Question GF Low

[L13]

Because you may never touch the line and then you’d never set the GF lo.

I imagine they set some arbitrary tolerance where if your distance to this stop is less than tolerance then you started your ascent for the purpose of GF factors.

They may very well do that, but no mater what tolerance you pick, someone may always not quite touch that new line. In the end it is all arbitrary.

Or maybe it’s based on your max ceiling?

As I understood it, when your depth exceeds the stop depth increment immediately below your lowest ceiling, it sets that depth increment as the GF-Low depth and runs from there. But I could be completely wrong, lol.

 

[BlueTrin]

They may very well do that, but no mater what tolerance you pick, someone may always not quite touch that new line. In the end it is all arbitrary.


As I understood it, when your depth exceeds the stop depth increment immediately below your lowest ceiling, it sets that depth increment as the GF-Low depth and runs from there. But I could be completely wrong, lol.

Totally agree, I am just curious

 

[inquis]

Out of curiosity, does any know when a Shearwater will decide you reached your first stop and will ‘anchor’ GFLo?

It's already been anchored by the maximum amount of tissue loading. [ETA: no "tolerances" to worry about.] The ceiling calculation, in fact, depends on that anchor. (Anchors, actually, as there is one for each tissue compartment.) If the max tissue loading in a compartment increases (as is typical before ascending), the anchor is reestablished (and the overall ceiling/stop depth is updated as needed).

 

[BlueTrin]

It's already been anchored by the maximum amount of tissue loading. The ceiling calculation, in fact, depends on that anchor. (Anchors, actually, as there is one for each tissue compartment.) If the max tissue loading in a compartment increases (as is typical before ascending), the anchor is reestablished (and the overall ceiling/stop depth is updated as needed).

That makes a lot of sense.

That makes it less dependent of the ascent speed too.

 

[KenGordon]

It's already been anchored by the maximum amount of tissue loading. [ETA: no "tolerances" to worry about.] The ceiling calculation, in fact, depends on that anchor. (Anchors, actually, as there is one for each tissue compartment.) If the max tissue loading in a compartment increases (as is typical before ascending), the anchor is reestablished (and the overall ceiling/stop depth is updated as needed).

What makes you say there would be independent anchor depths per compartment?

 

[L13]

It's already been anchored by the maximum amount of tissue loading. [ETA: no "tolerances" to worry about.] The ceiling calculation, in fact, depends on that anchor. (Anchors, actually, as there is one for each tissue compartment.) If the max tissue loading in a compartment increases (as is typical before ascending), the anchor is reestablished (and the overall ceiling/stop depth is updated as needed).

Do you have a reference document for separate anchors per tissue? Separate ceilings yes, but I can't find separate GF-Low anchors. Everything I am reading suggests only one anchor at the deepest ceiling for any tissue.

 

[BlueTrin]

Do you have a reference document for separate anchors per tissue? Separate ceilings yes, but I can't find separate GF-Low anchors. Everything I am reading suggests only one anchor at the deepest ceiling for any tissue.

Can you share what you have or mention the source of your documents? Just curious.

 

[inquis]

Do you have a reference document for separate anchors per tissue? Separate ceilings yes

The definition of the anchor is the maximum tissue tension seen by a given tissue during the dive. That varies per tissue, so...

With apologies, I don't have a document I can point you toward that states this. Ultimately, Baker's FORTRAN code perhaps. Or the Subsurface code. The Theoretical Diver blog linked earlier has some images that plot the per-tissue deco lines, IIRC.

 

[inquis]

What makes you say there would be independent anchor depths per compartment?

Because each tissue has its own maximum inert gas pressure. Whether you consider that pressure to be the anchor or the depth at which the per-tissue GFLow line equals that pressure is a bit immaterial since it's a linear mapping.

Basically, the ZHL-16GF algorithm computes EVERYTHING on a per-tissue basis, and then selects the "controlling" tissue based on the largest of the (per-tissue) calculated ceilings.

 

[L13]

Because each tissue has its own maximum inert gas pressure. Whether you consider that pressure to be the anchor or the depth at which the per-tissue GFLow line equals that pressure is a bit immaterial since it's a linear mapping.

Basically, the ZHL-16GF algorithm computes EVERYTHING on a per-tissue basis, and then selects the "controlling" tissue based on the largest of the (per-tissue) calculated ceilings.

I reviewed Bakers paper on GF's and The Theoretical Diver blog, and they both seem to indicate that the depth used for the linear interpolation of GF between GF-Low and GF-High for all tissues is the depth where the limiting tissue reaches GF-Low.

I.E. Say you are using GF 40/80 and on your ascent the limitiing tissues's GF_Low ceiling is 100 ft.At 90 ft is the move to 80 ft limited by all tissues being bellow GF=48 at 80 ft, or does each tissue have a different GF limit to go to 80 ft? At 30 ft is the move to 20 ft limited by all tissues being bellow GF=72 at 20 ft, or does each tissue have a different GF limit to go to 20 ft?

From The Theoretical Diver:
"GFlow is supposed to be applied at the first stop depth"
and:
"...in Subsurface we use the deepest ceiling encountered during the dive as the anchor depth for GFlow (also since for logged dives there is no clear distinction between the bottom part of the dive and the ascent)."

Apparently, at least in SubSurface, GF-Low is anchored at the same depth for every tissue.

UNLESS: maybe you and I are referring to something different by "anchor" with regard to GF-Low.