Question GF Low

[L13]

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

Baker's paper, where he presents the GF algorithm. The Theoretical Diver Blog, written by the lead implementer of the most well know open source implementation of the algorithm in SubSurface. What Shearwater actually does is harder to determine, given it is proprietary code.

 

[inquis]

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).

It would seem the difference is one of terminology. To my thinking, the individual tissue anchors are the point where each GFLow line intersects that tissue's maximum inert gas pressure. The Theoretical Diver calls that the "deepest ceiling" for that tissue. I can see how the generation of the ascent schedule can use the single anchor/deepest ceiling of the initial leading tissue. After the first stop, none of the other anchors matter since the ascent is underway. Running GF 50/75 with a first stop at 50 ft, you just wait until all tissues are such that none of them will be above 55% after ascending to 40 ft, then 60% after going to 30 ft, etc.

However, I still think the concept of an individual tissue's ceiling requires knowledge of its particular deepest ceiling or max tissue tension (what I've been calling its anchor). Say that some non-controlling tissue has inert gas pressure that intersects the GFLow line at 30 ft. Just because we stop at 50 ft (for the benefit of the controlling tissue) doesn't mean its ceiling is suddenly 50 ft. It's still 30 ft.

In summary, I agree the ascent profile can be generated with just a single "anchor" / starting depth. But, generally speaking, you have to keep track of each tissue's maximum tension throughout (or equivalently, the depth where its GFLow line equals that). Those are what I mean by the multiple "anchors".

 

[inquis]

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?

Specifically on this, with a first stop at 100 ft, there are 10 discrete stops (excluding the surface): 100 ft/40%, 90 ft/44%, 80 ft/48%, etc. as you allude. In order to leave 90 ft, all tissues must have a projected GF (at 80 ft) no higher than 48%. Similarly at other stops, it's all of the tissues compared to the same limiting value.

 

[inquis]

What Shearwater actually does is harder to determine, given it is proprietary code

However, when Shearwater's downloaded computations agree favorably with those of Subsurface (and they do), it's not a stretch to think they are very similar computations.

 

[elmo]

It would seem the difference is one of terminology. To my thinking, the individual tissue anchors are the point where each GFLow line intersects that tissue's inert gas pressure. The Theoretical Diver calls that the "deepest ceiling" for that tissue. I can see how the generation of the ascent schedule can use the single anchor/deepest ceiling of the initial leading tissue. After the first stop, none of the other anchors matter since the ascent is underway. Running GF 50/75 with a first stop at 50 ft, you just wait until all tissues are such that none of them will be above 55% after ascending to 40 ft, then 60% after going to 30 ft, etc.

However, I still think the concept of an individual tissue's ceiling requires knowledge of its particular deepest ceiling or max tissue tension (what I've been calling its anchor). Say that some non-controlling tissue has inert gas pressure that intersects the GFLow line at 30 ft. Just because we stop at 50 ft (for the benefit of the controlling tissue) doesn't mean its ceiling is suddenly 50 ft. It's still 30 ft.

In summary, I agree the ascent profile can be generated with just a single "anchor" / starting depth. But, generally speaking, you have to keep track of each tissue's maximum tension throughout (or equivalently, the depth where its GFLow line equals that). Those are what I mean by the multiple "anchors".

I remember when I actually had free time. Back then I was an active contributor to Subsurface. I can’t take credit for much but one of my more meaningful contributions was the code that defines the GFlow “anchor”. As indicated by the Theoretical Diver (definitely not me - Robert is far smarter than I am) blog, the anchor depth is the deepest ceiling for the dive. While not explicitly stated in the blog, the anchor is the same for all tissues. I doubt this has changed since I was involved (you could check the code since it’s public).
The original implementation/definition of gradient factors comes from Eric Baker’s Fortran code (not Bühlmann), and he took the anchor as the ceiling calculated using GFlow immediately prior to starting the ascent to the first deco stop. He also enforced a stop at this depth, even if the ceiling had cleared prior to reaching the stop - a depth that is necessarily the same for all tissues since they all do the same dive. That’s fine for a dive planner (although it produces odd results for extremely non-square profiles), but not much help for a dive log or computer, where the start of the deco phase of a dive is not defined. It just so happens that the deepest ceiling of a square-ish profile dive occurs about the start of the ascent to the first deco stop. So that is what we use in Subsurface.
Ed: can’t quote good
Ed: I can’t take credit for anchoring GFlow at the deepest ceiling. I confused myself with the equivalent parameter in the VPM code. Regardless, it’s the same value used for all tissues.

 

[BlueTrin]

Baker's paper, where he presents the GF algorithm. The Theoretical Diver Blog, written by the lead implementer of the most well know open source implementation of the algorithm in SubSurface. What Shearwater actually does is harder to determine, given it is proprietary code.

Ah thanks I always forget that I can check Subsurface code to see an implementation.

 

[Nelson_]

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

Very good question! That’s exactly the problem we run into if GF Low is not at a fixed amb pressure.

 

[Nelson_]

I remember when I actually had free time. Back then I was an active contributor to Subsurface. I can’t take credit for much but one of my more meaningful contributions was the code that defines the GFlow “anchor”. As indicated by the Theoretical Diver (definitely not me - Robert is far smarter than I am) blog, the anchor depth is the deepest ceiling for the dive. While not explicitly stated in the blog, the anchor is the same for all tissues. I doubt this has changed since I was involved (you could check the code since it’s public).
The original implementation/definition of gradient factors comes from Eric Baker’s Fortran code (not Bühlmann), and he took the anchor as the ceiling calculated using GFlow immediately prior to starting the ascent to the first deco stop. He also enforced a stop at this depth, even if the ceiling had cleared prior to reaching the stop - a depth that is necessarily the same for all tissues since they all do the same dive. That’s fine for a dive planner (although it produces odd results for extremely non-square profiles), but not much help for a dive log or computer, where the start of the deco phase of a dive is not defined. It just so happens that the deepest ceiling of a square-ish profile dive occurs about the start of the ascent to the first deco stop. So that is what we use in Subsurface.
Ed: can’t quote good

Aha! THAT answers my question! Thanks buddy

 

[Wibble]

Very good question! That’s exactly the problem we run into if GF Low is not at a fixed amb pressure.

GF-Low is the point on the ambient pressure v tissue tension graph for the first stop. That sets the slope of the "ceiling" with GF-Hi being the surface.

 

[Nelson_]

GF-Low is the point on the ambient pressure v tissue tension graph for the first stop. That sets the slope of the "ceiling" with GF-Hi being the surface.

Loud and clear!