Maybe the baby crib thing was worse.
The national 55mph speed limit, that was worse....
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
Maybe the baby crib thing was worse.
?..//...
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.
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.
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.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.
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.
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.