Help with Buhlmann ZHL-16c GF

[doctormike]

Yep. My mistake. Basically deleted that post a minute after posting it when I looked at the black and white lines a little bit closer, guess it wasn't fast enough.

Hah! Sorry, now I see that... AFTER I wrote my snotty response...

I guess we BOTH should find better things to do. Like DIVE!

But this weekend looks like a blowout, Hurricane Joaquin has cancelled my dive, and if it's raining, I don't even feel like going to the quarry.

 

[Kevrumbo]

What I like about the Petrel is that you can electively adjust the GFHi (surfacing Gradient Factor) real time during the dive if you want to extend your safety stop time (or O2 deco time if using Oxygen); For example, if you start with 45/95 GF's (Tech Mode), you can at your option change the "95" to a lower more conservative surfacing value like "60" during the dive if you wish; the Petrel will recalculate & update the Tissue Loading Bar Graph, CNS and reflect the new extended Time to Surface & Deco Time/Depth displays as needed.

Yes, I actually used part of that article in my discussion. I hate to add yet another link but for those readers interested in this discussion specifically as it relates to the Petrel, there is another function they might find interesting, the GF99...

[Conversely, the GF99 value can also be used to add conservatism. For example, if you knew that you had abeen exposed to factors that increase decompression sickness risk such as exercising during the bottom time or being colder on deco than the bottom, then you could use this value to extend shallow stops. Of course, you could always just add a few minutes to your last stop, but this gives a quantitative measure to correlate with “how you feel” after the dive.]

https://www.shearwater.com/shearwater-product-faq/what-does-the-gf99-display-value-mean/

I also need to add that none of the main characters in this debate, as far as I know, certainly not myself, really have the answers. Still hoping that one of the many decompression experts that frequent SB, or Shearwater themselves, will join the discussion.

Another instance where you might want to decrease the GF99 during a dive to a lower more conservative surfacing value is when doing repetitive deco dives over several consecutive days of a week or more --especially if using deep bottom mixes with high FN2 percentages like Air or Nitrox which can significantly load slow tissues with excess residual inert Nitrogen. By Day 4, I would start adding more O2 Deco profile time to washout this residual N2 at 6meters/20feet by reducing the Petrel's GF99 value from 85 to 70 or 60 --and sometimes even lower to 50 or 40 -- finishing with a very slow ascent to the surface (0.5m/min or 1.6 foot/min), and monitoring for signs & symptoms of slow tissue type I DCS. BE ADVISED THAT THE CNS OX-TOX VALUE AS TRACKED BY THE PETREL COMPUTER MAY BE UNCOMFORTABLY HIGH.

Better yet --to safely Off-Gas further those N2 saturated slow tissues (the Petrel Tissue Bar Graph clearly tracks & indicates that very nicely!) and to reset the CNS to zero as well-- take a Day-Off after three consecutive days of Deep Air bottom gas diving with 50% and Oxygen decompression gases. . .

http://www.scubaboard.com/forums/te...ndl-day-what-your-guidelines.html#post7456426

http://www.scubaboard.com/forums/te...187-deep-stops-increases-dcs.html#post7287638

 

[scubadada]

Thanks to everyone who has contributed to this thread so far, I'm learning

 

[rossh]

Look at it again, I think that you are misunderstanding the diagram.

That line with the stair steps underneath it isn't a GFL line or a GFH line. It's a line connecting GFL to GFH. Put another way, it's a line that shows the allowable overpressure gradient for any given depth, which with this algorithm starts at GFL (first stop) and rises to GFH (at the surface). It's the - for example - 30/70 line. So for 30/70, your overpressure (percentage of the distance between the ambient line and the M line) would be 30% on the first stop, and then (pulling numbers at random) 40% on the second stop, 50% on the third stop, and 70% when you surface. If by "breaching GFL" you mean "having an overpressure gradient greater than the GFL", then every dive using this algorithm involves continually breaching the GFL.



We may be discussing semantics here.... However: GFL and GFH are static in that you chose a set and put them into your algorithm to generate an ascent profile (although they can be changed on the fly). But the gradient factor (for the leading TC) is not static. At any point in the dive it is a measure of the driving force of decompression, and it ranges from 0 (ambient line, no decompression) to 100 (the M value) and beyond that if you blow off deco.

Jumping in a bit late here, but would like to touch on a couple of clarifications.

The ambient (GF 0) line is the water pressure line, and not inspired pressure or tissue pressure. If we follow an ascent at GF Lo 0, it will off gas at the rate of (ambient pressure - inspired pressure), and for an air dive that's a gradient of 0.21 (or 0.25 with water vapor included). But that is still too slow for any real use of course.


The observation that GF Lo has little meaning on an NDL is an interesting question. Some GF implementation are coded to ignore the GF Lo, until GF Hi has a limit to apply. But this is one of several compromises and conflicts in GF - how to initialize a deco, when the Lo wants to apply stops, but Hi does not? A GF Lo value (depending on what you select) usually wants to initiate stops before a GF Hi does.

For a dive planner, this conflict is handled without issue, but for a dive computer where the tissue loads build gradually, the result is that the stops begin with a surge - it proceeds from none to several levels of stops and stop time, all applied at one short instant. It's because the GF Lo stops where being held back until GF Hi kicked in, and then the requirement of Lo stops are shown. The solution to this is to make GF Lo and Hi the same value on NDL dives to take away the ambiguity. In some dive computers under the hood, that is what the method implementations do (which is why GF Lo only appears to have no effect on NDL).


GF was really designed for application to well formed deco dives, and the problems of NDL planning where not considered. This problem above (and two others) are not defined in the GF method design by Erik Baker.

 

[CptTightPants21]

Hah! Sorry, now I see that... AFTER I wrote my snotty response...

I guess we BOTH should find better things to do. Like DIVE!

But this weekend looks like a blowout, Hurricane Joaquin has cancelled my dive, and if it's raining, I don't even feel like going to the quarry.

Speak for yourself I'll be at Dutch at least Sat, potentially Sunday.

I'll be damned if a little water will hamper my end of season. I was there June 26/27 2015--which anyone who was actually at Dutch this year will remember well. It didn't stop.

 

[uncfnp]

Jumping in a bit late here, but would like to touch on a couple of clarifications.
...//...

GF was really designed for application to well formed deco dives, and the problems of NDL planning where not considered. This problem above (and two others) are not defined in the GF method design by Erik Baker.

Thank you for joining the conversation and adding some much needed clarification.

In case anyone reading this does not know, this is The Ross Hemingway, developer of the V-Planner program

 

[scubadada]

Thank you for joining the conversation and adding some much needed clarification.

In case anyone reading this does not know, this is The Ross Hemingway, developer of the V-Planner program

And MultiDeco, my favorite recent purchase. Particularly helpful in this thread as it runs Buhlmann ZHL16-C with GF MultiDeco dive decompression software - VPM-B and ZHL deco. Thanks Ross, it's teaching me a lot.

 

[Lorenzoid]

I'm going to have to read Ross Hemingway's comments several times for that to sink it. But it really seems to get to the heart of our confusion. So the "answers" may be implementation-specific. In other words, you'd have to ask the computer manufacturer how they handle it. Any of you Petrel Rec-mode users want to post the question(s) on the Shearwater forum?

Yes, I actually used part of that article in my discussion. I hate to add yet another link but for those readers interested in this discussion specifically as it relates to the Petrel, there is another function they might find interesting, the GF99...

https://www.shearwater.com/shearwater-product-faq/what-does-the-gf99-display-value-mean/

. . .

Thanks for reminding me about this bit of Shearwater documentation. I KNEW I had read this somewhere: "A value of 100% (Bühlmann’s originally allowed super-saturation limit) is now generally accepted to be too risky." I said something to that effect in another thread a few months ago, and a couple of people jumped on me and asked where I got that idea from, as they seemed to believe there is nothing "wrong" with following a "full Buhlmann" and that gradient factors are nice for conservatism but not an integral part of safe decompression for everyone in every case. But I couldn't recall the source. Deco for Divers doesn't go so far as to make such a statement. I know that Shearwater is not the oracle or something, but if Shearwater says it's "generally accepted to be too risky," I believe it. So would it be fair to say that "nobody dives full (100/100) Buhlmann"?

 

[uncfnp]

I'm going to have to read Ross Hemingway's comments several times for that to sink it. But it really seems to get to the heart of our confusion. So the "answers" may be implementation-specific. In other words, you'd have to ask the computer manufacturer how they handle it. Any of you Petrel Rec-mode users want to post the question(s) on the Shearwater forum?




Thanks for reminding me about this bit of Shearwater documentation. I KNEW I had read this somewhere: "A value of 100% (Bühlmann’s originally allowed super-saturation limit) is now generally accepted to be too risky." I said something to that effect in another thread a few months ago, and a couple of people jumped on me and asked where I got that idea from, as they seemed to believe there is nothing "wrong" with following a "full Buhlmann" and that gradient factors are nice for conservatism but not an integral part of safe decompression for everyone in every case. But I couldn't recall the source. Deco for Divers doesn't go so far as to make such a statement. I know that Shearwater is not the oracle or something, but if Shearwater says it's "generally accepted to be too risky," I believe it. So would it be fair to say that "nobody dives full (100/100) Buhlmann"?

Yes. Sometimes we amateur theorist forget that dive computer manufacturers all use various implementations of the algorithms. There is a thread with a post by RonR that has a very good discussion of this somewhere.

I have invited Shearwater by pm. No bites yet...

 

[dmaziuk]

GF was really designed for application to well formed deco dives, and the problems of NDL planning where not considered. This problem above (and two others) are not defined in the GF method design by Erik Baker.

So, what are the two others? (With thanks for the post of course.)