Science behind GF

[dave22387]

OK guys and gals science question. The basic concept (not to get overcomplicated) of a Buhlman deco model is to achieve supersaturation of the tissues in the case of the ZHL16 and when one of the 16 compartments reaches the supersaturation limit a stop is generated to off gas that compartment to below the limit level and continue said procedure until all compartments are slightly above the supersaturation level.

My question to you fine folks is this... Is the rate of off-gassing at a constant pressure (depth and therefore gas pressure) completely linear??? Meaning it would increase due to a greater pressure differential between the absorbed gas in the tissue and the ambient pressure. In theory the greater the differential the faster the off-gassing. In other words, the more supersaturated you are the faster you would offgas said compartment. Is that rate of off-gassing linear from the supersaturation line to the supersaturation max limit for the algorithm?

The reason I ask is say for ease of discussion you have a GF of 50/50 using ZHL16C, If the rate of off-gassing is not a linear increase from the supersaturation line to the max line are we not just playing with fire and being test dummies??? If the rate of off-gassing at 50/50 isn't exactly 50% slower than 100% then how can we be certain we are doing the right thing mathematically?

Now don't get me wrong I like and use GF and I also agree that adjusting them to how you feel after a dive is a good thing but I also know guys who have gotten a hit who didn't necessarily deserve a hit based on their profiles. I know this was long winded but any input on this discussion would be appreciated as I believe the more you know about this stuff the better chance you have. Thanks in advance!

 

[boulderjohn]

Does this help?

 

[dave22387]

Not really to be honest. I completely understand conceptually gradient factors. The question is if you are at 80% of your M value at a deco stop are you really off-gassing at 80% of the pre-derived off-gassing rate that was determined when they came up with the original algorithms or actually off-gassing at a slower rate than 80%? Before GF's came into play the rate of off-gassing was measured at the M-Value I assume. Does the rate of off-gassing increase in a completely linear fashion from the point of supersaturation to the M line? I know it probably doesn't matter a great deal but I would just like to know. I tend to believe that a study of the rate change of off-gassing (Ambient/tissue pressure differential) from the supersaturation line up to the M-value line was never probably tested since GF's are a 3rd party idea.

 

[RyanT]

Does the rate of off-gassing increase in a completely linear fashion from the point of supersaturation to the M line?

No it doesn't. Biological systems are inherently non-linear. We don't know what the curves actually look like, so we estimate them using linear models. Fortunately, the linear models do a pretty good job of estimating the off gassing rates for most divers on most dives (as demonstrated by the huge number of safe dives). But, there is still a lot that don't know about what those physiological curves actually are from point A to B.

 

[dave22387]

No it doesn't. Biological systems are inherently non-linear. We don't know what the curves actually look like, so we estimate them using linear models. Fortunately, the linear models do a pretty good job of estimating the off gassing rates for most divers on most dives (as demonstrated by the huge number of safe dives). But, there is still a lot that don't know about what those physiological curves actually are from point A to B.

That is what I assumed. Thanks for the reply! So in theory by using GF to add conservatism we are also going a little outside of what little bit of scientific research has been conducted on decompression theory. Again, I like and use GF, I just like to know what I'm doing to the best of my ability.

 

[lermontov]

Not really to be honest. I completely understand conceptually gradient factors. The question is if you are at 80% of your M value at a deco stop are you really off-gassing at 80% of the pre-derived off-gassing rate that was determined when they came up with the original algorithms or actually off-gassing at a slower rate than 80%?

well in mathematical terms no but all things being equal then yes thats my understanding assuming your using 21% if your using a lower N2 content than 79% then it will be faster

 

[leadduck]

At first, ambient pressure is not relevant for off-gassing in the models (Buehlmann, VPM); off-gassing rate is determined by the difference between inert gas partial pressure in the tissue vs inspired inert gas partial pressure.

You might want to look into linear-exponential model (VVAL18) by Thalmann. Thalmann algorithm - Wikipedia
There's the assumption that bubbles slow down off-gassing, with the effect that a higher gradient does not increase off-gassing rate. That's the opposite of your concern: you assume that GF100 will have a much higher off-gassing rate than GF50, making GF50 less safe than assumed, whereas in the VVAL18 model assumption, bubbles make GF50 off-gassing stronger than assumed and almost the same as GF100.

 

[dave22387]

At first, ambient pressure is not relevant for off-gassing in the models (Buehlmann, VPM); off-gassing rate is determined by the difference between inert gas partial pressure in the tissue vs inspired inert gas partial pressure.

You might want to look into linear-exponential model (VVAL18) by Thalmann. Thalmann algorithm - Wikipedia
There's the assumption that bubbles slow down off-gassing, with the effect that a higher gradient does not increase off-gassing rate. That's the opposite of your concern: you assume that GF100 will have a much higher off-gassing rate than GF50, making GF50 less safe than assumed, whereas in the VVAL18 model assumption, bubbles make GF50 off-gassing stronger than assumed and almost the same as GF100.

Very good info thanks!

 

[rjack321]

Does the rate of off-gassing increase in a completely linear fashion from the point of supersaturation to the M line? I know it probably doesn't matter a great deal but I would just like to know. I tend to believe that a study of the rate change of off-gassing (Ambient/tissue pressure differential) from the supersaturation line up to the M-value line was never probably tested since GF's are a 3rd party idea.

1) Don't forget there are actually 16 different M-lines and since each one has a different slope as the controlling Mvalue changes you end up with a curve.
2) The offgassing rates as a % of m-value could in theory be tested in a wet chamber by monitoring offgassing. But how you would decide which tissue was on/off gassing independent from the others is pretty much impossible, they are only theoretical tissues and don't necessarily relate to a biological tissue directly. The net product of GFs have been tested but only after surfacing with doppler and other measurements of residual stress.

 

[hroark2112]

The other things to remember:

Deco theory is just that - theory
Offgassing and decompression are different for each diver depending on body composition and physiology
Deco may be different for you from day to day, depending on hydration and physical factors

Bottom line is to use your GF as a baseline, and adjust your dive plan and deco according to your condition on the dive. Adding extra time to your shallow stops may mean the difference between a good nights sleep and a miserable night!