Is conventional wisdom just conventional?

[knotical]

I first posted this in the "Advanced Scuba Discussions" forum. (see: http://www.scubaboard.com/forums/ad...10-conventional-wisdom-just-conventional.html )

Conventional wisdom has said:
- bounce dives,
- saw-tooth dives,
- "backward profile" dives, and
- reverse profile dives
are all bad.

But are they really bad? How much science is behind those assertions?

I suggest that as long as we stay within the limits of whatever model we are using, these practices are no more dangerous than the degree to which they put us near those limits.

Bounce dives are just dives with rapid descents, short bottom times, and short ascents. As long as the ascent is sensible, plug them into your model and carry on.

Saw-tooth dives: Is deep-shallow-deep really bad? As long as we follow our model's ascent rules, it should be OK shouldn't it?
And what defines a saw-tooth? How many feet of re-descending does it take to qualify as a saw-tooth?

"Backward Profile" (dive that starts shallow, but gets deep toward the end of the dive): Shouldn't a proper ascent be sufficient to make this safe?

Reverse Profile (deeper dive after shallower dive): This one has some studies that say the earlier prohibition was unfounded. See:
Divers Alert Network : http://www.diversalertnetwork.org/membership/alert-diver/article.asp?ArticleID=674

A remaining issue with reverse profiles is the reduction of bottom time.
To see this, consider (for example) the following dives on PADI tables:
Dive A: 90 feet for 21 minutes
Dive B: 60 feet for 37 minutes
Surface interval: 60 minutes
If you do Dive A before Dive B, no problem.
But if you try to do Dive B before Dive A, the model says you shouldn't do the second dive.
The model would let you do the deeper dive after the shallower dive only if you reduce your bottom time for Dive A to 14 minutes, or extend your surface interval to 144 minutes.

I welcome the usual opinions and anecdotes, but it would be nice to see some citations of authoritative sources, too.

thanks,

k

Responders seemed to concur that reverse profiles and "backward profiles" were probably OK, but bounce and saw-tooth drew concern. Two theories were advanced as to why, but neither seemed quite right to me. And then the discussion fell silent. Perhaps posting in this, the "Ask Dr. Decompression" forum is more appropriate, with its somewhat different audience, especially Dr. Deco.

So I'm asking my questions again here. How wrong am I? Are there studies / references / empirical data that can be cited?

Thank you.

 

[AzAtty]

I'll respond with something clarifying one of your unanswered questions based on what I understand.

If you bounce dive after diving, you could recompress a bubble in the blood allowing it to cross to the arterial side.

I've never understood this. If a small bubble can cross to the arterial side, why didn't it cross when it was small the first time? Do you have a reference that discusses this?

The simplest answer is: the bubble did cross to the arterial side when it was small. But you're looking at the wrong bubble, which is why you never understood the explanation given.

A bounce dive profile looks like a spike--down and up pretty quickly. Bouncing on the first dive, as I understand it, isn't a huge issue if you're descending and ascending at a reasonable rate. My understanding is that bouncing AFTER a previous dive poses a potential issue, particularly with staged decompression dives. The classic cases are bouncing to pull a hook from a wreck or to retrieve discarded deco or stage bottles.

When we surface, we're still offgassing. The bubbles in our system when we surface are generally the happy-go-lucky sort that don't harm us, and are quite cheerfully reunited with their other chums in the atmosphere as they are filtered out of venous blood by the lungs.

BUT bubbles tend to increase in size for some time after surfacing. That may be one reason why some DCI symptoms manifest after leaving the water. It's the larger bubble that has come into existence after you surfaced that concerns us.

What would happen if the larger bubbles that came into being during your surface interval were compressed to the point that they could bypass the pulmonary filter and get into the arterial side? Descending compresses the larger bubble, the "crushed" bubble could bypass the pulmonary filter, and could transit to the arterial side. Now you have a large 1 ATM bubble compressed to, say, 3 ATM, running amok on the arterial side. Boyle's law won't be your friend when you start to ascend from the bounce and that 1 ATM bubble expands to is former size. But this time it's on the arterial side of things. Congratulations, you have a do-it-yourself AGE.

Now, were this just a regular repetitive dive, the bubble would most likely filter out on its own while at depth. But with a bounce, you crush the bubble and re-inflate it in rapid succession, and that's a problem.

That's the concept JeffG was explaining. It's just that most people look at the "wrong" bubble when trying to wrap their head around the concept.

I'll leave the rest to Dr. Deco, since he's smarter than I am.

 

[DocVikingo]

Excellent thread topic, knotical.

Thanks,

Doc

 

[mongodives]

I'm kind of curious if there is any studies etc. as well, especially about saw tooth profiles.
I've always been curious what kind of deviation and frequency was actally used/tested, or if its just an academic application of the gas laws?

 

[DevonDiver]

I was alays under the assumption that bounce dives (and free diving) after diving lead to recompression of non-symptomatic bubbles...allowing them to move from tissues into the circulation, bypass the lung filter and enter the arterial circulation. At that point, even re-expanded, the bubbles remain non-symptomatic. BUT as they travel freely around the arterial side, they are more likely to meet and merge into large symptomatic bubbles.