Why 2 gradient factors ?

[Nelson_]

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

 

[scubadada]

Hi @Nelson_

Read some basic background and see if you still have questions Flexible Control of Decompression Stress - Shearwater Research

For no stop diving, only the GF high is applicable and is the percentage of the M-value that cannot be exceeded before surfacing. GF low applies to decompression diving and is the percentage of the M-value used to determine the depth of the first deco stop during ascent.

Edit: sorry, I see that your question is why are we more conservative at depth during decompression than we are when surfacing? Sorry for my premature response. I will defer your question to someone more knowledgeable.

 

[Nelson_]

@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.

 

[lowwall]

@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 you shouldn't?

GF Lo was created as a way to implement a decompression hypothesis that had since proven to be largely incorrect. Many of its adherents are only slowly accepting this which is why it's still out there, but the recommendations are clearly moving from the large early disparities like 30/70 towards parity.

 

[Nelson_]

Because you shouldn't?

GF Lo was created as a way to implement a decompression hypothesis that had since proven to be largely incorrect. Many of its adherents are only slowly accepting this which is why it's still out there, but the recommendations are clearly moving from the large early disparities like 30/70 towards parity.

Deep stops (Pyle) ?

> moving from the large early disparities like 30/70 towards parity.
Aha! That makes sense. What parity are you suggesting? 50/50? 70/70?

 

[Rich1280]

I believe the link is to the controlling tissue compartment, with the shorter time compartments tolerating an overpressure better than the slower compartments. As the middle compartments are likely to be controlling deeper (if memory serves), then we can be more conservative there and come closer to pure Buhlman (100/100) shallower as the tissue should tolerate it better. I'm sure there's a more technical and clearer explanation that someone can do for you, but that's what's loitering in the back of my head.
Caveat, I generally don't worry so much as down to 50m and 30ish minutes of deco it makes minimal difference, 2-3 minutes or so overall so I just stick with what we were taught and the team agreed on.

Rich

 

[Smache]

My understanding is that there was a period of time (90s?) when it seemed like bubble models might be a better way to plan decompression. The bubble models prescribed a deeper first stop and less time at shallower stops than the Bühlmann model. However, since the bubble models were not publicly available like the Bühlmann model, divers approximated the shape of the bubble model by starting with a lower gradient factor and gradually increasing it as they ascended.

The evidence for bubble models has kinda fallen apart, and I think recent research has found that they don't have any practical benefit to divers. Many divers now use similar numbers for their hi/lo gradient factors.

 

[scubadada]

See article by David Doolette Gradient Factors in a Post-Deep Stops World

 

[Nelson_]

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

thanks a lot buddies!

 

[Nelson_]

See article by David Doolette Gradient Factors in a Post-Deep Stops World

Very informative!