My Journey into UTD Ratio Deco

[Dan_P]

Hi Storker,

Interesting. What kind of data and correlations do you have that supports an exponential relationship (i.e. a factor of ten for each x units of cold exposure) between cold exposure and offgassing? I'd love to see them.

To the best of my knowledge, the relationship across thermal state and its effect on offgassing, is pretty much uncharted territory - that is, we have very little to go on to support whether linear or exponential, or if any "tipping points" exist.
Of course, one could argue that bodily functions rarely adapt themselves to accomodate man's preference for straight lines on a piece of paper, but that's no answer.

I was careful to phrase myself so as to be clear that this is not something I have apt substantiation to present as a fact:
("To the best of my knowledge, we'll all need to accept that there's no definitive answer to that question, whichever solution we go with. At least nothing scientifically satisfying.")
I certainly hope that I was succesful in that endeavour.

I've been trying to find conclusions, without luck, and subsequently strong datasets towards this question, for a while, also unsuccessfully.
I did, however, come across some interesting reads along the way, particularly for cold-water divers (obviously). I've added a few links below for your consideration:

Experience with moderate hypothermia in the treatment of nervous system symptoms of decompression sickness,
A. Erde.
Venous gas bubble production following cold stress during a no-decompression dive,
Dunford R, Hayward J
Effect of peripheral temperature on the formation of venous gas bubbles,
Mekjavić IB, Kakitsuba N.
Climatic and environmental factors in the aetiology of decompression sickness in divers,
Broom JR.


Best Regards,

Dan

 

[TrimixToo]

Interesting. What kind of data and correlations do you have that supports an exponential relationship (i.e. a factor of ten for each x units of cold exposure) between cold exposure and offgassing? I'd love to see them.

I'm in a pedantic mood, so I feel constrained to point out that there are exponents other than ten. (Perhaps you meant "e.g." and not "i.e." in your post?) Cold might increase the value of the exponent, I suppose, or not, and it's an interesting question.

If I get cold during deco I'll pad my stops a bit, but by no means do I plan gas that allows me to pad them exponentially. I suspect, without any more proof than I think Dan has, that the effect is more linear than exponential. Otherwise, my reasoning goes, we would have widespread reports of bent divers who got cold on deco rather than the rather smaller number of reports we seem to hear about. But perhaps those of us who pad stops when cold are really just eating into the safety factors we use. For example, maybe we are surfacing with actual GFs of 97 rather than the calculated 70s our computers are set to.

I'd love to hear more-informed opinions here.

 

[Storker]

I'm in a pedantic mood, so I feel constrained to point out that there are exponents other than ten.

Sure, but since e is a somewhat weird and geeky number while ten is well known, and other logarithms than natural and Briggs' are pretty rare, I took the liberty of oversimplifying the issue. Perhaps excessively.

(Perhaps you meant "e.g." and not "i.e." in your post?)

I did. Sorry.

 

[Michael Guerrero]

Do you?

I don't Dan. None of us do. What I do have are several human trials that indicate that profiles favoring shallow stops over deep stops produce conditions that are more favorable to limiting decompression stress and likely the probability of DCS. Advocating for a "strategy" that moves deco time around in a profile, rather than emphasizing more time in shallower stops on higher O2 concentrations flies in the face of the science we have. Purporting that divers are "engaging" in their deco diving because they are arbitrarily moving stops around in ways unsubstantiated by science is simply endorsing voodoo. I think it is irresponsible and a shame.

 

[Dan_P]

I don't Dan. None of us do. What I do have are several human trials that indicate that profiles favoring shallow stops over deep stops produce conditions that are more favorable to limiting decompression stress and likely the probability of DCS. Advocating for a "strategy" that moves deco time around in a profile, rather than emphasizing more time in shallower stops on higher O2 concentrations flies in the face of the science we have. Purporting that divers are "engaging" in their deco diving because they are arbitrarily moving stops around in ways unsubstantiated by science is simply endorsing voodoo. I think it is irresponsible and a shame.

So you're asking me if I've found backing to answer how hypothermia impacts off-gassing, and when I say I don't have it either, you jump straight from there into "voodoo!", "irresponsible" and "a shame" because you have a different approach to decompression?
You can't even make that appraisal and claim to have support by scientific evidence.

Actually, what are you asking for here - agencies should tell their students to use a computer with an algorithm, and which algorithm to use and it has to be the one that you've landed on because otherwise it's "voodoo" - is that what you're getting at?
You can use RD with any given level of deep stop emphasis, just like you can use a computer with any given GF.

What you're doing is decontextualizing a concept and then making deductions about it outside of context.

 

[rjack321]

How so?

Here, check out the pictures in Doppler's blog (he left SB awhile ago, wise man lol), they explain it better than words.
Gradient Factors… a simplified primer

Bottom line, 100/70 is not mathematically possible. By going up to the M-value to initiate offgassing (that is what the 100 is), you automatically violate the GF70 high - which is supposed to be your ceiling aka maximum allowable overpressure gradient. You can't do both at the same time.

 

[Storker]

100/70 is not mathematically possible

It sure looks to me as if it's possible on this graph:

(Image from Gradient Factors | Dive Rite )

 

[dmaziuk]

You're quoting DiveRite's article which states High Gradient Factor defines the surfacing value. @rjack321's quoted source says setting the HGF is a question of how close to the 100 percent M-Value limit we are willing to venture. If that were true at every point of the ascent instead of only at the surface...

 

[Storker]

Thanks, I wasn't aware of that definition

 

[rjack321]

You're quoting DiveRite's article which states High Gradient Factor defines the surfacing value. @rjack321's quoted source says setting the HGF is a question of how close to the 100 percent M-Value limit we are willing to venture. If that were true at every point of the ascent instead of only at the surface...

Dive rite inaccurately described that. It is how close you are willing to venture at any point in the ascent.
See Erik Baker's original work on this topic
http://www.ddplan.com/reference/mvalues.pdf

But in the interest of discussion, if implemented, it would look like this figure.

This figure might make sense visually, but it requires your computer to predict the future and decide (at some point in the ascent) that 100% GF is no longer ok and it should start dialing the maximum overpressure gradient back so that no pressure group exceeds 70% by the time you surface.

If you wanted to actually do this ascent you could do it by setting the shearwater to 99/99, do the ascent through to 20ft(?) then reset the GF high to 70%. You'd then discover that you missed stop time at 30 and/or 40ft though since the GF99 had you ascend past the newly entered GF70. So you'd be in a ceiling violation. GF100/70 basically creates a situation where you are continually exceeding the 70% safety buffer on the m-value on at least one tissue (maybe more) throughout the whole ascent. That's why I said it wasn't mathematically possible (well maybe it is, but shearwater is not going to write code that creates a continuous deco ceiling violation).