New deco planning tool

[inquis]

Wouldn't that just push your first stops shallower and shorter, and make your later stops much longer?

Subsurface is one tool that allows GFLow > GFHigh. A comparison for a 60 min dive to 100 ft on 32% for the same surfacing GF:
50/75: 40 min TTS (3 min/30 ft, 13 min/20 ft, 21/10 ft)
80/75: 38 min TTS (10 min/20 ft, 25 min/10 ft)

 

[David Carron]

orry to break it to you, but ChatGPT doesn't realize that greater supersaturation at depth is an intentional characteristic of ZHL-16. (Even GFs like 70/80 yields greater supersaturation at depth than near the surface.)

You put me to think (a lot actually about this).

I concluded that GF Low has to be lower that GF High - not because the math doesn't work but because it may not make sense to what the decompression tries to achieve.

I had a little conversation with ChatGPT and this was its conclusion too:
If GF low > GF high, it leads to a logical contradiction: you’re allowing more supersaturation at depth than near the surface—which violates safe decompression logic. Most dive software will either block it or behave erratically.

Still, great question to debate!

Perhaps unexpectedly, it's more GFlo < GFhi that leads to "logical contradiction". Here's a plot of the 30/80 M-values with the first stop at 9m (the x axis labels got messed up converting atm to m). Take a look the dark blue 5 minute tissue and consider that it's defining two distinct ceiling depths.

The french military tables actually have quite a few entries which can only be reproduced with GFlo > GFhi.

 

[pstrika]

Sorry to break it to you, but ChatGPT doesn't realize that greater supersaturation at depth is an intentional characteristic of ZHL-16. (Even GFs like 70/80 yields greater supersaturation at depth than near the surface.)

The fact is that something like 80/70 (yes, GFLow greater than GFHigh) still has 20% less supersaturation in the fast tissues than Buhlmann felt was required to avoid DCS. The amount of margin will always be a personal call.

No worries, no need to be sorry. Great discussion was triggered. Coming back to probably 10 posts ago, divers-hub.com was programed to have GFL < GFH. If the conclusion is that it'd be useful to break this rule, I can easily correct that in the tool.

 

[inquis]

If the conclusion is that it'd be useful to break this rule, I can easily correct that in the tool.

Your tool, your call. I don't foresee many people doing it, but there's no physics-based reason to have that constraint. In fact, if Eric Baker (who came up with GFs) had called them GFdeep and GFsurface, then I'm sure that no one would conceive of such a constraint.

 

[David Carron]

Your tool, your call. I don't foresee many people doing it, but there's no physics-based reason to have that constraint. In fact, if Eric Baker (who came up with GFs) had called them GFdeep and GFsurface, then I'm sure that no one would conceive of such a constraint.

That's a very good observation.

 

[pstrika]

Subsurface is one tool that allows GFLow > GFHigh. A comparison for a 60 min dive to 100 ft on 32% for the same surfacing GF:
50/75: 40 min TTS (3 min/30 ft, 13 min/20 ft, 21/10 ft)
80/75: 38 min TTS (10 min/20 ft, 25 min/10 ft)

Interesting... in both scenarios surface if 75% (so leading tissue should be at 75% supersaturation below M-Value; but time was shorter in 2nd case and with 1 less stop at 30ft. I'm wondering if the leading tissue at the end of the dive is the same...?

 

[inquis]

I'm wondering if the leading tissue at the end of the dive is the same...?

Very likely the same, but you can turn on "all tissues" in the profile window to see which tissue is controlling the ceiling.

 

[dmaziuk]

Your tool, your call. I don't foresee many people doing it, but there's no physics-based reason to have that constraint. In fact, if Eric Baker (who came up with GFs) had called them GFdeep and GFsurface, then I'm sure that no one would conceive of such a constraint.

I wouldn't be so sure about Erik Baker: he knows math, not just physics.

Think of what GFs do to Buhlmann equation, then consider what happens when you are calculating your NDL using GF High of, say, 42, and once you're over the NDL: calculate your first stop depth using GF Low of, say 84.

Report back where your first deco stop is.

 

[David Carron]

I wouldn't be so sure about Erik Baker: he knows math, not just physics.

Think of what GFs do to Buhlmann equation, then consider what happens when you are calculating your NDL using GF High of, say, 42, and once you're over the NDL: calculate your first stop depth using GF Low of, say 84.

Report back where your first deco stop is.

It would be at 3m. I'm not sure why you think it would be anywhere else or why that case might be problematic.

 

[inquis]

consider what happens when you are calculating your NDL using GF High of, say, 42, and once you're over the NDL: calculate your first stop depth using GF Low of, say 84.

Report back where your first deco stop is.

First stop is at 10 ft. Subsurface puts NDL at 100 ft on air at 5 mins. Staying for 6 mins has the ceiling at 2.4 ft when leaving the bottom and 1.9 ft when arriving at the 10 ft stop.

I understand you've setup a scenario where the tissue pressure is below the GFL-tolerated amount at the surface (therefore a negative GFL-ceiling) but above the GFH-tolerated amount. No argument that mandating GFL<GFH avoids this; however, Subsurface can clearly deal with it. The code is freely available if anyone is interested to see how they handled it.