Question GF Low

[Nelson_]

Hi,

Can someone tell me at what ambient pressure GF Low is defined? For GF High it is 1 bar, but I don’t find any info about the ambient pressure for GF Low. It is important because that influences the slope of the gradient line. The higher the pressure for GF Low, the steeper the gradient line, right?

Thanks for any help,

Serge

 

[Wibble]

Look at the ambient pressure vs. tissue tension graph. GF-Low affects the slope of the line; 30/80 in the diagram. It's more common nowadays to use something like 50:80 which will move the first stop shallower.

Surface GF and Other Teric Musings - Shearwater Research

Powerful yet Small Imagine having a tool that would give you a numerical percentage value of the decompression stress you would encounter if you were to instantaneously ascend to the surface. Not all dives are […]

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This diagram is key to understanding gradient factors. There's some good youtube videos of lectures by people such as Mark Powell. It is worth studying this as it's key to decompression theory.

 

[inquis]

Can someone tell me at what ambient pressure GF Low is defined?

It's a good question that most explanations gloss over. Typically, the vertical component of the deco line's right endpoint (see the blue arrow in the graph @Wibble posted) is fixed by the maximum inert tissue pressure during the dive. The horizontal (depth or ambient pressure) component is simply where that (horizontal) pressure line intersects the (angled) GFLow line.

The stop depth will then be that intersection depth rounded up to a multiple of 10 ft or 3 m. That means you might stop a little deeper than the intersection depth on a direct ascent. Alternatively on a multi-level dive, you will probably stop shallower than the intersection depth due to significant off-gasing during the shallower level(s).

 

[Nelson_]

I make myself more clear... Look at the GF images (from Green to Orange). The image for GF High is drawn at 1 bar (or 1 ATA). But where should GF low be located on the horizontal ambient pressure line? 5 bar, 10 bar? If you draw a vertical line from the GF Low image it crosses the horizontal pressure axis, but where? At what distance from 1 bar?

 

[tursiops]

I make myself more clear... Look at the GF images (from Green to Orange). The image for GF High is drawn at 1 bar (or 1 ATA). But where should GF low be located on the horizontal ambient pressure line? 5 bar, 10 bar? If you draw a vertical line from the GF Low image it crosses the horizontal pressure axis, but where? At what distance from 1 bar?View attachment 812706

GFhigh is NOT at 1 bar at surfacing; it is at 80% of the M-value (for that compartment), which is a number higher than 1 bar. Similarly, GFlow is at 30% of the M-value for whatever the ambient pressure is at depth.

 

[Nelson_]

GFhigh is NOT at 1 bar at surfacing; it is at 80% of the M-value (for that compartment), which is a number higher than 1 bar. Similarly, GFlow is at 30% of the M-value for whatever the ambient pressure is at depth.

I’m talking about the crossing of the vertical line through the GF Low/High points with the horizontal ambient pressure line (X-axis). Look at the vertical arrow at the right side please.

 

[tursiops]

I’m talking about the crossing of the vertical line through the GF Low/High points with the horizontal ambient pressure line (X-axis). Look at the vertical arrow at the right side please.

That is whatever ambient pressure you are at, at depth. During your ascent, your ambient pressure goes from (say) 5 bars at 40m to 1 bar at the surface.

 

[inquis]

But where should GF low be located on the horizontal ambient pressure line?

The GFLow endpoint of the deco line (i.e., the right endpoint) is not defined by an ambient pressure (a vertical line as you have drawn). It is fixed vertically by the tissue loading, as I mentioned above. The position on the horizontal axis is just the intersection of that tissue (horizontal) line and the GFLow% line (angled).

Perhaps the picture & explanation in this post will better explain things:

how to calculate where your first decompression stop is

I'm going to get crap for this. so much crap. why I decided to even post it, not totally sure, but nobody else would probably know but this forum. I've tried looking it up online and can't find really anything. to preface this, it's only for brain exercise and academic purposes. I am NOT going...

scubaboard.com

 

[Nelson_]

Still not at the same frequency… let me put it this way: the more you move the GF Low to the left, the flatter (more horizontal) the ceiling line becomes. The more you move it to the right the steeper. So, the place where you put GF Low influences the ceiling line.

 

[inquis]

the more you move the GF Low to the left

Again, you do not move GFLow "to the left", at least directly. Your choice of GFLow dictates the angle of an entire line angling upward. See the lower "dotted" line in this image (taken from the post I linked). [ETA: that line is always GFLow% of the vertical distance from the dashed ambient line to the solid M-Value line. In this example, GFLow was 40%.]

Your tissue tension (on this particular example) is the vertical component at the blue star on the very right (at 100 ft depth). The intersection of the GFLow line with the horizontal tissue tension is the anchor point for the deco line. The depth at which that intersection occurs is simply a consequence. You do NOT choose a depth as the determining factor of that anchor point.