcome back Trey! (sarcasm)

 

[PfcAJ]

. . .The more important issue is that technical divers do not do air decompression dives, they use oxygen-accelerated decompression. If decompression stops are conducted using a breathing mixture with a low inert gas fraction, then, of course, there is less gas uptake into the relatively slow compartments. The effect of this is to increase the depth at which stops become “bad” deep stops.

David Doolette

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.

 

[Diver0001]

You guys still locked in a "believers" and "non-believers" discussion?

That's really too bad, tbh. This thread started with an absolutely clear and scientifically well grounded article that demonstrated without any shadow of a doubt where the needle should point in the "deep stops" vs "gas washout" issue

and the two of you (with help from some others) have turned it into trench warfare about ratio deco.

R..

 

[Subcooled]

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 ----------

Deep stops on backgas are bad.

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.