Deep Stops Increases DCS

[rossh]

It clearly needs some fiddling. We already established that VPM is broken as risk of DCS goes up as deco time goes up.

ZHL is broken, too, but you can fiddle with the GFs to make it less broken.

Ah well. Ok so you want to incorporate a pDCS risk component. Good.. David is the man for that. If you ask him, he will likely say, that risk always goes up with increasing depth and time, even in mil tables. Its the nature of the task.

Or you could just use an existing military table set or model, and be done with it. Bye.

 

[PfcAJ]

Ah well. Ok so you want to incorporate a pDCS risk component. Good.. David is the man for that. If you ask him, he will likely say, that risk always goes up with increasing depth and time, even in mil tables. Its the nature of the task.

Or you could just use an existing military table set or model, and be done with it. Bye.

Some of what they're using is 'flat risk' tables that maintain a consistent risk level.

What gets me is that you *know* risk goes up, and you *know* that risk is decreased by more deco, but *still* try and peddle VPM which produces less deco total with less time on oxygen then the GFs people are proposing.

That's looney tunes dude.

Its probably ok for relatively short BTs at normal tech depths (10-30mins at 150-250ft, kinda normal run of the mill activities) but anything outside of that its just unacceptable.

 

[David Doolette]

Well the problem is, adding up 16 puts a bias towards shallow stop profiles. Which then make this method invalid for comparing shallow and deep profiles.

As you fully know, if a slow tissue was to some how on gas enough, it would rise to top and become the limit. And it works this way already. if you look carefully, you can see how the deeper on gassing has given a higher surface supersaturation. The problem is solved already by Haldane / Schriener equations - it doesn't need fiddling.

.

Again you are talking about a compartment becoming the controlling (or limiting, or leading) compartment - that is a characteristic of classical algorithms used to generate schedules. So again, you are confusing generating a schedule with an algorithm and an algorithm-independent evaluation of a schedule by looking at the sum of integral supersaturation in all compartments. In the latter case it is entirely appropriate to look at the sum across all compartments.

 

[Storker]

Kudos to @EFX. Admitting misunderstanding and changing POV because of that is seen way too seldom. Usually, people dig down in their trenches and refuse to be swayed by rational arguments, particularly in online discussions. It's a relief to see the opposite.

 

[rossh]

Again you are talking about a compartment becoming the controlling (or limiting, or leading) compartment - that is a characteristic of classical algorithms used to generate schedules. So again, you are confusing generating a schedule with an algorithm and an algorithm-independent evaluation of a schedule by looking at the sum of integral supersaturation in all compartments. In the latter case it is entirely appropriate to look at the sum across all compartments.

I'm not confusing it. The pressure lines in the graphs, are half times common to all the samples shown. These are algorithm-independent. The program can show and draw all 16 concurrently, if you want. These values shown are independent of the model that made the stops.

 

[rossh]

Some of what they're using is 'flat risk' tables that maintain a consistent risk level.

What gets me is that you *know* risk goes up, and you *know* that risk is decreased by more deco, but *still* try and peddle VPM which produces less deco total with less time on oxygen then the GFs people are proposing.

That's looney tunes dude.

Its probably ok for relatively short BTs at normal tech depths (10-30mins at 150-250ft, kinda normal run of the mill activities) but anything outside of that its just unacceptable.

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.

.

 

[UWSojourner]

Well the problem is, adding up 16 puts a bias towards shallow stop profiles. Which then make this method invalid for comparing shallow and deep profiles.

As you fully know, if a slow tissue was to some how on gas enough, it would rise to top and become the limit. And it works this way already. if you look carefully, you can see how the deeper on gassing has given a higher surface supersaturation. The problem is solved already by Haldane / Schriener equations - it doesn't need fiddling.

.

This was all hashed out (i.e. many posts correcting Ross' misunderstandings) on RBW. Please see this post and the few following.

 

[EFX]

Big mistake there... VPM uses Haldane gas tracking...... the ascent is dictated by Haldane / Schriener equations.... Bubble models DO make longer times for the deep stops...its the same formula that is in the ZHL and powers your GF as well. If it works for one, it works for the other too. Also, VPM-B is LONGER than ZHL for most plans - yes longer, up to about 3 or 4 hours. So what do think now?.

I meant total deco time not just the shallow stops are shortened. Your statement contradicts what BRW said in his summary in Kevrumbo's post #36:

"One thing about these bubble models, as they are collectively referenced, that is common to all of them is deeper stops, shorter decompression times in the shallow zone, and shorter overall deco times. And they all couple dissolved gases to bubbles, not focusing just on bubbles or dissolved gas. . ." [BRW]

So, is he wrong about VPM-B?

 

[rossh]

I meant total deco time not just the deep stops are shortened. Your statement contradicts what BRW said in his summary in Kevrumbo's post #36:

"One thing about these bubble models, as they are collectively referenced, that is common to all of them is deeper stops, shorter decompression times in the shallow zone, and shorter overall deco times. And they all couple dissolved gases to bubbles, not focusing just on bubbles or dissolved gas. . ." [BRW]

So, is he wrong about VPM-B?

Well that might be an RGBM thing - I don't much about them. But here in VPM-B land, It does not get "shorter".

As I said, and you can see for yourself, just about any VPM-B plan is longer than its real ZHL-C counterpart, up to about 3 hours. So the worry about VPM-B being too short, is n/a.

 

[UWSojourner]

Well that might be an RGBM thing - I don't much about them. But here in VPM-B land, It does not get "shorter".

As I said, and you can see for yourself, just about any VPM-B plan is longer than its real ZHL-C counterpart, up to about 3 hours. So the worry about VPM-B being too short, is n/a.

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.