3 or 5 minute Safety Stop?

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For recreational dives, I think you can say that, if you stay within your no-deco limits for the model you are using (whatever it is), and you control your ascent rate, your likelihood of DCS is extremely low -- so low that no one can really tell you whether 3 or 5 minutes at 15 feet is better.

Damn data!! Screwing with everyone's misconceptions. I have never seen so much worry in people over a non-existent injury/death rate in recreational diving. Maybe the baby crib thing was worse.

For anyone that may have gotten an " undeserved hit" or for other reasons may feel they are at an increased risk for DCS, the answers to the OP's questions may be more then just academic interest and they may feel that any options that can lower their personal risk may be worth discussion.
 
Yes, IMHO, the last 20' are where most of the damage is done.

I've always been amused by 'altitude divers' and their attention to detail...
 
Well, as I said in my first post, I don't think you can argue that you don't get SOME benefit from time spent in the top 1.6 ATA, in terms of offgassing. I just think that it is not possible to prove that the benefit is sufficient to recommend the practice across the diving population. If I had gotten a undeserved hit myself, I'd first get myself tested for a PFO, then I'd switch to diving the richest Nitrox mix possible for any given dive, and I'd plan stops at 30, 20 and 10 for all dives where I could do them in the water conditions. But of course, I ALREADY do those stops -- so I'd just make them longer, and plan my gas accordingly.

But whether that would really reduce my risk of another hit or not, I don't know -- maybe it is something I did much deeper in the dive that caused the problem in the first place. We really don't have good information on this.

I would highly recommend to anyone who finds this kind of discussion fascinating, the GUE DVD "The Mysterious Malady". It's a fascinating series of interviews with the folks who actually do research in this area, and what you learn is rather chilling . . . we really don't know all that much about DCS, and if you don't know the pathophysiology of a disease, it's awfully hard to know either how to prevent it or how to treat it.
 
A lot of the problems with these discussions are illustrated in just these few posts.

Definitions aren't clear -- What IS a "deep stop"?

Stops when executing deeper, staged decompression dives are different from stops executed when the entire dive is done within NDLs.

Ascents rates often aren't clearly discussed when discussing stops.

One of the biggest problems with recreational dives is the lack of control in the top ATA, especially in the top 15 feet AFTER the safety stop. Thalassamania has often stated that the safety stop was instituted to enforce some kind of ascent rate control in the shallows; one could also consider that the safety stop ensures that the fast tissues offgas to a sufficient degree during those 3 to 5 minutes (and note that 5 minutes is the half-time for the fastest compartment) that poor ascent rate control from there to the surface is less likely to be injurious.

DEEP stops, however, are quite different from the original question, regarding whether the shallow stop ought to be 3 or 5 minutes. Deep stops are posited to provide bubble growth control -- allowing elimination of nitrogen while still under sufficient ambient pressure to keep bubbles from forming, or from expanding if they have formed. Recommendation for the depth and length of deep stops are very variable. NAUI recommends, if I remember correctly, 2 minutes at half maximal depth for a recreational dive. UTD teaches one minute stops at 80% of your average bottom time ATA, which is very deep. The DAN Symposium on Technical Diving included at least one speaker who presented data suggesting that stops that deep are counterproductive; making such stops on a recreational dive are pretty obviously absurd, and really convert an ascent into a multi-level dive.

It all gets even more interesting, and frustrating, when you realize that the incidence of DCS in recreational dives done within no-deco limits is EXTREMELY low, which means you have to study a LOT of dives to see anything make a significant difference -- and those studies are expensive, and in view of the low incidence, not studies anybody is really all that interested in doing. So today, we use Doppler bubble studies as a proxy for DCS, which is a very flawed proxy, as high bubble grade and symptoms do not correlate all that well. I do believe studies are ongoing, however, in measuring bubble grades in divers using various ascent strategies, but primarily on technical dives.

For recreational dives, I think you can say that, if you stay within your no-deco limits for the model you are using (whatever it is), and you control your ascent rate, your likelihood of DCS is extremely low -- so low that no one can really tell you whether 3 or 5 minutes at 15 feet is better.


And....if you decide you are doing deep stops on a 90 foot recreational dive, then the amount of gas you need for yourself and buddy--your rock bottom, goes way up...This is a concept that could either push people into not being able to use 80 cu ft al bottles that are ON THE BOAT....and maybe even push some to doubles when they really ought to be able to use a 100.
 
And....if you decide you are doing deep stops on a 90 foot recreational dive, then the amount of gas you need for yourself and buddy--your rock bottom, goes way up...This is a concept that could either push people into not being able to use 80 cu ft al bottles that are ON THE BOAT....and maybe even push some to doubles when they really ought to be able to use a 100.

I guess you'd have to define a "90 foot recreational dive". On AIR my Hollis DG03 only gives me 19 minutes at depth. My Suunto Mosquito gave me 22 minutes. My SAC rate is .65 (not exceptional) so that's about 46CF or 1800 PSI for the bottom time portion of the dive. That's 1200 PSI remaining. 90 * 10 + 300 = 1200 so that's not a bad PSI to be ascending with. When I pop up to 45' now my rock bottom is 750. I've done the math and it should be fine.

Now if you throw in 36% and a longer bottom time... sure, bigger tanks can be needed.
 
The topic of ascent strategy for recreational dives often ends up with one foot in science and one foot in religion. I try to stay in the former. For those with an inordinate interest in the science, I found the the following to be of interest, including support for the traditional 3 minute safety stop Modern Decompression
 
Start with 80% for the high limit and 30% for the low limit.

All the reasons and thinking are in Mark Powell's book. In addition, get a copy of the Six Skills by Steve Lewis. Finally, for another great "mindset" read, get a copy of Sheck Exley's "Basic Cave Diving -a blueprint for survival."

All of which draws the cloak off of the DIR types. (don't push back unless you are willing to post all the specifics of ratio deco). They start out in their education with an all encompassing ascent plan, argueably not optimal, but far better than the "NOW WHAT???" that most of the other agencies use if NDL is violated. It is interesting to resolve their plan against Buhlman, RGBM, deco on the fly, etc, etc, etc.

There is more than one way to the top.:wink:
I have both books. I will go back and reread the appropriate sections. I don't have Sheck's but I've been eyeing it. Wish it was available electronically but I can sweet talk my husband into "ripping" it up and scanning it into a PDF file for my IPAD.
 
And....if you decide you are doing deep stops on a 90 foot recreational dive, then the amount of gas you need for yourself and buddy--your rock bottom, goes way up...This is a concept that could either push people into not being able to use 80 cu ft al bottles that are ON THE BOAT....and maybe even push some to doubles when they really ought to be able to use a 100.

Going out on a limb here cause I hate math but I think for the average diver, say a SAC of .7, adding a 2.5 min stop at roughly 50 feet adds about 4.5 cf of gas then say another 2 cf of for an extra 2 minutes at 15 feet? Don't think I'd need doubles. And I would not include this in a rock bottom or buddy share air calculation since most likely if in OOA situation, I'd not be too worried about the extra safety margin of added or prolonged stops.
 
I guess you'd have to define a "90 foot recreational dive". On AIR my Hollis DG03 only gives me 19 minutes at depth. My Suunto Mosquito gave me 22 minutes. My SAC rate is .65 (not exceptional) so that's about 46CF or 1800 PSI for the bottom time portion of the dive. That's 1200 PSI remaining. 90 * 10 + 300 = 1200 so that's not a bad PSI to be ascending with. When I pop up to 45' now my rock bottom is 750. I've done the math and it should be fine.

At first I couldn't figure out your maths, but then I figured you were only accounting for you. By definition, "rock bottom" is enough gas for two divers to ascend at the normal rate and do standard safety stops. Maybe there needs to be another term for minimum gas needed for a solo diver?

And I would not include this in a rock bottom or buddy share air calculation since most likely if in OOA situation, I'd not be too worried about the extra safety margin of added or prolonged stops.

Rock bottom, as I was taught it (by the inestimable JoeT himself), always includes all stops.
 
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