Why 2 gradient factors ?

[Nelson_]

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

Screen capture from the Shearwater manual…

 

[Brett Hatch]

Screen capture from the Shearwater manual…

If you think this is a gotcha, you might want to ask yourself whether the authors of the Shearwater document are the same people as the researchers linked above. Grant's point directly answers your question: "conservatism" is a slightly misleading term here, because a lower GF is not the same thing as higher conservatism. Posts #19 and #20 explain why.

At any given point in the dive, the fast, middle, and slow tissues are doing completely different things. A lower value GFLo makes things easier on the faster tissues, and harder on the slower tissues. It is unclear whether this is a win for health outcomes, but it is quite clear that it results in a longer total deco time. Based on that information, different divers make different choices, knowing full-well that the supporting evidence is thin, and there could be a better choice out there.

If you haven't read it yet, Deco for Divers gives a very approachable overview of this, and is widely available for around $40.

Edit: I should flag that I don't have any skin in this game, since I am not trained for, and don't do deco dives. If somebody who knows better disagrees with me below, they are probably right and I'm probably wrong.

 

[rjack321]

Screen capture from the Shearwater manual…

So? as mentioned multiple times, its a pair of numbers. And bigger vs smaller GFs don't automatically mean more conservative or less conservative.

 

[dmaziuk]

So? as mentioned multiple times, its a pair of numbers. And bigger vs smaller GFs don't automatically mean more conservative or less conservative.

It's established use: more overpressure -> more aggressive off-gassing, less overpressure -> more conservative off-gassing. Every DC manual calls it "conservatism" factors so I wouldn't split too many hairs over it.

 

[inquis]

I think it's uniformly agreed that GFHigh (the second of the pair) is inversely related to conservatism. For NDL dives, the second GF is the only one that is used, and the lower it is the more conservative the setting.

What is far from settled is what goes on with the GFLow (the first of the pair, which controls the depth of the first stop). As the Shearwater excerpt states, refer to other sources for further explanation. Current prevailing thought is that a GFLow of, say, 20 is MORE risky than, say 50 because of the additional on-gassing during the deeper stops it requires.

 

[rjack321]

I think it's uniformly agreed that GFHigh (the second of the pair) is inversely related to conservatism. For NDL dives, the second GF is the only one that is used, and the lower it is the more conservative the setting.

What is far from settled is what goes on with the GFLow (the first of the pair, which controls the depth of the first stop). As the Shearwater excerpt states, refer to other sources for further explanation. Current prevailing thought is that a GFLow of, say, 20 is MORE risky than, say 50 because of the additional on-gassing during the deeper stops it requires.

This is what I was getting, but for keep in mind that we've discussed some particularly unusual GFs here in the past and there's pretty much nothing known about 100/70 for instance. So GF low bigger than 10 is more conservative except once it higher than _____ the data are non-existent and we have no idea anymore. GF high is a bit more intuitive at least

 

[Tracy]

This is what I was getting, but for keep in mind that we've discussed some particularly unusual GFs here in the past and there's pretty much nothing known about 100/70 for instance. So GF low bigger than 10 is more conservative except once it higher than _____ the data are non-existent and we have no idea anymore. GF high is a bit more intuitive at least

How would 100/70 work?
Your high has to be equal or higher, the math doesn't work any other way.

 

[inquis]

How would 100/70 work?

It works just fine. The deco line starts on the M-Value line at depth. The other end is no different (70% between ambient & the M-Value line at the surface).

 

[Tracy]

It works just fine. The deco line starts on the M-Value line at depth. The other end is no different (70% between ambient & the M-Value line at the surface).

How are you planning it? The math doesn't work for me. Multi deco won't accept it, shearwater won't accept it.

 

[lowwall]

GF Low can't be higher than GF High by definition. GF High is the maximum percentage of any M value that is allowed at any point during the dive.

For example, here's how the builders of OSTC technical dive computers use GF High in their calculations.

GF high

With a GF high value smaller than 100% we reduce the maximum permissible overpressure in the tissues that the OSTC uses for calculation of the ceiling depth and the no-stop limit.

Put simply: If you set a GF high of 80%, then the ceiling depth shows that depth at which the leading tissue has a saturation of these same 80% of the Bühlman-maximum value. In the same way the no-stop limit is calculated so that when reaching the surface exactly at the end of the no-stop limit no tissue will have more saturation than 80%. Without gradient factor it would be 100%

Maybe someone is confusing GF Hi with Shearwater's Surface GF?