Question about Shearwater default GF low settings

[PR_SMITH]

As @Valyngar says: probably for hysterical raisins. AKA "it sounded like a good idea at the time".

Check also: Deep-stops vs shallow-stops: an interesting read.

Yeah, gotta watch out for those "hysterical raisins"

 

[LFMarm]

According to the latest thinking by David J. Doolette (Gradient Factors in a Post-Deep Stops World), GF low is optimal when it is 83% of the GF high. Using this rule, the 3 Shearwater default factors should be: 60/75, 70/85 and 80/95. I am personally even more conservative and opt for 50/60 with the exception of very big dives.

 

[L13]

According to the latest thinking by David J. Doolette (Gradient Factors in a Post-Deep Stops World), GF low is optimal when it is 83% of the GF high. Using this rule, the 3 Shearwater default factors should be: 60/75, 70/85 and 80/95. I am personally even more conservative and opt for 50/60 with the exception of very big dives.

I think that is overstating Doolette's position a little. He uses 83%, but admits that the evidence for that particular value isn't as strong. The evidence is much stronger that GF-Low should be at least >= 50%.

 

[L13]

The evidence is much stronger that GF-Low should be at least >= 50%.

I wish even that evidence was stronger than it is.

But the evidence is strong enough that I also pick a GF-low that is a "nice number" that is close to 80-85% of GF-High. Basically the numbers @LFMarm listed.

 

[inquis]

The point of the 83% was to make the GF lines mostly parallel to the ambient pressure line (a characteristic of the Navy's current algorithms). Consequently, fast tissues have the same degree of supersaturation reduction as slow tissues. This backs off of Buhlmann's position that faster tissues could tolerate greater stress, but not as far as VPM or other "deep stop" models which tried to "protect" the fast tissues. Seems like a good middle ground to me, but no one claims that it is optimal to my knowledge.

 

[L13]

The point of the 83% was to make the GF lines mostly parallel to the ambient pressure line (a characteristic of the Navy's current algorithms). Consequently, fast tissues have the same degree of supersaturation as slow tissues. This backs off of Buhlmann's position that faster tissues could tolerate greater stress, but not as far as VPM or other "deep stop" models which tried to "protect" the fast tissues. Seems like a good middle ground to me, but no one claims that it is optimal to my knowledge.

I personally have a suspicion that Bulmann may be right here, but have no confidence in it. The difference between 60/75 and 75/75 isn't significant unless you are doing really big (for me) dives.

 

[dmaziuk]

Here's the study suggesting that the "confidence" interval is GF High 80..87 and GF Low 72..84 (-ish): Fraedrich follow-up – The Theoretical Diver

He did, however, only run them against DCIEM and VVAL.

OTGH Robert's latest post rather neatly shows the problem with where the models were tested vs. where we apply them, so how confident we are in that confidence is a bit of a question.

 

[L13]

Here's the study suggesting that the "confidence" interval is GF High 80..87 and GF Low 72..84 (-ish): Fraedrich follow-up – The Theoretical Diver

The values for GF High and GF low that you give do not occur anywhere in the linked "study", nether does the words "confidence" or "interval". It is a re-analysis of existing data. Basically he analyzed 5 other well known and experimentally validated models(Van Liew and Flynn, VVAL-79, DCIEM, and SAUL) and determined the range of GF's that would produce the same results as those models. The theory is that those models were well validated, so the GF's they agree on would also be "good".

Despite what I just said, I think you are right that it is an extremely valuable analysis. It is probably the best analysis of GF's with currently available data, but does not add any new data. It adds significant weight to modern trend of selecting GF's in the ranges you stated.

He did, however, only run them against DCIEM and VVAL.

As noted above, he also ran against SAUL, Van Liew and Flynn as well.

OTGH Robert's latest post rather neatly shows the problem with where the models were tested vs. where we apply them, so how confident we are in that confidence is a bit of a question.

You and Robert are absolutely right about that, but we don't have anything better.

 

[Kendall Raine]

According to the latest thinking by David J. Doolette (Gradient Factors in a Post-Deep Stops World), GF low is optimal when it is 83% of the GF high. Using this rule, the 3 Shearwater default factors should be: 60/75, 70/85 and 80/95. I am personally even more conservative and opt for 50/60 with the exception of very big dives.

He did not say "optimal." He wouldn't because he of all people knows no one knows what"s "optimal" across divers/dives. He simply explains his rationale for choosing a particular GFLo.

Here's what Doolette actually said: "With this information in mind, I set my GF low to roughly counteract the ZH-L16 “b” parameters (I have been using Shearwater dive computers with ZH-L16 GF in conjunction with my tried and true decompression tables for about three years). In ZH-L16, the average of “b” parameters is 0.83. I choose my GF low to be about 83% of the GF high, for instance GF 70/85. Although the algebra is not exact, this roughly counteracts the slope of the “b” values. This approach allows me to believe I have chosen my GF rationally [my emphasis] , is not so large a GF low as I am unable to convince my buddies to use it, and satisfies my preference to follow a relatively shallow stops schedule."

 

[LFMarm]

Thanks for the clarification. I am using his approach to picking the slope