Question about Shearwater default GF low settings

[scubadada]

If you set SurfGF to 85, then your dive computer will apply that as the threshold for all tissue compartments, slow or fast, and it’s not going to matter whether the planned dive was a NDL or DECO dive. You computer will suggest a “safety stop” or a “deco stop” until your computer calculates a SurfGF at or below 85 for all abstract tissue compartments. Whether the dive was NDL or DECO being irrelevant for this calculation.

Is this too obvious ?

Or are you asking a different question ? For slower tissues, there may be higher N readings on a DECO dive compared to an NDL dive, but your dive computer will suggest stop lengths to bring all tissue groups below SurfGF.

You can't set your SurfGf, you set your GF high. Shearwater computers do not take the safety stop into consideration as it is not required. A safety stop will reduce your surfacing GF below that of a direct ascent. I would imagine that you are close to your GF high at the end of your last deco stop.

My GF high is set at 95. When I am very close to NDL or doing light deco, I use SurfGF to surface with a GF of <80.

 

[Umuntu]

My question can not be simplified anymore.

You are positing they are equivalent risk, others are taking a different stance.

I don’t know , and would like actual data but I don’t think it exists.

No, not equivalent risk.

For a NDL dive your SLOW tissues may be well below your SurfGF because they haven’t reached that level on your dive, but your FAST tissues may be just below your SurfGF threshold of 85 when your dive computer indicates that it’s okay for you to surface.

For a DECO dive, at the end of your deco stops, your FAST tissues may be well below your SurfGF, because they are fast, but your SLOW tissues may be just below your SurfGF threshold of 85 when your dive computer indicates that it’s okay for you to surface.

These are quite different scenarios. Hard to say which has greater risk since the tissue models are theoretical.

 

[Miyaru]

Would ending an NDL dive with a surfGF of 85 be equivalent risk to ending a deco dive with surfGF of 85?

According to your computer: equivalent risk

According to your body: probably an equivalent risk.

This is where that warning in the manual kicks in: this computer has no brain. You have to do the actual thinking yourself.
So at this point you add in all the factors that influence on/off-gassing.
With your own brain.

 

[lostsheep]

According to your computer: equivalent risk

According to your body: probably an equivalent risk.

This is where that warning in the manual kicks in: this computer has no brain. You have to do the actual thinking yourself.
So at this point you add in all the factors that influence on/off-gassing.
With your own brain.

View attachment 884022

The empirical nature of all decompression models has bothered me since I first learned that fact.

That’s said, I’ll take statistics over nothing and I fall back on what my DP instructor said, “if you don’t want to get bent, do more deco.”

 

[dmaziuk]

The empirical nature of all decompression models has bothered me since I first learned that fact.

You have to define "risk" first: since the outcome is defined as "symptoms of clinical DCS", all these SurGFs, ISSes, and even bubble scores, are snake oil as far as the model's concerned. The outcome is 100% empirical: you either need a chamber ride or not.

 

[lostsheep]

You have to define "risk" first: since the outcome is defined as "symptoms of clinical DCS", all these SurGFs, ISSes, and even bubble scores, are snake oil as far as the model's concerned. The outcome is 100% empirical: you either need a chamber ride or not.

Not only is the word risk commonly used when comparing decompression strategies but the intended meaning can easily be ascertained by context in prior posts.

I think you know exactly what we mean.

 

[dmaziuk]

Not only is the word risk commonly used when comparing decompression strategies but the intended meaning can easily be ascertained by context in prior posts.

Have you read the DSAT report yet? Part B.1..3, in particular?

 

[Kendall Raine]

No, not equivalent risk.

For a NDL dive your SLOW tissues may be well below your SurfGF because they haven’t reached that level on your dive, but your FAST tissues may be just below your SurfGF threshold of 85 when your dive computer indicates that it’s okay for you to surface.

For a DECO dive, at the end of your deco stops, your FAST tissues may be well below your SurfGF, because they are fast, but your SLOW tissues may be just below your SurfGF threshold of 85 when your dive computer indicates that it’s okay for you to surface.

These are quite different scenarios. Hard to say which has greater risk since the tissue models are theoretical.

Agreed. Very little empirical data outside of doppler studies, which are in themselves problematic.

That said, I found Kevin Watt's heat maps of respective tissue (compartments) proximity to critical supersaturation using various deco regimes very interesting in conjunction with the only study that used DCI as an endpoint (NEDU 2005 study).

 

[leadduck]

Agreed. Very little empirical data outside of doppler studies, which are in themselves problematic.

That said, I found Kevin Watt's heat maps of respective tissue (compartments) proximity to critical supersaturation using various deco regimes very interesting in conjunction with the only study that used DCI as an endpoint (NEDU 2005 study).

Bühlmann did a ton of experiments, dives with varying depth and duration, in order to come up with the a/b values in the ZHL model. Slow vs fast compartments may differ in how sensitive they are to DCS, but this is reflected in the model coefficients because they were fitted to experimental data.
Uncertainties remain of course regarding dive profiles that were not tested by experiments. Bühlmann's experiments all had rectangular dive profiles AFAIK, hence increased risk from yo-yo diving is probably not covered, neither are multiple repetitive NDL dives. They did dives at altitude, and deep dives with helium mixes.

 

[scubadada]

Bühlmann did a ton of experiments, dives with varying depth and duration, in order to come up with the a/b values in the ZHL model. Slow vs fast compartments may differ in how sensitive they are to DCS, but this is reflected in the model coefficients because they were fitted to experimental data.
Uncertainties remain of course regarding dive profiles that were not tested by experiments. Bühlmann's experiments all had rectangular dive profiles AFAIK, hence increased risk from yo-yo diving is probably not covered, neither are multiple repetitive NDL dives. They did dives at altitude, and deep dives with helium mixes.

How were the revised coefficients for Buhlmann ZH-L16 B and C arrived at relative to A? Thanks