Slow tissue on gas from stops

[UWSojourner]

Thanks for the link and clarification. Didn’t mean to assign labels. For the dive I was talking VPM-B and ZHL-B GF 30/80 produce a first stop deeper than ZHL-B 50/80. Although not Dr Mitchell’s intent 50/80 has become a bit of a measuring stick.
For the dives I am concerned with, and I think are applicable to a lot of divers, within the limits of Tec50 and AN/DP, VPM-B and ZHL-B 30/80 produce very similar profiles. So in that context VPM-B and GF 30/80 are similar enough to each and both dissimilar enough to GF 50/80 to be categorized together. I’m sure at greater exposures the differences are more pronounced.
Again, didn’t mean to label anything.

Fair enough. I didn't see that you were talking about "light deco". You're correct that in technical diving where the deco obligations start to accumulate the problems in VPM-B, RD, etc. are more clearly seen.

 

[tridacna]

But in a study conducted in Adelaide 15 years ago,..

I say nuts to that data. Any study in which evidence is derived from a place that considers "pie floaters" haute cuisine must be discounted immediately. I went there once to present a paper and was almost 15 minutes late because of the Britannia Roundabout. And a local chap was driving.

 

[rossh]

Ross,

You have never quite got your head around the concept of decompression efficiency (the least risk for the same decompression time), or that this deep stops debate has always been all about that.

You believe in VPM-B. Fair enough. Let's say you plan a decompression dive using VPM-B (whatever conservatism level you choose), and it tells you that the total decompression time is X. Then someone else comes along and plans the same dive using a GF approach with shallower first stops, but with the GF adjusted so that the total decompression time is still exactly X; the same as your VPM-B approach.

It is an indisputable fact that your VPM-B approach will result in less supersaturation in faster tissues early in the ascent, and more supersaturation in slower tissues later in the ascent (because of the issue being discussed in this thread). In contrast the GF approach will result in more supersaturation in fast tissues early in the ascent, and less supersaturation in slower tissues later in the ascent. Both you and the GF diver believe their approach is optimal. Its like two deluded people both claiming to be Jesus; at the very least they cannot both be right (and most probably they are both wrong).

Which is why your statement....

....is nonsense. ​

They only "properly compensate" to satisfy the assumptions of the person who wrote the model. And in case you are tempted to launch off into a speech about your belief that GF is not a model, that would be missing the point. You clearly believe that VPM-B is a model, and I have articulated one of a number of ways in which someone might generate a decompression profile of exactly the same length but which distributes the pattern of supersaturation across the range of tissues very differently. I repeat, they can't both be right.

So which is right? This is where we come back to the current state of our available evidence, which suggests that decompression approaches prescribed by bubble models (and RD) incorporate deep stops that are too deep for the most efficient decompression (least risk for the same decompression time). I would remind you that all that evidence is pointing in the same direction, and there is none that supports the degree of emphasis placed on deep stops by bubble models. Relating that back to the subject of the thread, the protection of faster tissues early has not fulfilled the promise that many thought it would, and protection of slower tissues later in an ascent appears more important than many assumed.

Simon M

Simon,

You continue to use the words "efficient" and "efficiency" in invalid ways. Efficient in decompression planning and modeling is the "shortest" possible time while still generating a safe plan. But no one dives like that now, and its not been tested in the last 20+ years in any model we would use. All diving we do now is loaded with extra padded time, which makes them all deliberately "inefficient". As a result of all the extra conservatism and padding, every dive plan type works just fine, and so no one is testing the limits of efficient decompression.

When you try to argue efficiency, it is a red hearing. NO one today is testing the edge of safe deco. The entire emphasis today is to find and make excuses to justify longer and slower deco. Its under that umbrella concept that you falsely accusing VPM-B of problems that don't matter, or don't exist, and trying to influence a change based on a faulty premise.

No, the Nedu did not test efficiency - they did a "go slow" race between two shallow stop profiles with double elongated time, to see who could tough out the cold the longest - no deep stops to be seen here, and no efficiency test either..

If we wanted to organize our existing by efficient measures, for most of our tech dives, the real ZHL-C model would be the most efficient one. Then next its VPM-B (because its plans are longer than most ZHL-C examples).

But profiles with GF appended onto a ZHL-C plan can never be included in this, because GF has no basic definition or "standard" settings. No two people agree on what GF x/x values to use, so its an open ended dimension, which changes year over year. The very nature and purpose of GF adjustments is to take a model plan (ZHL-C) and modify it in such way, that it pleases and satisfies the wishes of the diver. Its use today is to make "dial a plan" profiles, that don't look anything like the underlying model, and that clearly changes the attributes and concept of plan away from the underlying model design. The application of GF over ZHL-C is so simplified, that its mathematically inconsistent, and cannot be used as a datum or baseline measure against other models.

A decompression model incorporates attributes and concepts that is given in its planning techniques and profile output, but GF has none of that, so it can never be classed as a model.


*********


Now *IF* you think VPM-B gas tracking is invalid or wrong, then YOU have to show why 40+ years of traditional model formula; Haldane Schriener, equations, (as used in VPM-B and ZHL) are in your opinion, no longer valid. These standard formula are used throughout most dive planning, and most dive computers. But until you can actually show that is somehow wrong, then you have NO complaint against VPM-B or its use of gas tracking formula. Of course this would also invalidate all your GF and ZHL planning too....good luck.

"...then from a DCS risk point of view, ..."

The universally accepted method of DCS risk reduction, is to lower supersaturation levels in the dive. That is how all decompression models function - a balance of this pressure vs time. The safest ascent possible is the one with no supersaturation. i.e . an ascent with a GF of 0/0. (which is really one big elongated deep stop). Of course all these realities conflict with your concept of "safe".



The basic theory of a VPM-B model, is it keeps the supersaturation lower across the whole ascent, and it finishes with about the SAME supersaturation levels as a traditional model would use. Yes, it is NOT essential to do this, and we can all tolerate high initial supersaturation as millions of traditional model ascents show that shallow stops are just fine.. But it does NO harm to do the deep stops either.


How much does deep stops really cost? What does it really add in terms of extra on gassing? Lets examine that in some examples:

Using EFX's dive planner tool and his sample dive:


This is the in-dive changes (tissue gradients):

link: hhssoftware.com/images/stop_ongas_compare_w.png


This is the surface supersaturation (gold line), and tissue gradients (blue / red line):

link: hhssoftware.com/images/deepstopeffect.png


Simon you, are making an enormous fuss about a tiny-tiny elevated surfacing supersaturation from deeper stop use, and its all trivial amounts.



Your entire argument about deep stop on-gassing, can be nullified and eliminated, by a 2 min change in bottom time. That's all your arguing about - the equivalent of a 2 minute bottom time change.


A 15 minute increase in bottom time, has increased the surface supersaturation, 7 times more than the deep stop effect....


Simon, You have all these people worked up in a tizzy, about some frivolous and meaningless on gassing change.... which is so small it is lost to the gray noise of deco practice, and completely covered and catered for, by one single step of conservatism.


So it seems you failed to do your math homework on this subject too.

.

 

[PfcAJ]

Simon,

You continue to use the words "efficient" and "efficiency" in invalid ways. Efficient in decompression planning and modeling is the "shortest" possible time while still generating a safe plan.

Ross, a key component you’re missing is defining “safe”.

If the DCS risk goes up with one ascent schedule vs another, even through they are the same length, then one of those is more efficient than the other. One schedule is a more efficient use of time than the other.

 

[UWSojourner]

Efficient in decompression planning and modeling is the "shortest" possible time while still generating a safe plan.

Not quite. An "efficient" plan is the shortest route to the surface for the same level of risk. Or, one plan is more efficient than another if it has lower DCS risk given the same decompression time.

So the NEDU study leads us to believe that, for example, VPM-B is not efficient due to the deep stops it imposes on divers. What the NEDU study seems to say is that you can take the surfacing time that VPM-B recommends and redistribute some of that time to shallower stops and you'll reduce your decompression risk. Same time, lower risk.

Now, what I think we've shown pretty conclusively is that the GF model IN ALMOST ANY CONFIGURATION does a pretty good job reallocating time to shallower stops when compared to VPM-B. It just naturally better reflects the NEDU's results than does VPM-B.

 

[Patoux01]

Ross, a key component you’re missing is defining “safe”.

If the DCS risk goes up with one ascent schedule vs another, even through they are the same length, then one of those is more efficient than the other. One schedule is a more efficient use of time than the other.

You have to at least acknowledge he improved his definition of "efficient" since the last (or was it a few before?) time. He no longer considers skipping deco to be efficient.

 

[TrimixToo]

Ross has a great product. It's flexible, convenient, and works well. It offers more than one algorithm to choose from and supports gas mixing and top-off predictions reasonably well (even if I sometimes wish for a bit more flexibility there). His licensing policy for new devices is very reasonable, and there is a smartphone version (even if it misses a few features of the PC version that I'd like to have on a boat). I use it for gas planning, even though I no longer do I use it for the actual schedules, since the actual dives are never square anyway and I have two computers that know what I'm breathing and exactly how deep I have been for how long. I use it for mixing, too. I have not encountered anything better yet. Though I'd be happy to learn about it if it existed, Multi-Deco and V-Planner before it are good enough that I probably wouldn't want to pay another license fee to get it.

All that said, I do wish he would stop arguing with Simon. We're all running out of popcorn, and Simon could be writing posts about other things we would doubtless all benefit from knowing more about.

Just another 2 PSI.

 

[rossh]

You have to at least acknowledge he improved his definition of "efficient" since the last (or was it a few before?) time. He no longer considers skipping deco to be efficient.

Rubbish. You are getting confused..... What you are describing comes from the RD and DIR classroom theory. VPM-B has never followed those ideas.

VPM-B adheres to the recognized and accepted scientific decompression formula and principles, and its peer reviewed. This is why we can say that VPM-B correctly adjusts for any on / off gassing in ascent or deco or any other part of the dive, just like any other model does, and your dive computer does now.

Ross has a great product. It's flexible, convenient, and works well. It offers more than one algorithm to choose from and supports gas mixing and top-off predictions reasonably well (even if I sometimes wish for a bit more flexibility there). His licensing policy for new devices is very reasonable, and there is a smartphone version (even if it misses a few features of the PC version that I'd like to have on a boat). I use it for gas planning, even though I no longer do I use it for the actual schedules, since the actual dives are never square anyway and I have two computers that know what I'm breathing and exactly how deep I have been for how long. I use it for mixing, too. I have not encountered anything better yet. Though I'd be happy to learn about it if it existed, Multi-Deco and V-Planner before it are good enough that I probably wouldn't want to pay another license fee to get it.

All that said, I do wish he would stop arguing with Simon. We're all running out of popcorn, and Simon could be writing posts about other things we would doubtless all benefit from knowing more about.

Just another 2 PSI.

Thank you for the compliments.

Yes Simon can be interesting and does find new relevant subject matter in diving to discuss,

Most of his new information is not peer reviewed, so someone has to check up on his ideas and math. In the area of decompression, Sadly that task seems to fall to me alone.

.
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[Patoux01]

@rossh , do you have a list of published articles you (co-)authored ?

 

[Dr Simon Mitchell]

Hello Ross,

Simon, You continue to use the words "efficient" and "efficiency" in invalid ways.

You appear to be the only person who does not get this. The only useful definition of efficient in a discussion about how to distribute your decompression stops across depths is "the least risk for a given amount of decompression time". You have been told this by true experts in the field time after time. Indeed, it was clearly stated by the author off the NEDU study right back in the early pages of the rebreather world deep stops thread:

The Navy Experimental Diving Unit (NEDU) “deep stops” study (Doolette DJ, Gerth WA, Gault KA. Redistribution of decompression stop time from shallow to deep stops increases incidence of decompression sickness in air decompression dives. Technical Report. Panama City (FL): Navy Experimental Diving Unit; 2011 Jul. Report No.: NEDU TR 11-06) was undertaken to determine if deep stops decompression schedules, such as those prescribed by bubble decompression models, are more efficient that the traditional shallow stops schedules prescribed by “Haldanian” models. More efficient in this context means a decompression schedule of the same or shorter total decompression time has less risk of decompression sickness (DCS) than an alternative schedule.

You have always tried to redefine efficiency to avoid the uncomfortable focus on the fact that your model appears to over-emphasize deep stops and in doing so does not optimize safety for a dive of given length.

As a result of all the extra conservatism and padding, every dive plan type works just fine, and so no one is testing the limits of efficient decompression.

If that were true we could stop worrying about having recompression chambers.

Now *IF* you think VPM-B gas tracking is invalid or wrong, then YOU have to show why 40+ years of traditional model formula; Haldane Schriener, equations, (as used in VPM-B and ZHL) are in your opinion, no longer valid. These standard formula are used throughout most dive planning, and most dive computers. But until you can actually show that is somehow wrong, then you have NO complaint against VPM-B or its use of gas tracking formula. Of course this would also invalidate all your GF and ZHL planning too....good luck.

and....

VPM-B adheres to the recognized and accepted scientific decompression formula and principles, and its peer reviewed. This is why we can say that VPM-B correctly adjusts for any on / off gassing in ascent or deco or any other part of the dive, just like any other model does, and your dive computer does now.

Ross, this particular line of argument sits alongside your recent insistence that it is tissue half times and not tissue perfusion that determines tissue gas kinetics, as unimpeachable evidence that you have very little knowledge of a field in which you portray yourself to be expert. It is all the more strange because you contradict yourself constantly on the subject. For example, in the very same post you say:

If we wanted to organize our existing by efficient measures, for most of our tech dives, the real ZHL-C model would be the most efficient one. Then next its VPM-B (because its plans are longer than most ZHL-C examples).

Here we have two algorithms which by your own admission are legitimate models, which by your own admission use the "Haldane Schriener, equations, (as used in VPM-B and ZHL)", and which by your own admission give decompressions of different lengths for the same dive. It is very unlikely that the profiles are iso-risk.

So, for the umpteenth time, yes all models use standard equations to track inert gas tensions and calculate supersaturation in tissues during a dive and decompression. But what the different models do with those calculated values (how much supersaturation they allow in which tissues and when) is (by your own admission above) different. You are making a monumental assumption that all related approaches are somehow equivalent and will get you to the same risk end point, but the truth is that different approaches will almost certainly have different risks, as for example, we saw in the NEDU study and have seen in other test dive programs. Which is why it is incredibly naive to argue that VPM's use of standard gas tracking formulae, of itself, legitmizes its decompression predictions, and somehow makes it equivalent to other approaches that use these formulae.

Parenthetically, it is also relevant to point out that you have very much changed your tune on this particular issue in comparison with years gone by when you used to trumpet VPM's superiority. How could it be superior in the context of your latest claims that use of the standard gas tracking formulae renders everything somehow equivalent?

The basic theory of a VPM-B model, is it keeps the supersaturation lower across the whole ascent, and it finishes with about the SAME supersaturation levels as a traditional model would use. Yes, it is NOT essential to do this, and we can all tolerate high initial supersaturation as millions of traditional model ascents show that shallow stops are just fine.. But it does NO harm to do the deep stops either.

Ross, this is why it is so important to understand the concept of efficiency. The question is, if someone was prepared to do the same length of decompression as prescribed by the VPM-B model for a particular dive, would they be safer if they did less deep stopping. The answer, based on the best (and only) evidence we currently have is yes.

How much does deep stops really cost? What does it really add in terms of extra on gassing? Lets examine that in some examples

I can't make sense of your graphs. They appear to compare the (alleged) supersaturation in a single leading tissue at a single point in time after surfacing. This is not an adequate analysis for comparing the impact of deep stops on subsequent slow tissue supersaturation. It is not only the single leading tissue (which is what I assume you mean by "significant cells") that contributes to risk. The sum of an integral of supersaturation and time across all tissues is what you need to calculate and compare, and some of this supersaturation occurs before surfacing. This was done for real-world type profiles by UWSojourner back in the deep stops threads here. The small differences between different GF configurations were noted then, but there is a bigger difference when comparison is made with a VPM-B+4 profile.

But even if we took your figures at face value, you at least appear to admit that there is some degree of extra slow tissue supersaturation occuring as a result of more deep stops. How would you know what degree of extra supersaturation in decompression can be written off as trivial? No one is arguing that as a result of doing deep stops everyone who does the dive will get bent. The real question is, what if 1000 divers did the 50:85 profile, and 1000 divers did the 20:85 profile. Would we see a difference in outcome? Based on current evidence it certainly seems possible. It would seem very unlikely that there would be benefit from doing the deep stops.

Simon M