Re-Evaluating My GF

[boulderjohn]

... some of the Doppler studies have shown big improvements in changing the Lo from say 30 to around 50, ...

Can you cite these studies? (I am writing an article on the current thinking on this topic, and I want to include everything.)

 

[JonG1]

From memory there is a lot in this thread, but don't know if the studies are/were published:

My DCS Hit

Quel Trimix pour quelle plongée : exemples et (…) - Club de plongée d Aix les bains, lac du Bourget (Savoie)

 

[JohnnyC]

Pyle's anecdotal data (remember the plural of anecdote isn't data) on his stops is REALLY old. Like, early 90's if I remember correctly. And it was never an empirical study, he simply stated that he felt less fatigued when he stopped on the way up to deflate swim bladders.

The NEDU study certainly isn't analogous with normal tech diving practices, remember that it was an experiment designed to eliminate as many variables as possible. However, what the profiles showed was that an approach that favored starting decompression with deeper stops led to increased nitrogen loading, and thus increased decompression stress, for a given time. The Spisni study played that out ever further (albeit with profiles more akin to modern day technical diving practices) where the UTD guys running their ratio deco profiles started their decompression stops deeper, and despite increased shallow O2 time, still had higher levels of decompression stress as measured post-dive.

Dr. Mitchell's use of high GFLo, and why it isn't currently higher, is, and I'm paraphrasing here, simply backing off the deeper stop profiles in a conservative manner by moderately increasing his GFLo as opposed to jumping right up to say, 80/80, etc. There's nothing to say that 80/80 isn't potentially the "right" answer, just that he's careful not to make too big a leap at this point.

We can extrapolate that ANY doppler study is going to show less bubble formation with a higher GFLo (say 50/80) when compared to older approaches (30/70 for example) because there is less on-gassing during the deep stops portion of the ascent.

I'd be interested to see COMEX's extrapolated data if you have a link available. Ultimately we all do what works, but the 50% of max depth doesn't really have any evidence grounded in empirical data it would seem. And we need to take everything in context (NEDU study being the most apparent) if we're going to start changing our practices.

ETA: The French link you posted still heavily penalizes the use of helium in mixed-gas diving. More current research discredits the helium penalty as adding additional decompression time based on the composition of the gas. Dr. Mitchell is actually of the opinion, and again paraphrasing, helium doesn't require more deco, however with the implementations of the current "helium penalty," we're probably doing the right amount of decompression for those dives, just for the wrong reasons. The helium isn't what's requiring increased decompression obligations. Lots of understanding has occurred since those papers were published, and even then, those papers referenced didn't take into consideration much of the current thinking.

FWIW I dive 50/80, up from 30/80, and do feel better after decompression dives. I don't add any additional deep stops. Anecdotal for sure. And I would NEVER dive 20/70. Waaaaay too deep based on current research.

 

[JonG1]

Pyles stuff was empirical though because it was based on experience and observation, the definition of empirical is:

"based on, concerned with, or verifiable by observation or experience rather than theory or pure logic"

DAN's study was old too and is referenced in Deco For Divers (pg 143) but both Pyle and DAN were at or around 50%, I just wonder if everyone got a bit carried away and started stops deeper and deeper, without compensating shallow and that's where things went wrong.

Comex is linked in my previous post and yes have read that SM, is kind of splitting the difference in the absence of any better understanding between the 2 poles at this point.

 

[JohnnyC]

Pyles stuff was empirical though because it was based on experience and observation, the definition of empirical is:

"based on, concerned with, or verifiable by observation or experience rather than theory or pure logic"

DAN's study was old too and is referenced in Deco For Divers (pg 143) but both Pyle and DAN were at or around 50%, I just wonder if everyone got a bit carried away and started stops deeper and deeper, without compensating shallow and that's where things went wrong.

Comex is linked in my previous post and yes have read that SM, is kind of splitting the difference in the absence of any better understanding between the 2 poles at this point.

I'd argue that it isn't empirical because it's neither verifiable nor observational by any quantifiable means. It's certainly anecdotal, but there's no quantifiable data to justify his observation. He simply says that he felt less fatigued. There's no way to actually measure any of that. Who knows if he had an extra cup of coffee in the morning, or if he got an extra hour of sleep. What if his buddy feels MORE fatigued? The problem with Pyle stops is that they have always been inherently unscientific. There's no data to actually back up anything that he suggested. In fact, all of the theoretical data that supposedly bolstered his position is being disproven the more studies are conducted and the more we understand about our physiology. As a whole the industry is moving away from bubble models, even RGBM hold outs like Suunto are introducing Buhlmann with gradient factors as options on some of their computers as consumers move away from those types of models.

Regardless, it's pretty old news and the current data disproves the idea that his stops are beneficial. At best they don't do anything, at worst they contribute to increased gas loading. Either way, as long as you're not getting bent doing those stops, drive on.

I'm not really sure that it's splitting the difference in understanding. The physiology works the same no matter what, and the helium penalty was all theoretical in the first place. I think the bigger issue is that we're trying to mathematically model a physiological process that has an untold number of unaccounted for variables. I'm sure eventually we'll crack the egg with a dive computer that realtime analyzes our actual inert gas loading and generates a decompression plan based on what's actually occurring in our bodies. Maybe then we won't have to bother with any of this anymore. Ultimately, we dive the profiles we're comfortable with and as long as we remain on the right side of DCS, we're all doing well.

 

[boulderjohn]

From memory there is a lot in this thread, but don't know if the studies are/were published:

My DCS Hit

Quel Trimix pour quelle plongée : exemples et (…) - Club de plongée d Aix les bains, lac du Bourget (Savoie)

In the first case, I read through the first 150 of the more than 400 posts in the thread and did not see any references to what you described. I saw a very familiar thread, one I have read before. If you think the studies you are remembering come from Simon Mitchell, then I already have them, and don't recall any that match your description. (I am collaborating with Simon on the writing of the article.)

In the second case, my French is not up to the task for good understanding, and the translation I used was not much better. I do not fully understand the source of the beliefs expressed in the article. I further see it as to some degree contradicting the idea and calling for pretty deep stops on the deeper dives.

 

[boulderjohn]

From memory there is a lot in this thread, but don't know if the studies are/were published:

My DCS Hit

I am still working my way through this thread, but I highly recommend it. It has a lot of useful information.

One aha! in it was that the diver whose DCS is being examined used a much faster ascent rate than the normally recommended 30 FPM, and the impact that had on supersaturation was remarkable. There is more to ascent than the choice of GF. (BTW, I know the OP in this thread uses a normal 30 FPM ascent rate.)

 

[JonG1]

This is another relatively recent thread:

Deep Stops

JohnnyC why do you dive 50/80 given your comments about recent research, wouldn't 80/80 be more reflective if current thinking?

 

[JohnnyC]

This is another relatively recent thread:

Deep Stops

JohnnyC why do you dive 50/80 given your comments about recent research, wouldn't 80/80 be more reflective if current thinking?

I'm conservatively working my way that direction, just backing off slowly. It's been a little while since I've been in the water on deco so I'm evaluating my increases in GFLo over time. Like anything tech diving related, I'm working my way cautiously from what worked, to something theoretically more efficient and safer. We'll see how it goes.

 

[Celt]

I think you may looking at the wrong part of your dive for the reason for the bend. Maybe you should have a long think about your bottom time and how you we’re feeling physically. I always shorten my bottom time when I don’t feel 100% and increase my deco.