Deep Stops Increases DCS

[UWSojourner]

In Erik Baker's VPM code of 2001,
ftp://decompression.org/Baker/VPM%20Decompression%20Program%20in%20Fortran.pdf
you will find the parameters
Critical_Radius_N2_Microns, Crit_Volume_Parameter_Lambda, Surface_Tension_Gamma, Skin_Compression_GammaC, Regeneration_Time_Constant
to be the same as in Yount's 1986 paper.

When adding Boyle's law adjustment, Baker saw that the VPM-B algorithm became more conservative and suggested to reduce the critical radius and lambda. Great. Tune internal parameters so that the runtime looks OK to the unsuspecting user, but don't worry about parameter calibration by controlled experiments, DCS risk, efficiency or anything.

Yep. I'll just add one thing. When asked about why VPM had inverted gradients (i.e. higher allowed supersaturations in slower compartments), Dr. Yount said:

"You are interpreting things much more literally than I ever interpreted them. For example, I would be reluctant to associate different compartments with specific sites or tissues. As another example, you have shown that the radial distributions must be different in different compartments following a 3-10 min compression, but this presumes they were the same to begin with. You don't know that they were the same, and they certainly wouldn't be the same if the surface tensions were different in the different tissues. As yet another example, Hennessy and Hempleman worried a lot about the solubility of various gases in watery tissue versus fatty tissue. It's not wrong to worry about things like that, but if you do worry about things like that, you will very quickly get into a regime where there are so many unknown parameters you can't make a practical model. You might as well do a maximum likelihood calculation and not try to understand the underlying physics or physiology. It is amazing that VPM works as well as it does in spite of all the details that have been left out ... If this works, we're in business. If it doesn't work, we need to start over and make some more assumptions."

The goal was to get a workable model based on the physics of bubble formation, not tune it to physical outcomes. And in order to get a workable model a lot of simplifying assumptions had to be made as you point out.

I give Dr. Yount a lot of credit for the last sentence.

[This link is the source of Dr. Yount's quote. See the bottom of the page.]

 

[RyanT]

Why do want to start looking at VGE now? There is no new problem to solve, we do not have any crisis in injury numbers. Its not going to improve anything.

Let's draw an analogy. If you look at U.S. automobile fatalities, the rate has declined substantially since 1975. This is due in large part to better automobile safety technology. So if automobile fatalities are declining, then there is no need conduct additional research to make cars even safer. It's hard to imagine anyone would make that argument!

 

[tursiops]

I get 23 minutes for 170 feet. My own 'home-brew' curve fit is 666,666 divided by depth and that number divided by depth again. (divide by D squared) But this is only my U/W guesstimate for the first real hard stop. (Also known as: NDL.).

I sincerely hope this is sarcasm. At least, that seems to be the popular explanation for someone having said something really stupid.

 

[RainPilot]

I sincerely hope this is sarcasm. At least, that seems to be the popular explanation for someone having said something really stupid.

Took me the 3rd time to realise. Beautiful and I'll spring for the new keyboard.

 

[PfcAJ]

I
For example, as a population each of us may have a 4% chance of having a genetic illness but on an individual basis you either have it or you don't. If you dive in a way that works for you, it SHOULD continue to work for you. That being said, adjusting your deco strategy to take advantage of new research as well as dialing up your conservatism as you age is probably the best way to go.

I don't think thats an accurate thing to say. Frequency of a genetic illness isn't really the same thing as saying a dive profile has an x% chance of resulting in DCS.

What you get away with one day doesn't affect what you'll get away with on another day.

 

[orange_diver]

One thing that's interesting to note, the original concept of "Pyle Stops" had those deep stops being added to the total bottom time for decompression calculations. So for a 30 minute dive to 150', with Pyle Stops added at 90' and 60', a person would shift to either the 35 minute or 40 minute decompression schedule, depending on how long those stops were. This means that when conducting "Pyle Stops" the original way, you're actually going to be lengthening your shallow stops, not shortening them.

Thank you. This whole time I was wondering why the deep stops that worked for Pyle are not working anymore in the NEDU and other tests. This explains it.

 

[lowviz]

I sincerely hope this is sarcasm. At least, that seems to be the popular explanation for someone having said something really stupid.

No sarcasm. Just a really stupid mistake on my part. Use six 2's NOT six 6's and you will get a nice approximation to NDL's using the Navy Air Tables.

The part that is indefensible is that 23 minutes at 170 feet on air didn't scream out at me, too caught up trying to follow the conversation to check my own typing...

 

[David Doolette]

That's what happens when you try and do long division at 170 fsw on air

 

[tursiops]

No sarcasm. Just a really stupid mistake on my part. Use six 2's NOT six 6's and you will get a nice approximation to NDL's using the Navy Air Tables.

The part that is indefensible is that 23 minutes at 170 feet on air didn't scream out at me, too caught up trying to follow the conversation to check my own typing...

LOL
If you are actually following the conversation it is a wonder you have any brain cells left!
Isn't NDL=120-D even easier?
Hmmm, if both formulae are right, then they agree exactly at 62 ft and 96 ft. interesting.

 

[David Doolette]

...but that is really cool. How did you come up with 222222?