Deep Stops Increases DCS

[KevinNM]

A lot of decompression research was done to support high altitude aircraft in WW2. Iirc, that is where the dehydration/DCS connection was made.

 

[boulderjohn]

A lot of decompression research was done to support high altitude aircraft in WW2. Iirc, that is where the dehydration/DCS connection was made.

A lot of decompression research is still being done for high altitude flying. In the ScubaBoard Ask Dr. Decompression forum, Dr. Decompression is Michael Powell, who helped create the PADI tables but who chiefly worked studying decompression for NASA. In relation to these continuing debates, I had the opportunity to talk with a diver who consults with NASA today and who also consults with the Pentagon on decompressing U2 pilots. He gave me a lot of insights on the issues that were over my head on these threads.

 

[lowviz]

...//... 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. ...//...

I agree completely with everything that you write. Not splitting hairs here. What I'm searching for is a point somewhere at depth (during final ascent) where everybody agrees that you when taken as a system are just beginning to lose your nitrogen load.

So I repeat: what is the deepest point at the beginning of your ascent from a given dive where the net effect on your body is off-gassing? That point has to exist and it could very well mark the first depth where one may wish to linger for a minute or two. Even more importantly, it could be a point from which to label 'deep' or 'shallow'. I won't make a big deal of this, just a question that I'm not able to answer by my own research.

Yes, indeed, some tissues will be on-gassing and others will be off-gassing. I would expect that. But the result of ascending to this point puts you at the very beginning of off-gassing when you are taken as a single compartment. That is a first-order approximation. The vagaries of fast and slow compartments can now be added as higher order approximations.

I'm looking to see if there is a place to start. That is the entire intent of my post.

 

[KevinNM]

Tissue loading varies by model. Some are very, very different than the ones we usually think about.

 

[cerich]

Hello,

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.


Simon M

the other factor that came into play for many at the same time we started to incorporate "deep stops" was more Helium use. Because of that there may be some misinterpretation of exactly what was making us feel better. But feel better was a pretty common comment.

 

[Ayisha]

what is the deepest point at the beginning of your ascent from a given dive where the net effect on your body is off-gassing? That point has to exist and it could very well mark the first depth where one may wish to linger for a minute or two. Even more importantly, it could be a point from which to label 'deep' or 'shallow'.

This is not based in science at all, but personally, for "deep" dives within the recreational realm, I like to do my first 1 minute stop no deeper than 50 feet. Back in the day when there were recommendations of doing the first deep stop at 55% or 60% of your maximum depth, I still kept it to 50 feet max. Then around 2009, there were recommendations to increase the deep stop to 2 - 3 minutes, and the stories were coming out right away about people getting bent using deep stops on dives to technical depths.

I figure while there is some on-gassing at 50 feet or less, there is also off-gassing. Again, not based on anything but personal conjecture. Perhaps it's a moderate stop as opposed to a deep or shallow stop? YMMV.

 

[KenGordon]

What I'm searching for is a point somewhere at depth (during final ascent) where everybody agrees that you when taken as a system are just beginning to lose your nitrogen load.

So I repeat: what is the deepest point at the beginning of your ascent from a given dive where the net effect on your body is off-gassing? That point has to exist and it could very well mark the first depth where one may wish to linger for a minute or two.

Since the compartments are a fiction and do not represent particular organs or tissue we can't really compare between them. So if 1 to 3 are off gassing but 4 to 16 on gassing is that a net on gas or off gas?

Also, it may be that the slowest compartments are so far away from having enough gas load that they might prevent immediately surfacing that we really do not care if they are net on gassing or off gassing.

The maths is such that each tissue compartment has an effective saturation depth. That is the depth which would neither lead to on gassing or off gassing. That depth tends towards the current depth but generally doesn't catch up except in the faster compartments in a typical recreational deco dive (I'd non saturation). The fastest compartment will catch up and as soon as you go up will start to off gas.

In the gas content models the fast and slow compartments differ by half time (typically from 4 to 600 minutes) AND by the limiting over pressure, which is generally lower for the slower tissues.

Having a play with the numbers may lead to an 'ah, I see' moment, depending on how you think.

 

[The Chairman]

Thanks to everyone for their thoughts and insights in this thread with special thanks to those who answered my questions directly. I think it was Uncle Ricky who once opined that deco theory was tantamount to measuring with a micrometer, marking with a chalk and then cutting with an axe. Or as I like to categorize it; a SWAG. While we all agree that what goes in, must come back out we seem to be at odds in regards to the mechanism, the timing and which tissues are most critical.

@cerich, how many tables have we cut? I'm at about 50. You're probably way, way ahead of that. When I first started, 'plan your dive, and dive your plan' was essential and PDCs were seen as a quick way to the chamber. I think the common mantra was "Computers rot your brain". Today, people still plan, but they do it right on their PDC to determine how much gas they need. After that, they just dive their PDC. While I've heard a couple of instructors lament this trend, it's more and more an accepted practice. Personally, I like where we're at and where we're going.

But, that still leaves our tech wannabe/light tech/pushing the NDL diver out in the cold. How does this affect them? I remember how Deep stops really made a difference for me when I first incorporated them into my diving. These divers usually have plenty of gas to handle deep stops or even a short incidental deco stop.But, for the most part, they are ascending within NDL: no stops required. Hell, I fit into this group quite often when I'm ocean diving. Is there a reason why I shouldn't a deep stop in this case? By deep stop, I'm meaning half of my max depth for for a minute or two. Should I change that to a third, now? Should I discontinue them? To clarify, I do 5 minute safety stops, not 3. I think the extra two minutes are important when you're riding the NDL with a PDC.

 

[Dr Simon Mitchell]

But, that still leaves our tech wannabe/light tech/pushing the NDL diver out in the cold. How does this affect them? .

Hello Pete,

To be clear, the debate about "deep stops" as taking place here is really about decompression diving as you have pointed out. The status of "half depth stops" (also sometimes referred to as deep stops) on ascent from sub-NDL dives is not really part of this discussion. What I can tell you is that there is conflict in what little relevant literature exists on that topic. There have been two studies that looked at the effect of this sort of "deep stop" on venous gas emboli numbers after surfacing and they showed opposite results (that is, one showed a deep stop reduced bubbles and one showed that it increased them).

Not much help I know. Sorry.

Simon M

 

[leadduck]

I agree completely with everything that you write. Not splitting hairs here. What I'm searching for is a point somewhere at depth (during final ascent) where everybody agrees that you when taken as a system are just beginning to lose your nitrogen load.
So I repeat: what is the deepest point at the beginning of your ascent from a given dive where the net effect on your body is off-gassing? That point has to exist and it could very well mark the first depth where one may wish to linger for a minute or two. Even more importantly, it could be a point from which to label 'deep' or 'shallow'.

I think a concept that comes close to what you are looking for is called a "profile dependent intermediate stop" (PDIS) in Uwatec/Scubapro computers. It is the depth to which you have to ascend before the medium-fast compartments switch from on-gassing to off-gassing. That's about the halftime that is commonly associated with the sensitive spinal chord and I find it an interesting idea. Bubble models insert deep stops to limit the growth of their imaginary bubbles and may create stops at a depth where medium compartments are still on-gassing; whereas PDIS tells you the depth to which you have to ascend at least to get into the realm of off-gassing. I don't use it like a mandatory stop, it just tells me: get up quickly to this depth, and from then on ascend slowly.