Gradient Factors in Use

[kensuf]

40/80.

 

[mathauck0814]

I'm running 30/70 on anything deeper than 200 feet and 30/85 all others. I had to play around with it to find what worked for me. I suspect it's different for everyone based on your physiology. Not sure there are any wrong answers. I like to have my standalone/backup computer set to a more aggressive GF in has the SHTF and I need out of the water; not quite GF99, but not far from it.

 

[Michael Guerrero]

So, 100/100 is straight Buhlmann ZHL-16C right (in the Petrel anyway)? Using GFs, we are intentionally introducing conservatism, moving ourselves away from the M-value line and toward the ambient pressure line. Using the same factor for low and high (e.g., 70/70) would produce a consistent supersaturation level for the leading compartment (whichever one it happens to be during a particular phase of the ascent) across the entire ascent.

So if we don't want to introduce "deep stops", but want to add conservatism to Buhlmann, shouldn't we be diving something like 100/70 Low/High? This would put us at the m-value line according to the algorithm at the start of the ascent, well into supersaturation but theoretically not into critical supersaturation, but would keep us further away toward the end with longer shallow stops. Isn't anything less than 100 for GF low by definition introducing "deep stops" (i.e., stops deeper than the first stop prescribed by the algorithm you're running)?

Next question, if Buhlmann produces some certain % of DCS risk when run unmodified across some spectrum of dives (anyone happen to know what % that is by any chance?), what method do we use to pick our factors in the low and high areas based on our desired outcome of less DCS. Presumably, any modification which keeps us further from the m-value would mean that it's safer, but the NEDU study and others seem to contradict that notion as evidenced by higher incidence of DCS in a profile which introduced "deep stops". The theory being the slower compartments continue to ongas more than they would otherwise with the algorithm while we're keeping the fast tissues from getting too crazy.

It seems to me that by adding GF Low we are trying to make Buhlmann look more like a bubble model, which typically starts stops deeper, but without doing all the free phase math and what not.

 

[Michael Guerrero]

Anyone have any thoughts on this?

 

[waterpirate]

All this math is way above my paygrade. The concept is not. I began my deco by " feeling my way out of the water". Maximum conservancy produced a mountain of deco obligation. I then started dialing it back and listened to how my body felt post dive. This produced far less deco obligation, and a good post dive feeling. Every dive is different, I typically add more deco on high p02 gas if the dive was stressfull, I do not feel like getting out, there are fish or turtles around 20 fsw. Dive, dive dive, and listen to your body post dive.
Eric

 

[Redshift]

Presumably, any modification which keeps us further from the m-value would mean that it's safer, but the NEDU study and others seem to contradict that notion as evidenced by higher incidence of DCS in a profile which introduced "deep stops". The theory being the slower compartments continue to ongas more than they would otherwise with the algorithm while we're keeping the fast tissues from getting too crazy.

It seems to me that by adding GF Low we are trying to make Buhlmann look more like a bubble model, which typically starts stops deeper, but without doing all the free phase math and what not.

Could someone point me to where I can find that NEDU report and DAN's presentations about it?

I have some problems understanding how an algorithm can compute the gas absorption and release at any time of the dive and then screw up when people do deep stops. If a slower tissue is still ongassing, it's either still not enough for it to be a concern or it makes that tissue become the leading tissue and therefore control the following stops, always maintaining the % of the M-values we forced whichever software we use to adopt.

I agree, GFs are a way to try to mimic results from bubble models while using Buhlmann. We know bubbles are formed although we can't mathematically describe them. Even bubble models adopt a priori parameters which are not based on anything real. So I don't dislike the idea of taking a widely used model (Buhlmann) and build upon it something which is a bit empirical (as seen by the posts here, it takes a bit of trial and what feels good to us), but that forces that model to behave a bit more like what we'd expect from a bubble model (without relying completely on a bubble model witch has its own problems).

 

[Michael Guerrero]

Here's the study:

Redistribution of decompression stop time from shallow to deep stops increases incidence of decompression sickness in air decompression dives

The video from the DAN conference is on this page:

https://www2.dan.org/research/conference/2008TechnicalDiving/Decompression_Workshop.aspx

I have some problems understanding how an algorithm can compute the gas absorption and release at any time of the dive and then screw up when people do deep stops. If a slower tissue is still ongassing, it's either still not enough for it to be a concern or it makes that tissue become the leading tissue and therefore control the following stops, always maintaining the % of the M-values we forced whichever software we use to adopt.

That's the theory. But you continue to bubble out of the water for several hours post dive. And just because the model thinks it's safe, doesn't make it so.

In the navy study, the took two different profiles, one that looked like Buhlmann (skew toward shallow, long stops), one that looked like a bubble model (skew toward longer deep stops), and ran them in extreme conditions. They did 170' dives for 30min bottom time, and a fixed amount of decompression for both (I think 174 minutes). With this fixed amount of deco, the two algorithms then were allowed to distribute stops according to their algorithm. The divers worked at depth, were cold during deco, and did deco on air. Pretty harsh conditions.

Both algorithms resulted in DCS, but the one with deeper stops resulted in more (about 5% I think) DCS than the one that started stops shallower (about 2.5% or something). More details in the study.

 

[Omisson]

Here's the study:

Redistribution of decompression stop time from shallow to deep stops increases incidence of decompression sickness in air decompression dives

The video from the DAN conference is on this page:

https://www2.dan.org/research/conference/2008TechnicalDiving/Decompression_Workshop.aspx



That's the theory. But you continue to bubble out of the water for several hours post dive. And just because the model thinks it's safe, doesn't make it so.

In the navy study, the took two different profiles, one that looked like Buhlmann (skew toward shallow, long stops), one that looked like a bubble model (skew toward longer deep stops), and ran them in extreme conditions. They did 170' dives for 30min bottom time, and a fixed amount of decompression for both (I think 174 minutes). With this fixed amount of deco, the two algorithms then were allowed to distribute stops according to their algorithm. The divers worked at depth, were cold during deco, and did deco on air. Pretty harsh conditions.

Both algorithms resulted in DCS, but the one with deeper stops resulted in more (about 5% I think) DCS than the one that started stops shallower (about 2.5% or something). More details in the study.

The DAN study draws the same conclusions that many multi-compartment models have always had as a latent theme- namely the correlation between the CONTINUED on-gassing at depth during ascent and optimum deco schedule to avoid DCS risk. Because they are just "models" and the three types of tissue compartments are not proportional all we can say is that the body continues on gassing while at deeper depths - this outweighs most benefits of a slower ascent and deeper stops with longer hold times.

The problem is finding the optimum ascent rate and stop locations to minimize on-gassing while not blowing out off-gassing nitrogen to the point of damage to the body. Since we still don't even understand the actual physiological process it's hard to really answer the question. We just keep revising deco algorithms as more data becomes available and more studies give us information to use. But so far it's still a lot of theory... Not a lot of fact.


Dan-O

Sent from my iPad using Tapatalk 2

 

[Dsix36]

The DAN study draws the same conclusions that many multi-compartment models have always had as a latent theme- namely the correlation between the CONTINUED on-gassing at depth during ascent and optimum deco schedule to avoid DCS risk. Because they are just "models" and the three types of tissue compartments are not proportional all we can say is that the body continues on gassing while at deeper depths - this outweighs most benefits of a slower ascent and deeper stops with longer hold times.

The problem is finding the optimum ascent rate and stop locations to minimize on-gassing while not blowing out off-gassing nitrogen to the point of damage to the body. Since we still don't even understand the actual physiological process it's hard to really answer the question. We just keep revising deco algorithms as more data becomes available and more studies give us information to use. But so far it's still a lot of theory... Not a lot of fact.


Dan-O

Sent from my iPad using Tapatalk 2

There is absolutely no reason for you to be posting all of this common sense and practical knowledge all over an otherwise foollhardy and incorrect internet forum discussion.

 

[Michael Guerrero]

foollhardy and incorrect internet forum discussion.

Huh???

Are there incorrect statements in this thread?