Why 2 gradient factors ?

[grantctobin]

All replies make a lot of sense and support my point of view about more conservatism at depth.

thanks a lot buddies!

You’re using that word “conservatism,” but I doubt any of the researchers linked would use that word to describe it.

 

[Nelson_]

You’re using that word “conservatism,” but I doubt any of the researchers linked would use that word to describe it.

Well, in my opinion ‘gradient factors’ are a way to implement some safety, hence conservatism, opposed to using plain M-values. No?

 

[L13]

@scubadada good article, but it only explains what gradient factors are. It doesn’t explain why we should be more conservative at the deeper stops.

Because modern evidence has invalidated the theory that we should be more conservative at deeper stops..

 

[scubadada]

The second edition of Mark Powell's Deco for Divers, 2014, has a long chapter 2, Decompression Principles. Tissue compartments, tissue half-lives, supersaturation, critical supersaturation, and M-values are all discussed. There is no mention of gradient factors.

Gradient factors are discussed over 2 1/2 pages in chapter 6, Deep Stops and Bubble Models. GFs are described as a way to increase safety by staying further away from the M-value. There is no specific discussion regarding choosing a GF low lower than the GF high. An excessively low GF low is discussed as potentially resulting in continuing on-gassing of slower compartments, requiring longer shallow decompression to clear. I get the impression that a low GF low is largely a remnant of Pyle stops and bubble models. Perhaps a third edition of Deco for Divers would have an expanded discussion of gradient factors, incorporating more recent information.

See the article by Doolette that I cited in post #8

I am no expert and am only interested in and have followed the topic. Personally, I only do light back gas deco on approximately 5% of my dives of almost always less that 10 min. I have had no stops deeper than 10 ft. I would be glad to see more discussion.

 

[BlueTrin]

This video may be interesting

 

[boriss]

Well, in my opinion ‘gradient factors’ are a way to implement some safety, hence conservatism, opposed to using plain M-values. No?

They're just variables you use to manipulate your decompression profile based on an algorithm. You're not increasing conservatism if your GFLow is set too high for a decompression dive. You do increase conservatism if your GFHigh is set lower, but then you're just spending more time in the water.

In truth nobody knows what the ideal GF is for a given person on a particular dive. Understanding the factors that contribute to DCS is tangentially important; so if you haven't seen this information before it may be worth a read: The Many Factors in Decompression Stress

 

[Tracy]

I wonder why we use 2 gradient factors (GF low and GF high). Is there a reason to be more conservative at depth than near the surface? Why are we using 35/70 instead of 50/50 ?

thanks,

Nelson

Who says we are? Gradient factors are customizable for the diver. My high and low have been the same number for at least five years. I have played with them lots of different ways and have settled on ones that work well for me and the diving I do

 

[LFMarm]

0.83 is the ratio between the GFs that seems to make sense according to David Doolette (Gradient Factors in a Post-Deep Stops World). I typically decide my GF high based on the dive I am doing (conservatism, gas need, etc.) and calculate the GF Low with the 0.83 factor.

 

[dmaziuk]

Well, in my opinion ‘gradient factors’ are a way to implement some safety, hence conservatism, opposed to using plain M-values. No?

In the sense that lowering the "allowed overpressure" value adds conservatism. Remember that overpressure drives gas exchange so reducing it increases your off-gassing time. And with multiple compartments, some of them may still be on-gassing and the whole "safety" thing becomes... not quite that simple.

 

[rjack321]

Well, in my opinion ‘gradient factors’ are a way to implement some safety, hence conservatism, opposed to using plain M-values. No?

You are missing the point that as you slow down and stop earlier with a smaller low GF, to be more "conservative" for some tissues, other tissues are on-gassing and have more dissolved gas to eliminate later so your plan is less "conservative" for those tissues.

In the past this was conceptualized as "protecting the fast spinal tissues" from bubbles via deep stops - to in theory reduce DCS type II rates (which people consider to be 'worse' than type 1 DCS in 'slow' tissues). Which has all proven to be incorrect and not actually conservative if your end point is DCS rates overall.