GF lo logic - decrease in presets with increased conservatism (Shearwater)

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Divectionist

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Location
Gold Coast, Australia
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I just don't log dives


A ScubaBoard Staff Message...

This thread has been moved from the Basic Forum to Advanced, as a better location for this technical discussion.
Specifically on Shearwaters, but in general as far as conservatism presets are concerned, I would like to understand the reasoning behind reducing the GF lo value as conservatism increases.

So whilst the GF hi percentage obviously decreases to have the diver surface with a greater margin of safety, why does the GF lo also step down in 5% increments for each 10% decrease in the hi value?

Perdix AI presets in OC Rec:
45/95 (Low conservatism)
40/85 (Medium conservatism)
35/75 (High conservatism)

Does starting/clearing your deco deeper on a high conservatism ascent provide for a marginally safer profile? Is there a sweet spot for NDL diving/accidental(=minimal) deco diving where slow tissue saturation is not as pronounced as during proper extended deco diving? Does this differ from intentional deco diving?

I would appreciate any input re the above for the benefit of those intending to customise their GFs. I am personally using a custom 50/75 setting in OC Rec on the Perdix because I still take the GF lo value seriously for NDL diving in case of accidental deco and looking at the general evidence, I would rather have a shallower stop than a deeper stop. But there is still a correlation that I would like to be clear on to feel like I make the best possible choice. I am not trying to edge towards a more aggressive setting as I am a conservative diver and choose a large safety margin despite being under 30 years old and physically fit. It is really more about making a decision for a 'set it and forget it' scenario that can be a constant principle going forward into some mild deco tech diving experience, until new studies/evidence comes along.
 
I have wondered about this also. These are Rec presets. Not only have I wondered why they decrease, but I have also wondered why they are just so low.

My primary computer for the last 9 1/2 years has been an Oceanic VT3 running DSAT. I dived a Dive Rite Nitek Q backup for about 3 years to learn Buhlmann with GF. About 95% of my dives are no stop, the remainder are light deco, almost all less than 10 min. I settled on running 75/95. The GF high of 95 matches DSAT reasonably well. The GF low of 75 kept my short deco stops shallow, I've not had a stop deeper than 10 feet. I would do 5 min SS for dives very close to NDL and I would do 3-5 min after clearing deco. I do the last part of my ascent at 15 ft/min.

I've now had a Teric for the last 9 months. I dive it at 95/95, still matches the VT3. Now, l use SurfGF to guide my SS or the padding of my deco so that I surface with a GF of no higher than the mid 80s, often much lower than that. Turns out my previous strategy was not far off.

Maybe @Shearwater Will comment on the choice for the rec presets
 
I suspect the reason why they've increased GFLow numbers when increasing GFHi numbers is to minimise the slope difference between the modified and un-modified m_values (lines) so as not to compel relatively deeper and longer stops (compared to not increasing the GFLow numbers) - mostly only relevant for tec divers.

That's the theory I think, but when I play with the 3 preset GFs on a 60m dive, and also take the extremes of 45/75 and 30/95, there's not much in it.

I used to be worried about accidental (i.e. very light) deco, and what would be an appropriate stop depth / time. Now, I don't think it's too much of a worry because [quote below]: (TL;DR - it doesn't have much / any real affect on stop depth and SurfGF is what really matters).

FWIW I'm diving with 99/99 for GF. My SurfGF is usually in the 50's, sometimes 60's, seldom in the 70's. Which mean's I am diving a reverse GF*. In practical terms it's probably averaging 80/55. I tend to take it very easy from 10m and shallower, and super-slow from my 6m SS (5 min) to the surface. On deeper squarish dives to ~40m, gas isn't an issue, so as long as I achieve my SurfGF I'm not too worried about seeing NDL=0. It's dives with a shallower average depth that I need to check I've got enough gas to get my SurfGF down even though NDL is farther from the limit; with 50bar reserve, my NDL and gas limits intersect at an average depth of 16m on GFHi of 99 (SAC is 1bar/min for AL80).

I wanted to somehow assess the wisdom of this seemingly aggressive GFHi and for the occasional dives where NDL get's pushed below 5min remaining. Here's my thought process:

At max Rec depths (or slightly beyond) on Air, a 40m dive for 20min produces stops from 9m for GFLo's between 60-83 with 24-27mins of deco using a GFHi of 70. GFLo83+ is 6m stop depth.

Reducing the deco time to something closer to 'light' deco, say a 40m dive for 15mins, with a GFHi of 70, a GFLo of 65+ results in 12min of total deco starting at 6m. With a GFHi of 60, 6m stops are produced from GFLo of 70+

Given GLo has less of an effect on the depth of the first stop as max depth becomes shallower, at 30m for 25min produces 15mins of deco starting from 6m for GFLo 53+

To me, this reinforces SurfGF targeting, long SS and super slow ascent as per Recreational Ascent Rate in the last 15 feet

That in turn (perhaps) defocuses NDL at depth ... because:

The difference between GFHi of 90 & 99 is 3 mins at 30m, 2min at 40m. Between GFHi 80 & 99 it's 6 and 3mins respectively. i.e. not massive differences at depth and it's just a question of gas supply and time availability at/near SS depths. Secondly there's seems almost/no difference between doing what I'm doing and say very 'light-deco' on a lower GFHi and a higher GFLo provided I'm slow from 10m and shallower.

re Reverse GFs: *The question of whether the actual (self adjusted) m_value line I'm following is a reverse hockey-stick (like merely padding the SS), or is a modified linear mayn't be too relevant for Rec depths.

(p.s. I realise/agree on a NDL dive GFLo plays no effective role).

Thoughts? Holes in my logic? and who else dives a similar NDL/SurfGF style?

from: What's your SurfGF and how does it compare to your (Rec) GFHi?

The above answer doesn't change much / if at all for light deco (air / low Nitrox / Rec depths).

It does for change a lot for Tec (dare I say) real deco, whereby GFHi is the surfacing target.

I take comfort that the above 'works' given Tec's current typical GFHi, and Rec's current typical SurfGF (both ~70).
 
Specifically on Shearwaters, but in general as far as conservatism presets are concerned, I would like to understand the reasoning behind reducing the GF lo value as conservatism increases.

So whilst the GF hi percentage obviously decreases to have the diver surface with a greater margin of safety, why does the GF lo also step down in 5% increments for each 10% decrease in the hi value?

Perdix AI presets in OC Rec:
45/95 (Low conservatism)
40/85 (Medium conservatism)
35/75 (High conservatism)

Does starting/clearing your deco deeper on a high conservatism ascent provide for a marginally safer profile? Is there a sweet spot for NDL diving/accidental(=minimal) deco diving where slow tissue saturation is not as pronounced as during proper extended deco diving? Does this differ from intentional deco diving?

I would appreciate any input re the above for the benefit of those intending to customise their GFs. I am personally using a custom 50/75 setting in OC Rec on the Perdix because I still take the GF lo value seriously for NDL diving in case of accidental deco and looking at the general evidence, I would rather have a shallower stop than a deeper stop. But there is still a correlation that I would like to be clear on to feel like I make the best possible choice. I am not trying to edge towards a more aggressive setting as I am a conservative diver and choose a large safety margin despite being under 30 years old and physically fit. It is really more about making a decision for a 'set it and forget it' scenario that can be a constant principle going forward into some mild deco tech diving experience, until new studies/evidence comes along.
Fashion at the time. There was a belief that deep stops helped, VPM etc produced deep stops and GF was designed to mimic those profiles on the cheap. Then the NEDU study came along and fashions changed.

It doesn’t generally matter for no stop diving so nobody really cares. Also internet people generally assume ZHL16/GF is not deep stops and anything that mentions bubbles is, so GF can be all things to all people.

Why do you want to change your GF numbers? As in, what are you trying to achieve?
 
Why do you want to change your GF numbers? As in, what are you trying to achieve?

I set the GF hi at 75 for conservatism on my NDL display and adjusted the default GF lo of 35 up to 50 because of the 'new fashion' of shallower stops. This does not matter much at the moment because it is very unlikely that I enter accidental deco, in which case the extra stop would be minute, but I'll get into some planned deco diving next year and would like to 'set and forget' values that make sense as per the current state of knowledge. Reading up on the subject, as well as googling for various forum discussions, I feel that 50/75 is a safe and sound setting.
 
I would like to understand the reasoning behind reducing the GF lo value as conservatism increases.

Hello,

I would say Ken is right: "fashion at the time".

In the light of the literature as it currently stands there is no logical reason to decrease the GFLo to achieve greater conservatism (at least, not from a starting point of 45).

Something more like 50/90 (low); 50/80 (medium); and 50/70 (high) would be more confluent with the current state of the science in my opinion.

Simon M
 
Hello,

I would say Ken is right: "fashion at the time".

In the light of the literature as it currently stands there is no logical reason to decrease the GFLo to achieve greater conservatism (at least, not from a starting point of 45).

Something more like 50/90 (low); 50/80 (medium); and 50/70 (high) would be more confluent with the current state of the science in my opinion.

Simon M

Very interesting. I'm really curious about the recent literature / science to support those 'high' GFHi numbers (~80-90~). I'm assuming you're referring to Tec diving meaning that GFHi is ~obtained on the surface assuming no padding of last stop(s). If there's more you could share it would be appreciated.

p.s. another fan here of your presentations too (and the great jokes too).
 
tagging this thread for answers.
 
I see, easy setting to update with the next firmware perhaps @Shearwater
They have had plenty of chances to revise these numbers. I suspect they will not bother. 1) they don’t make much difference, 2) how can they be sure they are better? 3) now they might be being said to recommend those new numbers - when someone gets bent with them are they responsible?
 

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