Question Why GF high not lower that GF low?

[andrebreton93]

Hello

I wanted to know why the GF high is always lower or equal to the GF low.
the GF low is always lower or equal to the GF high.

Some studies tell us that deep stops are not that great compared to shallow stops.

Why not have a GF high lower than GF low to spend longer time on the last stop?

 

[scubadada]

Hi @andrebreton93

This seems a little mixed up. GF high is equal to or greater than GF low. GF low dictates the depth of the 1st stop when having a deco obligation. The GF high dictates the surfacing GF, whether from deco or no stop. A GF low=GF high will give you the shallowest profile.

 

[Capt Jim Wyatt]

I wanted to know why the GF high is always lower or equal to the GF low.

GF Low is normally Lower than GF High.

At the risk of sounding like an Alpha Hotel I think you need a better understanding of GFs.

 

[andrebreton93]

GF Low is normally Lower than GF High.

At the risk of sounding like an Alpha Hotel I think you need a better understanding of GFs.

I understand GF.
I think that you didn't understand my question as it's not that clear.

For example, we usually use flat GF 80/80, 70/70 or sometimes GF with deep stop 45/80.

But I wanted to know why not using 80/45 for example. Has someone already test this kind of GFs?

 

[harvalen]

The first line seems to be a typo and that's likely what the above replies got hung up on.

Here is an analysis of a NEDU study that compares heliox and trimix, N2 vs. He, what’s the difference? – The Theoretical Diver . The profile used more or less matches Buhlmann with GF Low 110 and GF High 36. I think the shallow profile in the NEDU deep stop study would've had higher GF Low than GF High if it was modeled with Buhlmann, but I might be remembering wrong there. So there might be a reason to dive profiles that you would get with GF High lower than GF Low, and NEDU seems to be using profiles like that.

Currently the experts, including those running the NEDU studies, seems hesitant to recommend anything other than GF Low slightly lower than GF High though. I guess you could get close to 80/45 on many dives by running 80/80 and just sticking around at 3-6 m until you get down to 45, so it's probably not far off from what many people do on benign reef dives where there is a lot to watch in the shallow.

 

[andrebreton93]

The first line seems to be a typo and that's likely what the above replies got hung up on.

Here is an analysis of a NEDU study that compares heliox and trimix, N2 vs. He, what’s the difference? – The Theoretical Diver . The profile used more or less matches Buhlmann with GF Low 110 and GF High 36. I think the shallow profile in the NEDU deep stop study would've had higher GF Low than GF High if it was modeled with Buhlmann, but I might be remembering wrong there. So there might be a reason to dive profiles that you would get with GF High lower than GF Low, and NEDU seems to be using profiles like that.

Currently the experts, including those running the NEDU studies, seems hesitant to recommend anything other than GF Low slightly lower than GF High though. I guess you could get close to 80/45 on many dives by running 80/80 and just sticking around at 3-6 m until you get down to 45, so it's probably not far off from what many people do on benign reef dives where there is a lot to watch in the shallow.

Thanks for the reply.
Oh yeah I f.cked up my first sentence, my bad.

 

[scubadada]

SurfGF gives us many options.

 

[L13]

A lot of time and gas is spent for each reduction in GF high as compared to reducing GF low.

Elevated GF is a risk, so your resources of time and gas are better spent reducing GF low once GF high drops to near GF low.

 

[inquis]

Why not have a GF high lower than GF low

Because the names wouldn't make sense!

But yes, some implementations will allow you to run a larger number to control the initial stop -- i.e., the first number of the pair. The main issue is there has been little testing at such settings (at least none that I'm aware of). The consequence is that the fast tissues (which are typically dominating during the initial ascent) are subject to a greater degree of supersaturation. However, you may not realize that the deco scientists back in the day were aware the faster tissues could tolerate more supersaturation, and this is *already* built in to the model. (The maximum supersaturation for a GF of 95/95 (or 85/85, 80/80, etc.) has a slope greater than 1 as the ambient pressure increases.)

Your suggestion of running a GF of, say, 85/75 would make the slope even greater than it already is, which is opposite of the direction current research has gone. I believe the more modern algorithms (e.g., those used by the Navy) are running a slope at or very near 1. Consequently, Doolette's practice for standard Buhlmann+GF is to set the first number as 0.83x the second in order to roughly approximate a slope near 1 on average (across the various tissues).

 

[dmaziuk]

I understand GF.
I think that you didn't understand my question as it's not that clear.

For example, we usually use flat GF 80/80, 70/70 or sometimes GF with deep stop 45/80.

But I wanted to know why not using 80/45 for example. Has someone already test this kind of GFs?

Workman's M-values are already greater at depth, and the argument behind deep stops was that they're "too much" greater, esp. for the fast tissues. Gradient actors were invented to "flatten" them.

You should look at Workman's work to see how exactly he arrived at these slopes.