Alert diver article on Deep Stops

[boulderjohn]

Okay, in that case, then why bother holding a 1-minute stop at half-depth? Let's just reduce ascension rate even more.

The problem is that there are no studies, no empirical data to back up the theory of deep stop.

There are studies, but there aren't enough. In fact, the results of some studies contradict the results of others. If you search the DAN or the Rubicon web site, you will find references to several studies. IMO, we still don't know enough for me to make any kind of firm commitment of my own.

Several studies have compared stops to slow ascent rates, and found that stops are more effective. One study cited by DAN (I am doing this from memory) compared three ascent rates (10 fpm, 30 fpm, and 60 fpm), with and without stops. The most effective combination was 30 fpm with a mid point stop and a safety stop. The least effective was a 10 fpm ascent. If I were to take a guess as to why that is try, it would be that you are not driving the gradient in the fast tissues as much, and you are ongassing the slow tissues more.

As for your earlier question about the difference between deep stops and multilevel dives, the difference is the length of time at the stops. The benefit of the stop (allowing faster tissues to offgas to a safe level) is accomplished relatively quickly. During that time, you are ongassing the slower tissues, but you are hopefully not there long enough to get that much additional N2. If you hang out at that level in a multilevel dive, you stop getting any benefit of any kind in the fast tissues because they near equilibrium quickly, and you are now ongassing the slow tissues more significantly.

EDIT: In deeper technical diving there is a lot of controversy about this as well. I know at least one theorist who believes that some agencies push deep stop practice much too far and are in fact creating a multilevel profile that does more decompression harm than good.

 

[fnfalman]

@Slamfire: You may want to read the article. The one that we're discussing just came out in the Winter 2010 edition of Alert Diver. It showed up in my mailbox last week.

Yep. This is the one that just barely came out. It also has a Q&A on hot tub after a dive, and an article on a person who didn't hit NDL, did safety stop, the whole shebang and still got DCS hit while his wife who has similar dive profile was fine.

 

[Sas]

an article on a person who didn't hit NDL, did safety stop, the whole shebang and still got DCS hit while his wife who has similar dive profile was fine.

Not unusual. More than half of recreational DCS hits are 'undeserved'.

 

[Slamfire]

Then I am discussing something I haven't read about. That issue is probably still on its way to my mailbox.

 

[MXGratefulDiver]

Not unusual. More than half of recreational DCS hits are 'undeserved'.

I always had an objection to that term ... of course, if you get DCS it's because you violated some limit of your body. The problem is that (a) there are a lot of different variables that determine those limits, and (b) nobody truly understands what the limits are, or how the variables interact to determine them.

Decompression is far from an exact science. That's why there's so much conservatism built into decompression profiles. Saying someone took an "undeserved" DCS hit is like saying someone lost an "undeserved" amount of money at the local casino ... what it really boils down to is you placed a bet on what your body would tolerate and you lost the bet.

... Bob (Grateful Diver)

 

[Bubbletrubble]

Not unusual. More than half of recreational DCS hits are 'undeserved'.

By "undeserved," you mean that the diver did not exceed NDLs and recommended ascent rates, right?

I cringe whenever people use that terminology, though. The term "undeserved" connotes that the DCS hit was a "punishment" for being reckless or following poor diving practice. I prefer using the term "unexpected," since it doesn't overstate how deco algorithms were developed and tested. Using dive computers and adhering to the implemented deco algorithm will only drive down the probability of DCS...but not eliminate it altogether.

 

[Sas]

By "undeserved," you mean that the diver did not exceed NDLs and recommended ascent rates, right?

I cringe whenever people use that terminology, though. The term "undeserved" connotes that the DCS hit was a "punishment" for being reckless or following poor diving practice. I prefer using the term "unexpected," since it doesn't overstate how deco algorithms were developed and tested. Using dive computers and adhering to the implemented deco algorithm will only drive down the probability of DCS...but not eliminate it altogether.

Sure, fair enough. I was just using common terminology as well as quotation marks.

 

[MaxBottomtime]

I had another thought. Making a deep stop helps to eliminate the fatigue some divers feel on air. Maybe the agencies don't want to mention deep stops so they can sell nitrox certs.

 

[MXGratefulDiver]

I had another thought. Making a deep stop helps to eliminate the fatigue some divers feel on air. Maybe the agencies don't want to mention deep stops so they can sell nitrox certs.

I'm not so sure it's that so much as inertia ... it's very hard to get training agencies to embrace anything they haven't been teaching for years.

... Bob (Grateful Diver)

 

[dumpsterDiver]

Context.

I doubt your diving habits are within the boundaries of which they are talking about.

Maybe, but I am getting older and if a deep stop lets me get away with some more aggressive diving, then I am going to do it, even if DAN thinks it is not necessary.