Deep Stops Increases DCS

[rossh]

VPM-B being shorter or longer isn't what's being discussed. Whether deep stops are more efficient (i.e. produces lower DCS risk for the same time) is the issue addressed by the NEDU study.

So you dial in your VPM conservatism setting and it produces a certain amount of deco time. The question is whether another profile (e.g. GF) can be found that uses that same amount of time but lowers decompression risk. In the NEDU study it was very clear the deep stops profile was inefficient (5% risk for deep profile vs 1.6% risk for shallow profile for the same deco time). I think the NEDU study (among other studies pointing in the same direction and discussed by Dr. Mitchell here) points toward the conclusion that deep stop models like VPM are inefficient in their allocation of decompression time. Their deep stops just skew too much time too deep.

More efficient? Really? How futile. A meaningless pursuit.


40/80 is more "efficient" than 30/85
50/70 is more "efficient" than 40/70
60/60 is more "efficient" than 50/70
70/55 is more "efficient" than 60/60
80/45 is more "efficient" than ...
90/40 is more "efficient" than ...


You get the idea? Little incremental changes - all better than the last one. This whole 'efficiency" drive is biased towards go more shallow. The way you try to measure this - all biased to shallow.


So just go straight to the end - GF 100/10, and be happy you have the most 'efficient' profile possible. Save us all the agony of listening to these self serving efficiency explanations.


But the reality is, you are not looking for "efficient" deco. If you were, you would be searching for ways to make deco shorter. No one is doing that.

No, this effort is all about looking for lame excuses to justify throwing on more unnecessary deco. No one is trying to be "efficient" here.


***************

Efficient: "achieving maximum productivity with minimum wasted effort or expense."



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[PfcAJ]

Yeah... let me quote again from David's intro to TR11-06

You see, your level of risk is your choice. We give you options for conservatism levels, GF settings and you can always pad on a big dollop of extra time. Some people like really fast, some like really slow, and some like it just... right. I can't say which is best for you.

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Except you've got no idea what the risk is nor do you know how much you're adjusting the risk. But if parameters are kept the same, risk goes up. That's not cool, and it shows that vpm doesn't produce "perfectly good schedules" like you said earlier.

 

[PfcAJ]

More efficient? How futile. A meaningless pursuit.


40/80 is more "efficient" than 30/85
50/70 is more "efficient" than 40/70
60/60 is more "efficient" than 50/70
70/55 is more "efficient" than 60/60
80/45 is more "efficient" than ...
90/40 is more "efficient" than ...


You get the idea? Little incremental changes - all better than the last one. This whole 'efficiency" drive is biased towards go more shallow. The way you try to measure this - all biased to shallow.

And GF? not a model - not a base line - a fudging tool only.

So just go straight to the end - GF 100/10, and be happy you have the most 'efficient' profile possible. Save us all the agony of listening to these self serving efficiency explanations.


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You're missing the point of "efficiency".

If deco time is constant, does changing the stop distribution effect frequency of dcs.

It's not about chasing the lowest time you can get the software to spit out at you.

 

[rossh]

You're missing the point of "efficiency".

If deco time is constant, does changing the stop distribution effect frequency of dcs.

It's not about chasing the lowest time you can get the software to spit out at you.

Efficient: performing or functioning in the best possible manner with the least waste of time and effort;

 

[PfcAJ]

Efficient: performing or functioning in the best possible manner with the least waste of time and effort;

Good job Ross. You looked up a word in the dictionary. Proud of you.

"Good job team. A+ for efficiency. Everyone was out of the water lickity split! Yeah, half are bent, but that's the price you pay for efficiency!"

You've got a career in middle management ahead of you for sure!

 

[rossh]

Except you've got no idea what the risk is nor do you know how much you're adjusting the risk. But if parameters are kept the same, risk goes up. That's not cool, and it shows that vpm doesn't produce "perfectly good schedules" like you said earlier.

The risk is reflected in the real world treatment numbers (1:5000 to 10000). And studies, and papers (1:3700).

All diving has risk - accept it, or stop.

 

[PfcAJ]

The risk is reflected in the real world treatment numbers (1:5000 to 10000). And studies, and papers (1:3700).

All diving has risk - accept it, or stop.

I'm no stranger to risk, but I'm also no stranger to mitigating risk. If I can mitigate some risk by redistributing my deco stops I'm going to do it, and I think any sensible person would have similar goals.

Again, do you even tech dive?

 

[UWSojourner]

Straight back to fantasy land I see.

Yes - they do show exact supersaturation values... In any pressure units you choose to select.

Ok. Then you can probably break those down into the N2 tissue compartment pressure and the surface pressure, right?

The points I see in the chart below as best as I can determine are:

surface ~38 kPa -- what is N2 tissue pressure for this one? And which compartment does it represent?
10ft ~ 42 kPa -- what is N2 tissue pressure for this one? And which compartment does it represent?
20ft ~ 28 kPa?? -- what is N2 tissue pressure for this one? And which compartment does it represent?

Feel free to correct my estimates. I had to eyeball them on the chart.

 

[RyanT]

I don't need to show anything beyond plain math.

Math is great, but it only grossly models physiology. Honestly, I have a statistical concern with the NEDU study and I could complain about that. BUT...if we look at the actual proportion of hits, the numbers do suggest that we should be thinking critically about the gospel of deep stops. I'd like to see a stronger empirical data set, but how often can you find folks lining up to get bent?

Also, Ross keeps complaining that the NEDU study didn't test deep stops and therefore isn't relevant. I'm going to just hazard a guess here, but it seems that if the "deep stop" study treatment had started their stops even deeper, then we would have seen even more hits due to additional slow-tissue on-gassing.

 

[Colliam7]

Honestly, I have a statistical concern with the NEDU study and I could complain about that. BUT...if we look at the actual proportion of hits, the numbers do suggest that we should be thinking critically about the gospel of deep stops.

A very reasonable statement. We HAVE to think critically about that 'gospel', and we HAVE to think critically about how to assess the validity of that 'gospeI', in no small part because the NEDU study was not designed to, nor could possibly, evaluate the validity of that 'gospel'. Personally, I understand the basis for the use of a one-sided test, given the stated hypothesis, although I could be persuaded that a more robust analysis would have involved a two-sided test. The trial starts with the hypothesis that was is / was currently being done was the gold standard. And, to make a decision to change to an alternative approach requires demonstration of superiority of that alternative.

I'd like to see a stronger empirical data set, but how often can you find folks lining up to get bent?

I think there would be general agreement with any desire to have a stronger dataset, for any trial. BUT, the NEDU study actually provides a pretty strong empirical dataset, probably as good as we are going to get with regard to the area of research focus. This was a d*** good study in many ways - the hypothesis was clearly stated, the rationale for the trial methodology was clearly stated, the methods employed were clearly stated, the results were clearly summarized, and the ONE, SINGULAR conclusion that the authors drew from the study - 'deep stop air decompression schedules are more efficient than shallow stop schedules' - was also clearly stated. Note that the conclusion did not explicitly state that deep stops were safer or more dangerous. They did not say that bubble model or gas content models were 'better' or 'worse'. The result are useful in the context of the hypothesis that was tested - deep stop air decompression schedules are more efficient than shallow stop schedules - and in the manner in which that hypothesis was tested. The study was actually about efficiency - and the two decompression schedules were designed around that premise.

Also, Ross keeps complaining that the NEDU study didn't test deep stops and therefore isn't relevant.

I am not sure he said it is not relevant. What he has been consistently arguing - as I understand his comments - is that the NEDU study did not test 'deep stops' that are consistent with contemporary technical diving practices. For example, I originally was taught that a 'deep stop' was made at a depth that was equidistant between the maximum depth of the dive, and the first planned deco stop. If I was using a 50% mix on my dive as the first deco gas (to which I would switch at 70 feet), and my bottom depth was 170 feet, then my 'deep stop' would be 120 feet. That is considerably deeper than the 70 ft maximum first stop depth evaluated in the NEDU trial. That isn't a criticism of the study, merely a statement of the parameters.

I'm going to just hazard a guess here, but it seems that if the "deep stop" study treatment had started their stops even deeper, then we would have seen even more hits due to additional slow-tissue on-gassing.

I suspect that is EXACTLY the hazarded 'guess' - an extrapolation of results not substantiated by the trial design - that Ross is particularly concerned about. I don't know what would happen if the "'deep stop' study treatment had started their deep stops even deeper". I could certainly speculate, but that speculation has no relation to any objective assessment performed in the NEDU trial. The NEDU study results CANNOT possibly address that issue, NOT because the study was somehow flawed, but because it wasn't designed to test that scenario. It is simply not 'fair' to the results of the trial to extrapolate and apply them to situations not assessed in the trial itself.

Unfortunately, some of the rhetoric in this thread, indeed the tone of the expression of some of the concerns, has compromised the opportunity for discussion of the science and applicability of the results. And, that is disappointing, because the study results merit serious consideration, as does the potential scope of applicability of those results.