Where Is Your GF?

[rjack321]

Now that's new to me. Granted, the sum total of my deco knowledge comes from SB, Deco for Divers, and a smattering of articles pointed out on SB. But I'm trying to learn more. I don't recall having seen "total dissolved gas" mentioned as a factor, only the concept of the leading compartment. So, for example, if you surface with a faster compartment that is 80% of the way to its M-value, then that is the (only?) one to be concerned about causing DCS, not another compartment that is 50% of the way to its M-value. Or so that has been my understanding. However, if this concept of total dissolved gas in the body is believed to be a factor in DCS, then I learned something.

In NEDU discussion post after post, the key factor has been that the total gas load on the deeper stop profile was larger than the shallower stop profile. Even if the deeper profile kept the divers further away from the m-values in the deeper portions of the ascent.

Its not really plausible, but imagine having all 16 compartments "full" of gas at the m-value vs only having one compartment "full" and the rest only "half-full". The former is (likely) a whole lot more stress on the body. The bubble showers that the faster compartments are releasing are going to combine with the showers from the "full" slower compartments and be more likely to produce clinical symptoms. Vs. if only one compartment is at the m-value the other "less-full" compartments are not as going to add as much and the overall stress is less.

 

[Lorenzoid]

Ah--the NEDU discussions that inevitably turn into arguments and cause me to tune out.

 

[kensuf]

Thank you for the whole post.

On this last point, isn't that pretty much the same thing that that article Shearwater put on their website a few months ago (about "the helium penalty") was saying? I.e. that there is no helium penalty, but that having one built into our algorithm(s) is what has been making it safe, and that what we really need is to adjust our plan for nitrogen to match what the helium "penalty" has been dictating?

Eliminating The Helium Penalty - Shearwater Research

That's not exactly what I believe.

What I believe is that in some circumstances, diving a 30/30 trimix while using an EAN30 deco schedule is safer than diving an EAN30 nitrox and using an EAN30 deco schedule. This is different than what Shearwater wrote.

Caveat emptor, my data point for my belief is only myself.

 

[The Chairman]

I believe that sub-clinical DCS is real and is why I do loooooong safety stops: 5 minutes minimum, even when diving a Shearwater. I don't like Scuba induced narcolepsy. At this point, I dive "default" GF.

 

[rddvet]

What I believe is that in some circumstances, diving a 30/30 trimix while using an EAN30 deco schedule is safer than diving an EAN30 nitrox and using an EAN30 deco schedule. This is different than what Shearwater wrote.
.

This is the new idea that I've been taught over the past few years and believe. Although, I was taught to use a 32% deco schedule (likely not much difference between 30/32 other than 30 is probably being more conservative).

Then I recently discovered CCDS is banking 30/30 for this reason. Though I pump my own at home.

 

[kensuf]

BTW, discussions about specific GF's are silly and are up there with Try-Athletes discussing the wattages they generate.

The GF that works for you may leave me a pretzel and visa-versa. The "safe" default used to leave a friend of mine bent quite regularly and he had to run something like a 40/50 or 40/60 to avoid getting hit (when he told me his story I suggested he get tested for a PFO, guess what he had!).

At the end of the day we're all physiologically different and everyone that engages in decompression diving needs to have a respect for their body, pay attention to it, and adjust their ascent schedules accordingly. Don't just adopt a GF because some nitwit on the internet suggested it.

 

[stuartv]

Now that's new to me. Granted, the sum total of my deco knowledge comes from SB, Deco for Divers, and a smattering of articles pointed out on SB. But I'm trying to learn more. I don't recall having seen "total dissolved gas" mentioned as a factor, only the concept of the leading compartment. So, for example, if you surface with a faster compartment that is 80% of the way to its M-value, then that is the (only?) one to be concerned about causing DCS, not another compartment that is 50% of the way to its M-value. Or so that has been my understanding. However, if this concept of total dissolved gas in the body is believed to be a factor in DCS, then I learned something.

I think, generally, the Buhlmann model is referred to, categorically, as a "dissolved gas model".... (as opposed to a "bubble model".

Also, I believe the CONCEPT of the compartments and M-values is as you described. Whichever one is the closest to its M value is the one you have to watch out for. But, that assumes that all the M values are exactly correct and all the compartments are defined exactly correctly (including the correct # of compartments). And implies that all the M values have a single value that is "correct" for each individual and all circumstances.

EDIT: And it assumes that the M value for any given compartment is the same, whether you do a really short dive or a really long dive. And, this particular assumption seems to be the easiest to cast doubt on via thought experiment. Imagine off-gassing at the end of a short dive. You have a fast compartment that is near full and it's next to a slow compartment that is almost empty. During off-gassing, SOME of the gas in the fast compartment can migrate into the slow compartment (until they achieve equilibrium). That helps the fast compartment off-gas more quickly. At the end of a long dive, that slow compartment is, itself, closer to saturation and so will absorb less gas from the adjacent fast compartment. So, the fast compartment will off-gas more slowly after a long dive than it would after a short dive. (maybe...?)

I like what @rjack321 said about it in his last post ^^^.

 

[stuartv]

That's not exactly what I believe.

What I believe is that in some circumstances, diving a 30/30 trimix while using an EAN30 deco schedule is safer than diving an EAN30 nitrox and using an EAN30 deco schedule. This is different than what Shearwater wrote.

Caveat emptor, my data point for my belief is only myself.

That seems to be almost the opposite of what Shearwater said.

My understanding of what Shearwater's post says (in very simple terms): We've been using the term Helium Penalty to mean having longer deco when telling our computer we're using helium versus just using nitrox. But, that extra time is what has made those dives as safe as they are. We've (they've) now figured out that there is no penalty to using helium. BUT, in light of that fact, what we should NOT do is shorten our deco to what the computer says if you set it for nitrox. What we SHOULD do is lengthen our deco when we're diving nitrox to match what our computer would dictate if it were set for trimix.

You seem to be saying the opposite. The Helium penalty means to do more deco when using helium. Shearwater says to do that extra time even when not using helium. You are saying to use helium but do the shorter deco prescribed for nitrox. You're suggesting a Helium Bonus.

Have I understood you (and Shearwater) correctly?

 

[Lorenzoid]

I think, generally, the Buhlmann model is referred to, categorically, as a "dissolved gas model".... (as opposed to a "bubble model".

Also, I believe the CONCEPT of the compartments and M-values is as you described. Whichever one is the closest to its M value is the one you have to watch out for. But, that assumes that all the M values are exactly correct and all the compartments are defined exactly correctly (including the correct # of compartments). And implies that all the M values have a single value that is "correct" for each individual and all circumstances.

I like what @rjack321 said about it in his last post ^^^.

Well I suppose it goes without saying that less dissolved gas in the body should theoretically correlate with a lower risk of DCS than more dissolved gas in the body, and that everything about a deco model is just an approximation as far as it applies to our individual bodies. I don't think anyone would question the logic of those statements. I just hadn't heard of any science to support the theory--studies suggesting that as a practical matter we should pay more attention to total dissolved gas over all compartments. I guess I need to wade into the NEDU study. Light reading.

 

[stuartv]

Well I suppose it goes without saying that less dissolved gas in the body should theoretically correlate with a lower risk of DCS than more dissolved gas in the body, and that everything about a deco model is just an approximation as far as it applies to our individual bodies. I don't think anyone would question the logic of those statements. I just hadn't heard of any science to support the theory--studies suggesting that as a practical matter we should pay more attention to total dissolved gas over all compartments. I guess I need to wade into the NEDU study. Light reading.

It's just my psuedo-theory based on the little bit I know. I didn't read any science to specifically support it.