Hypothetical question about decompression

[Gene_Hobbs]

Nice thread, but devoid of any real publications...

A nice starting point for the history of ascent rates and current recommendations is found here:
Lang, M.A. and G.H. Egstrom (eds.). 1990. Proceedings of the AAUS Biomechanics of Safe Ascents Workshop held Sept 25-27, 1989. Woods Hole, MA. 220p. RRR ID: 4241

Haldane's classic work:
Boycott AE, Damant GCC, Haldane JS. The prevention of compressed air illness. J Hyg. 1908;8:342?43. RRR ID: 7489
FUN QUESTION: If Haldane had not been related to the Minister of War, would Hill's work have been more fruitful?

Tables are only as good as the data they are based on... Changing the ascent rate for a table developed and tested using another is an interesting problem.
Excellent work on the PADI Table development:
Hamilton Jr, RW; Rogers, RE; Powell, MR. 1994. Development and validation of no-stop decompression procedures for recreational diving: the DSAT recreational dive planner. Diving Science & Technology Corp. RRR ID: 4228

The best publication I have seen on effectiveness of safety stops is here:
Uguccioni, DM. 1994. Doppler Detection of Silent Venous Gas Emboli in Non-Decompression Diving Involving Safety Stops. MS Thesis to University of North Carolina at Wilmington. RRR ID: 3430

The most recent look at ascent rates has only been published here (too much time working and not enough time writing... Few of my bad days at work are represented in this data. :depressed: ):
Vote, DAP; Vann, RD. 2003. DAN USA Ascent Rate Study (Abstract only work in progress) Journal of the South Pacific Underwater Medicine Society. Volume 33 Number 1. RRR ID: 7760

Happy holidays and ENJOY!!

 

[Blackwood]

So where does that leave the "required safety stop" if you come within three pressure groups of a "no-decompression" limit? if the distinction between a safety stop and a decompression stop is that the former is "recommended" and the latter is "required," then it looks to me as though the actual no-decompression limit is three pressure groups above the spot indicated as such on the RDP.

Clearly, as you said, it's a question of terminology. But the PADI terminology seems inconsistent.

There's no inconsistency.

The difference is that said "required" safety stop isn't required to satisfy the decompression model, it's to satisfy the author of the table annotation (and likely the lawyers turning the screws).

 

[wve]

Nice thread, but devoid of any real publications...

Yes! I was wondering when you were going to jump in and help with that...

Any references for Hill's original work?

 

[Gene_Hobbs]

Any references for Hill's original work?

Hill's book is the best I have seen...

Hill, L (1912). Caisson sickness, and the physiology of work in compressed air. London E. Arnold.

I added a few other refs to his life on the wikipedia page a while back so they might be easier for most folks to find: Sir Leonard Erskine Hill

SEARCH TIP: When looking for books, check WorldCat

...and this book is found here (OCLC Number: 14784965).

NOTE TO SELF: Pull and check copyright status...

 

[daniel1948]

There's no inconsistency.

The difference is that said "required" safety stop isn't required to satisfy the decompression model, it's to satisfy the author of the table annotation (and likely the lawyers turning the screws).

THANK YOU! That makes a lot of sense.

 

[wve]

Hill's book is the best I have seen...

No kidding! Hill's book (1912) was written after Haldane's (1908). So Hill got the chance to defend himself.

Hill found that goats were not the best animal for testing decompression theory. He tested lots of different animals and determined that pigs were actually a better model for a variety of reasons, not the least of which is that they tend to be fat and lazy -- like humans when diving.

So then he took the pigs and decompressed them in different ways. One week he would decompress them using a staged decompression. The next week he used a linear (or, as he called it, uniform) decompression. Finally, he used a method which was curvilinear. That is, as the pressure decreased, he slowed the rate of decompression. (Similar to going slower the nearer you get to the surface.)

Result: No difference. The same number of animals got sick or died regardless of the method used.

In his words:

The conclusion to be drawn, then, from experimentation on animals is that there is evidence in favour of stage decompression after short exposures, but no decisive evidence of its superiority after long exposures. The theory is a very captivating one, but experiment has not brought conclusive support which was to be expected, and there is no evidence to substantiate the statement ... [that] uniform decompression, to be safe, has to be so prodigiously slow that it is altogether an irrational proceeding.

So, as is so often the case in science, dogma derives as much from who you know as much as what you know. Haldane had the right connections and the good fortune to have his theory adopted by Navy divers so that it became the standard practice which was, in turn, perpetuated and built upon.

As it turns out, Haldane and Hill may have both been right. Haldane's staged decompression may have been better for diving applications (short exposure), and Hill's more applicable to commercial applications (coral harvesting, bridge building, etc).

Still, this whole historical situation is curiously similar to how we decide the correct dose of chemotherapy. Some time ask yourself, "Why do we calculate chemotherapy doses based upon how much skin a person has?"

Good question. The answer is: dogma. You can trace it back to one, single study that was done on animals decades ago. By default it became standard practice. Over time people found that it mostly works. And nobody has figured out a better, more convenient or practical way to do it. So we just keep on doing it that way even though there are numerous theories about how we could do it better. Pragmatism trumps theory again.

And the same goes for diving. As I said in an earlier post, when underwater, I don't really want to be that guinea pig. (Well, actually, guinea pigs do better with rapid decompression than goats or pigs, but you know what I mean.) I'll just stick with what works, even if it doesn't always make intuitive sense.

 

[DwayneJ]

Hill's book on Google...
Caisson Sickness: And the Physiology ... - Google Book Search

Thank you Gene - An amazing read.