Deep Stops Increases DCS

[rossh]

Hey Diver0001,

With regard to sufficient shallow time, I’ve seen it posted a lot that VPM "scales consistently and in the right proportions" as you go deeper and stay longer. I have another perspective on that idea. See the chart below.

View attachment 379689

Each of the lines shown on the chart represents the allowed supersaturations for each compartment upon surfacing from dives with progressively longer bottom times (all 200ft dives in this example). For this discussion the actual dive parameters (i.e. CCR or OC, depth, set point, gases) don't matter much. This pattern is reproducible in deeper dives when the effect of the critical volume algorithm (CVA) in VPM wears off. The CVA is that part of VPM that allows it to better reflect NDLs and lighter deco requirements. The important thing to understand about the chart is that we're seeing how VPM-B adjusts in the face of increasing gas loads due to the longer bottom times and deeper dives many tech divers are familiar with.

I added the black-dashed line to highlight something. Notice that the peak supersaturation that the diver is allowed to surface with doesn't change (or changes very little) as the bottom time increases (following CVA wear-off). Even staying within VPM theory you have to see the problem with that.

Suppose you execute the 40 min dive. Based on the chart the peak SS at surfacing is in compartment 11. If you move to a bottom time of 60 min C13 has the peak SS at surfacing. C13's half-time is about 63% longer than C11's. That is, C13 will stay supersaturated far longer because it's a slower compartment. But what has VPM done? It's kept the supersatuation at which the diver is allowed to surface the same (oh, there may be minor differences, but they are effectively equal as shown on the chart). VPM methodology allows the same supersaturation upon surfacing even though it must be endured for a much longer time. This approach can only do one thing --- increase risk. The diver, then, must know to increase VPM's conservatism or substantially pad shallow stops to adjust for this, or accept the certainty of increasing risk, because VPM is doing nothing on its own to make that adjustment. The lack of sufficient shallow time is directly related to this "feature".

So in response to the idea that VPM "scales correctly and in the right proportions" I would say the following. It's not so much that VPM doesn't scale correctly, it's that it doesn't scale AT ALL, at least in the sense discussed above. There is no attempt in the face of mounting gas loads in slower compartments to decrease allowed supersaturations to reflect the far longer SS exposure.

In comparison, GF does reduce allowed surfacing supersaturations as slower compartments enter the picture because Buhlmann's M-values get smaller as compartment half-times get longer. I'm not saying it adjusts perfectly to keep risk the same. But it does reduce allowed supersaturations as more gas is loaded in slower compartments. So in that sense GF "scales better and in better proportions" than VPM.

For comparison, see the charts below for GF and for a Navy Mk16 set of dives. Notice the negative slope in peak allowed surfacing SS as dives become more extreme and slower compartments become more important. Again, I can't claim the adjustment is enough to keep risk the same, but at least they're playing the right game.
View attachment 379690


For completeness, this pattern occurs when dives are extreme enough to minimize the effect of the CVA and long enough so that, for any given depth, the first stop isn't changing drastically as time increases-- basically technical dives with longer decompressions.

Ah yes... real models vs GF... 40/ 70, no this time use 60/ 85, ....... no it looks better if we use 65/72..... You see... all this GF fudging to make your graph look perfect...... "My stretched out GF , tailor made, fitted to perfection GF fudge looks just perfect".


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Mean while, back in the water, where the actual problem of DCS and Supersaturation stress begins.....

Don't forget the basics - an excess of supersaturation pressure, is the basis of all (type2) DCS injury. Here we see ZHL is (much) higher stresses, for the first 30 to 60 minutes of the ascent.

You can argue all you want about the necessity of reducing fast tissue supersaturation and its effect. But take a look above. VPM-B has lower supersaturation pressure for 1 hour of the ascent - that's long past 'fast tissue' state. This translates to a much lower in-dive stress factor, a lower over-all starting point for stress status when surfacing. This is clearly worth some allowance for permitting a little different stress on the surface.

A DCS injury is the product of both dive and surface stresses. ZHL-C and VPM-B go about doing the job differently.


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Added

You can see that both ZHL-C and VPM-B have both lowered the allowable surfacing pressure between the 30 and 60 minute examples. They are not equal amounts, but then neither is the in dive stress levels.

 

[UWSojourner]

Ah yes... real models vs GF... 40/ 70, no this time use 60/ 85, ....... no it looks better if we use 65/72..... You see... all this GF fudging to make your graph look perfect...... "My stretched out GF , tailor made, fitted to perfection GF fudge looks just perfect".

You obviously didn't understand the charts. It wouldn't matter at all if I'd picked GF80/40, 40/80, 100/100, 30/85, 90/75. The charts would still show that GF lowers allowed supersaturation as slower tissues come into play -- it's a function of Buhlmann's M-values. In contrast, VPM ... well, read the post here.

 

[rossh]

Oh, don't worry. I'm not implying. I'm saying it straight up. You have put a tremendous amount of effort over the years into vplanner and it's a unqualified success. Most people I know, even if they no longer use vplanner or VPM during the real-time part of the dive, still use it as a desk top planning tool. I do too.

You are, however, consistently coming across in these discussions like a mother bear protecting a cub. Your posts are argumentative sometimes to the point of being border line belligerent, you don't appear to be listening very well or answering straight forward questions with a straight forward answers and even when there is room to agree to disagree because there is a difference in interpretation you continue to attack.

Maybe that's your personality, but frankly, Ross, it's bad advertising. I don't know if it was an accident that you come across like that or what because we only know each other online and not very well but take it from me that this is how you are coming across. I'm sure I'm not the only one thinking it so take it for what it's worth and try avoid shooting the messenger.

And now we're on the track that this discussion should have been on the last X times it was had. Why are you communicating like this? Even in your last post you went at Simon and David again but playing the man isn't playing the ball and gets the discussion nowhere.

In my opinion.

Focus on the ball, would be my advice.

As for deco theory, it's all based on observation, guess work and making assumptions. It has been since Haldane was bending goats. We've been pretty darned good at making predictive models but no matter how you look at it some of the assumptions are slowly but surely being proven to be incorrect -- or at the very least aren't entirely correct. It means that model builders need to adjust their paradigms (slowly but surely) when confronted with new facts that help us fine tune our assumptions.

For Weinke it's too late. He's leveraged too deep on RGBM being a commercial success to untangle himself from that net and I'm sure he'll be willing to go down with the ship.

But you don't need to be on it when it does. You're not as irrevocably (and legally) tied to VPM so the question I would be asking in your position is how to take advantage of the connections you have with leading researchers in order to tune the algorithm to take these new insights into account.

The relevant question at this point in time (and we're almost 10 years on now so eventually someone has to ask the question!) "WHERE DO WE GO FROM HERE". and I think a LOT of technical divers are waiting for an answer other then "in circles".

You can see that, right?

R..

Hi Diver0001,

Thank you for your thoughtful well made post.

I understand what you wrote, and I wish it was that easy. It would be nice if the others played fair, but they don't. Just look at all the fakeness and artificial graphs and over bearing bullyness they apply, to get their message across to the public.

***

I'm happy to add any new model or dive practice we can to our programs. In the last few years, we added variable ascents, adjustable half times for "new' helium rates. Supersaturation checking, surface stress times, time to fly visual graphs. Confirmed our IBCD methods work with expected changes in accelerated deco computations, checked we conform with some recent Nedu tests on this.

Happy to be at the front of program development here.


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Circles? You know its almost unavoidable.

Think about this: we started many years ago with shallow fast models, then we had shallow slower models (or methods). Then deeper stops designs, both in models and adhoc form, that are both longer and shorter formats. Then the DIR people went even deeper and almost to the bottom, with various kinds of ascents to match.

There is no new territory or segment left to explore - its all been done.

Now its all about back tracking, trying to find something in the middle. But at the same time, its adding amounts of unnecessary extra time too, and looking for excuses to justify this. No more "decompression" derived deco... its past the extra margin deco. Now its about extra safety, on to of safety, on top of margin, on top of decompression requirements.

No wonder people feel like the process in going in circles - because it is.

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[rossh]

You obviously didn't understand the charts. It wouldn't matter at all if I'd picked GF80/40, 40/80, 100/100, 30/85, 90/75. The charts would still show that GF lowers allowed supersaturation as slower tissues come into play -- it's a function of Buhlmann's M-values. In contrast, VPM ... well, read the post here.

Your examples are focused on the blue circle in my diagrams (post above). The surfacing supersaturation is lowered in all the larger dive samples - both models, showing that obviously both models have lowered max SS limits with growing dive times.

Your use of GF amplifies the ZHL example, because GF causes an exponential increases in deco time, as the underlying deco time grows. A compounding effect. Your highlighting the fact that applying GF causes an unequal decrease in ss limit.


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Here is the after dive graph. We see peak supersaturation falling away. The VPM-B example is slighlty higher, or about 20 mins behind the ZHL examples. At the 90 minute mark (usually the time of max VGE), the peak pressure difference is just a small 5 kPa, on the 60 minute dive.

 

[UWSojourner]

Your examples are focused on the blue circle in my diagrams. The surfacing supersaturation is lowered in all the larger dive samples - both models, showing that obviously both models have lowered max SS limits with growing dive times.

Your use of GF amplifies the ZHL example, because GF causes an exponential increases in deco time, as the underlying deco time grows. A compounding effect. Your highlighting the fact that applying GF causes an unequal decrease in ss limit.

Why might it be a good idea (as highlighted in my post here) for a deco method to reduce supersaturations as progressively slower compartments come into play? Answer: Because the time you're exposed to supersaturation matters. The post shows that VPM keeps the same surfacing supersaturation gradients as you progressively move to slower compartments (within the constraints I described in the post).

It would be like saying, "Hey VPM. I will be exposed to supersaturation for 45 minutes. What SS pressure do you recommend?" VPM: "How about 600mb." Then you say, "Hey VPM, now I'll be exposed to supersaturation for 90 minutes. What SS do you recommend?" VPM: "How about 600mb." Pressure and time are the ingredients for bubble formation. If you lengthen the exposure time and leave the pressure unchanged, you just get more bubble formation.

That's fine if the diver 1) knows they need to adjust VPM's conservatism to reflect this new area of "the dive spectrum" (i.e. more bottom time/deeper dives), or 2) is willing to just accept more risk. In any case, I was just responding to Diver0001's post about Mark Ellyatt and insufficient shallow time -- there will be insufficient VPM time as you get progressively more extreme unless you know you need to compensate by adding shallow time or choosing more VPM conservatism, or both.

 

[rossh]

Why might it be a good idea (as highlighted in my post here) for a deco method to reduce supersaturations as progressively slower compartments come into play? Answer: Because the time you're exposed to supersaturation matters. The post shows that VPM keeps the same surfacing supersaturation gradients as you progressively move to slower compartments (within the constraints I described in the post).

It would be like saying, "Hey VPM. I will be exposed to supersaturation for 45 minutes. What SS pressure do you recommend?" VPM: "How about 600mb." Then you say, "Hey VPM, now I'll be exposed to supersaturation for 90 minutes. What SS do you recommend?" VPM: "How about 600mb." Pressure and time are the ingredients for bubble formation. If you lengthen the exposure time and leave the pressure unchanged, you just get more bubble formation.

That's fine if the diver 1) knows they need to adjust VPM's conservatism to reflect this new area of "the dive spectrum" (i.e. more bottom time/deeper dives), or 2) is willing to just accept more risk. In any case, I was just responding to Diver0001's post about Mark Ellyatt and insufficient shallow time -- there will be insufficient time as you get progressively more extreme unless you know you need to compensate by adding shallow time or choosing more VPM conservatism, or both.

We can see the actual difference in the surface charts I added. When we surface, all compartments fall away rapidly, for a good portion of time. Slow cells grow bubbles more slowly, and we have a longer time frame to deal with these.

There is no significant extra gas in slow compartments, even in the dramatic ZHL-C vs VPM-B examples I made. If it was there, it would become limiting.

The concern that elevated supersaturation in slower compartments comes down to a 20 minute spot difference, for this dramatic example, at any one point.


... might it be a good idea (as highlighted in my post here) for a deco method to reduce supersaturations as progressively slower compartments come into play?...

It happens already - see the examples. But even ZHL tends to flatten out like VPM-B does, once it gets longer. Remember - the deco comprises of both in dive and surface portions.

ZHL has an inflexible hard coded ceiling, that is applied progressively. Conversely VPM-B computes it limits on a per dive basis.


In the full scale of a dive plan, the tiny differences between 40/80 and VPM-B+3, are almost unnoticeable.

You keep trying to draw comparisons to GF plans, across different length dives. But GF does not scale evenly with dive time. Hence divers might start with 40/70, and then go to 40/80 and so on, as the dive gets longer (because 40/70 was too painfully long and unnecessary). That process negates your observations.


In VPM-B , adding conservatism, brings the allowed SS down across the whole dive and surface portions.

In ZHL-C, adding GF lo changes the allowed dive first stop SS (by the same % its set too), and GF hi does the same on the last stop end.
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[UWSojourner]

But the tiny differences between 40/80 and VPM-B+3, are almost unnoticeable in the full scale of a dive plan.

See this post. In that discussion VPM-B+3 created 30% more decompression stress at the surface and 20% more overall than a GF profile with the same runtime (GF60/75). Not quite "tiny".

You keep trying to draw comparisons to GF plans, across different length dives. But GF does not scale evenly with dive time. Hence divers might start with 40/70, and then go to 40/80 and so on, as the dive gets longer (because 40/70 was obviously wrong and too painfully long). That process negates your observations.

No. Read the post. VPM just keeps the surfacing supersaturation constant (within the constraints I discussed in the post). Are you exposed 45 minutes? VPM: Use SS=600mb. Are you exposed 90 minutes? VPM: Use SS=600mb . Are you exposed 120minutes? VPM: Use SS=600mb.

Obviously you will just get progressively more bubble formation (i.e. risk) as you do this. In contrast, GF tries to account for the longer exposure time by reducing SS (i.e. Buhlmann reduces M-values in slower tissues for this reason). That's rational.

 

[ajduplessis]

You obviously didn't understand the charts. It wouldn't matter at all if I'd picked GF80/40, 40/80, 100/100, 30/85, 90/75. The charts would still show that GF lowers allowed supersaturation as slower tissues come into play -- it's a function of Buhlmann's M-values. In contrast, VPM ... well, read the post here.

Are you saying that GF 100/100 is a better option than VPM?

 

[rjack321]

Are you saying that GF 100/100 is a better option than VPM?

For any given (fixed) deco time, where there's actual slow tissue saturation, yes. (so not a 5min bounce to 200ft with 15mins of deco)

But unfortunately there's incredible resistance to understanding that fixing the total deco time at X minutes was what made the NEDU study so powerful.

 

[rossh]

See this post. In that discussion VPM-B+3 created 30% more decompression stress at the surface and 20% more overall than a GF profile with the same runtime (GF60/75). Not quite "tiny".


No. Read the post. VPM just keeps the surfacing supersaturation constant (within the constraints I discussed in the post). Are you exposed 45 minutes? VPM: Use SS=600mb. Are you exposed 90 minutes? VPM: Use SS=600mb . Are you exposed 120minutes? VPM: Use SS=600mb.

Obviously you will just get progressively more bubble formation (i.e. risk) as you do this. In contrast, GF tries to account for the longer exposure time by reducing SS (i.e. Buhlmann reduces M-values in slower tissues for this reason). That's rational.

60/75 is the "new" GF today? Let me write that down (in pencil).

You are using the compounding errors of GF, as a panacea to get around the issue. GF is not the model. GF is a time fudge that does not scale correctly for underlying deco time. Your initial post says that VPM needs a conservatism change to make it match the already changed ZHL / GF. Its not a real comparison then.

ZHL-C is the model. That's where we look. And ZHL-C is almost the same as VPM-B in this regard.

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You are essentially correct in the observation. Deco models that we all use, have an increasing risk with growing exposure. Most mil models are the same too. The typical pDCS numbers - they go up with increasing depth/time exposure too. That is what you are looking at.

At the recent TeckDiveUSA, David described this as "the nature of the task".

Of course one can already plan and avoid all risk in any profile - just stay within the (green) band between inspired pressure and ambient pressure. But its not practical to do this, so we take on a (measured?) risk.

You talk about bubble formation on the surface?? That's applies to VGE (intra vascular), but VGE are not the issue - deco models do not get concerned with VGE !


The issue is tissue bubbles growth (extra vascular). The conventional theory that VPM-B / ZHL models are designed too, tells us that controlling the dive portion of tissue bubble growth, is sufficient to avoid DCS. And that's what they do. They control the dive portion to arrive on surface with DCS avoided. Of course deco has not finished at this point, and takes many more hours.

In the case of VPM-B the tissue bubbles is said to be stable at the point of surfacing, and the reducing tissue dissolved pressure (of all tissues concurrently), will shrink and dissolve remaining tissue bubbles.

From Dan Reinders VPM description: "If the pressure inside a bubble is greater than the pressure of the DISSOLVED gas in the surrounding tissue, the bubble will shrink.".

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