Deep Stops Increases DCS

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My observations as simply a guy that has been playing the tech diver game for a couple decades.

In the early 90's, bend and mend was common, "bigger dives" (now not so big) were quite experimental for non commercial/navy ops and we saw folks on big projects using custom tables generated by some of the deco guru's that were on the bleeding edge. In those days, many of us got bent to varied extremes pretty often. Skin bends and tingles/extreme fatigue was not uncommon for me and those I dove with. Around the later part of the 90's people started to talk deep stops, often pointing to Pyle. When most of us simple divers started to introduce deep stop we saw an immediate improvement in being able to get out of the water not bent. With some extreme exceptions (WKPP and a few projects), most of our tech diving was in the 160-300 range with run times of less than 3 hours and dang few of them on Trimix. Because we saw improvement using deep stops there is a bias towards them.

The pool of knowledge and the depth/duration of dives have continued to expand. For the dives like done in the old days (but trust me, you are MUCH better off with helium in th mix! :) ... yeah RGBM, VPM (whatever) seems to work and I wouldn't hesitate to continue to use, HOWEVER...if I was doing multiple day profiles to the depths and duration that have been opened up by CCR I would be looking at the latest of research combined with paying careful attention to my body on the work up dives to the seriously big dives possible and being conducted today. Regardless of any algorithm in use, the variables within our own bodies cause a great swing in deco efficacy. People doing these dives that require this much scrutiny of what algorithm to follow also need to understand that while you may get away with trying to buy a "turn key" deco solution sometimes, it is unlikely that you will get away with it always.

Ultimately, that is part of this thread discussion, consumers are looking for a solution they can just use and trust. If the science isn't changing when new evidence presents then why exactly should it be trusted? Regardless, even when you trust in deco, remember that all deco algorithms are theory, your body is reality and you get to cash that check.

We used to teach that helium went in fast and came out slow...wasn't that a special kind of stupid? There are divers and even instructors out there that still believe it.
 
Only if I'm saturated.

If I go up a foot on a typical dive, I'm still on-gassing...
what tissue? LOL
 
I think the quest for an exact definition of "deep stop" will come up empty. If you worked through the RBW thread, you'll see that at some point I started using the more relative terms "deeper stops" and "shallower stops", or "deeper stop profile" and "shallower stop profile". It was just my conclusion that a bright line test would likely fail no matter what line was drawn. ...//...
It seems that your 'deep' and 'shallow' refer to some fuzzy undefined concept that all the experts more or less acknowledge. That's fine for you guys, but could you transform that into something that the general diving population can understand?

So are your deeper stops above or below the depth that you just begin to off-gas? There has to be a marker somewhere. The point that I like (for comparison purposes) is the point at which one just begins to off-gas. Nothing is said about how efficient the off-gassing is, but at that point, the coin flipped. You go from on-gassing to off-gassing. I can't help but believe that such a point exists. I'll admit that there are way too many variables to nail it, but one can approximate...
 
It seems that your 'deep' and 'shallow' refer to some fuzzy undefined concept that all the experts more or less acknowledge. That's fine for you guys, but could you transform that into something that the general diving population can understand?

So are your deeper stops above or below the depth that you just begin to off-gas? There has to be a marker somewhere. The point that I like (for comparison purposes) is the point at which one just begins to off-gas. Nothing is said about how efficient the off-gassing is, but at that point, the coin flipped. You go from on-gassing to off-gassing. I can't help but believe that such a point exists. I'll admit that there are way too many variables to nail it, but one can approximate...
back to the tissues I would think, short exposures you are getting that coin flip easily, longer exposures not so much, more in play
 
In the early 90's, bend and mend was common, "bigger dives" (now not so big) were quite experimental for non commercial/navy ops and we saw folks on big projects using custom tables generated by some of the deco guru's that were on the bleeding edge. In those days, many of us got bent to varied extremes pretty often. Skin bends and tingles/extreme fatigue was not uncommon for me and those I dove with. Around the later part of the 90's people started to talk deep stops, often pointing to Pyle. When most of us simple divers started to introduce deep stop we saw an immediate improvement in being able to get out of the water not bent.

Hello,

Interesting comments, thank you, and also to the other discussants seeking a definition of deep stops. This is a very difficult one. Let me explain my take on it.

Lets start from a perspective we should all agree on. Consider a hypothetical decompression dive where a diver accumulates a significant decompression obligation and comes straight to the surface with no stops. They will almost certainly suffer decompression sickness. Coming straight to the surface is the ultimate (albeit ridiculous) "shallow stop" decompression. I think that we can all agree that some stops are necessary, and all of these will be "deep" in relation to the ultimate shallow stop decompression (coming straight to the surface).

Lets say that we have a fixed amount of time to use for decompression (because anyone can design a superb decompression if you can spend unlimited time decompressing). In a hypothetical world where we could conduct infinite test dives for decompression within that time frame and it didn't matter if we hurt the subjects, we could start gradually filling in stops for this hypothetical dive across multiple trials.... making them progressively deeper and / or longer and somewhere along the way we would hit the "truth in the universe" about optimal decompression for the dive and the correct depth for the deepest (first) stop. In that context, your story quoted above about stopping deeper and getting better outcomes is obviously plausible. But inserting even deeper stops (beyond our optimal deepest stop) would start to become disadvantageous or unnecessary or both. Equally, backing shallower from the optimal deepest stop would also become disadvantageous. The key point is that without doubt there is an ideal deepest first stop for any dive, but we don't know exactly how to predict what it is.

Different approaches to making that prediction have focused on different philosophies / beliefs but without much supportive science for any of them. In the late 90s / early 2000s the idea of focusing on "deeper deepest" stops began to take hold for a variety of reasons, not least of which was the attraction of the bubble model philosophy of preventing bubble excitement and growth early in the decompression. In attempting to achieve this the bubble model decompression algorithms such as VPM and RGBM prescribed deeper stops than typical gas content algorithms, and were promoted as being able to get you out of the water quicker as a result of this early control of bubble growth. Virtually everyone started using "deeper stops" in some form or another. Even those not using bubble models started manipulating gas content models such as Buhlmann to make them behave like bubble models (typically through the use of gradient factors). But referring back to my hypothetical dive analogy above, none of this was ever studied in a way that would allow us to know whether the bubble modelers had hit that "truth in the universe" optimal spot for the deepest stops.

That's where the currently emerging evidence becomes relevant. There is now a growing body of evidence from studies conducting direct comparisons that bubble models have probably taken the deepest stops beyond the optimal depth. This evidence has been discussed previously, and there is more to come (hopefully soon) in the published literature. The NEDU study is one component of that evidence. There are still people entering the various threads who cannot accept this based on the fact that that study used air dives and a bubble model that did not produce a profile that looked exactly like one produced by typical technical diving bubble models (for example by VPM). This issue, and why the NEDU study is nevertheless highly relevant, has been extensively discussed both in this thread and others. To those struggling with this I suggest you try reading the relevant posts (there are a number by uwsojourner) carefully.

This story leads me to conclude that bubble models unnecessarily over-emphasise deep stops. It does not mean they don't work, or that you should not use them. But the optimal deepest stop appears to lie somewhere shallower than typically prescribed by a bubble model; unfortunately we don't know exactly where it is. Our advice for changing practice has consequently been cautious but it is documented in various places. Getting to the point of the most recent posts on this thread, I hope you can see why it is incredibly difficult defining "deep stops" . Returning to my hypothetical dive above, perhaps the best we can do is to borrow from George Orwell and say that "all decompression stops are deep, but some are deeper than others".

Simon M
 
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It seems that your 'deep' and 'shallow' refer to some fuzzy undefined concept that all the experts more or less acknowledge. That's fine for you guys, but could you transform that into something that the general diving population can understand?

So are your deeper stops above or below the depth that you just begin to off-gas? There has to be a marker somewhere. The point that I like (for comparison purposes) is the point at which one just begins to off-gas. Nothing is said about how efficient the off-gassing is, but at that point, the coin flipped. You go from on-gassing to off-gassing. I can't help but believe that such a point exists. I'll admit that there are way too many variables to nail it, but one can approximate...

I suspect that the depth where ALL your tissues were off gassing would be pretty shallow indeed. The crux (as I understand it) of this discussion is exactly this point, that the fast tissues off-gassing and the slower tissues on-gassing at depth is a trade-off, and the NEDU study seems to point to the tipping point being shallower than the received "deeper stops are better" wisdom.

At least, thats what Ive picked up between the mud projectiles anyway so YMMV.
 
Simultaneous post with Dr Mitchell. He said it better than me.

Obviously.
 
It seems that your 'deep' and 'shallow' refer to some fuzzy undefined concept that all the experts more or less acknowledge. That's fine for you guys, but could you transform that into something that the general diving population can understand?

So are your deeper stops above or below the depth that you just begin to off-gas? There has to be a marker somewhere. The point that I like (for comparison purposes) is the point at which one just begins to off-gas. Nothing is said about how efficient the off-gassing is, but at that point, the coin flipped. You go from on-gassing to off-gassing. I can't help but believe that such a point exists. I'll admit that there are way too many variables to nail it, but one can approximate...
I don't want to ignore your questions. But I would refer you to Dr. Mitchell's post above. And RainPilot's too, I think, was quite good as a summary statement.

I just want to point out that the fuzziness you perceive in my language of "deeper stops" or "shallower stops" actually also resides within your "just begin to off-gas" language. Yes, there is a depth at which you "just begin to off-gas" if by that you mean the earliest depth at which your tissue model detects ANY supersaturation in ANY compartment.

But it's fuzzy language indeed to say "I'm off-gassing" when you consider that at that depth you're just barely supersaturated in your fastest compartments, let's say compartment 1, but the remaining 15 compartments are happily on-gassing as you say that. It might be akin to saying, "I've surfaced" when the first portion of your hair just pokes out of the water. You could say that, I guess, but most people would probably perceive that you're still in the water.

So to use the language "I'm off-gassing" at that depth is also fuzzy and I think obscures the true state of things relative to the other compartments which are still taking up gas.
 
You are saturated on the surface at 1 ATM of pressure (14.7 psi). Moving up to 18,000 feet cuts pressure by half 7.34 psi, and 34,000 feet halves it again 3.62 psi and this pressure change would create decompression as you reduce the ambient pressure. Anytime the pressure is greater than 14.7 psi you are on-gassing. I don't know why this is difficult to understand.
 
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