Gradient Factors - What is Everyone Using?

[NDonahue]

I’m writing this post to test my understanding with people who know more than me — I’m not ascerting that I’m right.

I often do my deco on a reef, so I typically adjust my rate of ascent to keep myself right on the GF line; this means that I usually don’t don’t have an actual stop obligation until I hit 6m. Because of this, I think about the difference between GF H and L as just changing the slope (rate) of my ascent, and like everyone else, I think of each GF% as my relative conservatism.
when GF Low is less than GF high I ascend more slowly than when GF L = GF H. The GF as a % 100% (M Val) just modifies how long I can stay at depth; and obviously if I were to set GF low to 100% I would have no flexibility to adjust my overall rate of ascent.

I think my continuous ascent is theoretically less conservative than ascents and stops because with stops a diver spends more time “below“ the GF line.

it’s definitely more interesting than staring at the same patch of coral and rock while I wait out a deco obligation.

I feel like slow continuous ascents have helped to better understand GF — although maybe someone is about to educate me that what I think I know is not correct. I would welcome being corrected.

 

[No_Stress]

It would be nice to see computer manufacturers introduce GFs which can be scaled to depth or total deco time.
NDL 85, then once exceeded that the plan shifts to 80, if more that X total deco shift to 75 etc.

Although with GF99 and surf GF it's easy enough to keep an eye on those. I'm not a tech diver (yet) but I keep an eye on both those pieces of data and will either slow an ascent and/or lengthen a SS based on that information.

But why would you want that???

 

[inquis]

like everyone else, I think of each GF% as my relative conservatism.
when GF Low is less than GF high I ascend more slowly than when GF L = GF H

It's clear that GF high is monotonically related to conservatism. GF low not so much. Ascending too fast OR too slow is less conservative.

The more modern approaches that the Navy use allow for a 1:1 pressure reduction. If the controlling tissue gas pressure drops by that of 1 fsw, you're allowed to go up by 1 fsw. Rather intuitive to my thinking. As Doolette indicates in one of the articles linked up thread, this can be reasonably achieved in Buhlmann+GF by setting GFlow=0.83xGFhigh.

 

[NDonahue]

It's clear that GF high is monotonically related to conservatism. GF low not so much. Ascending too fast OR too slow is less conservative.

The more modern approaches that the Navy use allow for a 1:1 pressure reduction. If the controlling tissue gas pressure drops by that of 1 fsw, you're allowed to go up by 1 fsw. Rather intuitive to my thinking. As Doolette indicates in one of the articles linked up thread, this can be reasonably achieved in Buhlmann+GF by setting GFlow=0.83xGFhigh.

Thanks, that all makes sense.

So if I understand your point, if GF High controls my overall conservatism, then GF Low controls the overall ascent rate (measured as max depth / TTS at max depth). The closer GFH minus GFL is to zero, the higher my overall ascent rate to exit at my target GF High. Maybe it’s easier to just say GF low only lengthens or shortens total run time to achieve a given GF High at exit.

I’d read the Doolittle article before and adopted the 83% guidance (85/70), but what I find interesting is that when I play with MultiDeco to look at the sensitivity of GFH-GFL to total run time. It appears that GF Low has almost no impact on total run time (and therefore overall ascent rate). The difference between 85/55 and 85/85 is only a couple of minutes of total run time for any normoxic / nitrox diving (down to ~55m) using 2 AL80s as max bottom gas volume.

This would maybe indicate that all but hypoxic divers (and maybe CCR divers not gas constrained) can just focus on GF High; and that GF Low doesn’t become material until you are very deep (>60 m) or at depth longer than 2 tanks of bottom gas can reasonably support. Is that too strong a statement?

 

[Wibble]

The GF-Low controls the starting point of your deco, i.e. where the first stop will be.
GF-High sets the length of time you stay on the last stop.
The slope of the line from GF-Low to GF-High applies to all other stops.

GF-Low is always less than (or equal to) GF-High. e.g. 50:80, 50:70, 95:95.

This might be of interest:

Evolution of Dive Planning - Shearwater Research

Plan the dive and dive the plan has long been the mantra employed in all areas of diving. Technical divers, in particular, spend more time planning their dives than many recreational divers. This is due […]

www.shearwater.com

 

[The Chairman]

The more I learn about deco and GFs, the less inclined I am to play with them. I am not smarter than Shearwater. I use their default religiously. The only thing I change, and it might be more out of habit than sense, is I do a single 1 minute deep stop if and only if I'm doing a single day of diving. No, I don't ride my NDL either. Usually, my dive is cut short by my hour time limit set by most dive ops, or I'm doing a real deco dive in the caves.

If I'm on a boat, I do a full 5-minute safety stop and then wait until the last moment to end it. That means I'm the last one up on the ladder. Then I sit for five minutes before I try to change my stuff over or even longer if it's the last dive of the day. If I'm in the Springs, I do a full minute 5-minute safety stop, and then rest another five minutes after surfacing before I exit the water. Mitigating circumstances may change this a bit.

 

[inquis]

if GF High controls my overall conservatism, then GF Low controls the overall ascent rate (measured as max depth / TTS at max depth).

I suppose that's one way to look at GFLow, although I view it mainly as determining the first stop depth. Its impact on total ascent time is a bit indirect or non-linear. You're right, for the profiles you indicated, there isn't much difference for various values of GFLow in the range of 50-GFhigh.

 

[dmaziuk]

But why would you want that???

To run out of gas and drown, presumably: if you planned your dive for 70:80 and your computer decided mid-dive you need 30:50... oops.

 

[huwporter]

To run out of gas and drown, presumably: if you planned your dive for 70:80 and your computer decided mid-dive you need 30:50... oops.

Ah, the infamous Suunto 'adaptive' approach:

"Hey there! You just violated one or more of the undocumented and unproven rules I have added to my code, so I'm going to go ahead and double your deco. You are fortunate I didn't decide to lock up completely and leave you on your own. Good luck!"

 

[exodusmaximus]

To run out of gas and drown, presumably: if you planned your dive for 70:80 and your computer decided mid-dive you need 30:50... oops.

I would hope your dive plan includes ample reserves...