Help with Buhlmann ZHL-16c GF

[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.

 

[CptTightPants21]

I only have common sense to work with but I get what you are saying. As I said it is an ascent factor limiting the percentage of M value and as you said that as BT increases so does tissue loading so theory would suggest that at some point the tissue loading allowed by the HGF alone would exceed a set LGF, unless of course the LGF is set at 100. At what point this would happen, I have no idea but I would suspect that for the average recreational diver, the effect would be minimal.

Correct. Although it is wrong to think of it as just an ascent factor (we may be having a communication issue with this term). It is directly related to how long your NDLs are. A lower GFL will have shorter NDLs than higher GFL---the tissue loading issue. While it is "minimal" and I don't have the software to run simulations I would suspect that running different GFL could affect NDLs on a deep recreational dive by as much as 5-10 minutes depending on depth, profile, etc


What I was alluding too when I said "safer" is not the right word. Is the difference between 45/85 and 45/95 worth it when you take into account O2 clock, water temp, sea conditions, gas supply. These are all things that a diver needs to consider when he decides what GF he wants to run for each dive. In a theoretical non-real world 45/85 is safer than 45/95, but then again 45/75 is safer than them both so why don't we use that

 

[Bonuss]

This has been a great discussion... even if it did get off to a rough start : ). I learned allot, and want a Petrel now..

Bob in CO

 

[Lorenzoid]

This has been a great discussion... even if it did get off to a rough start : ). I learned allot, and want a Petrel now..

I'm glad others have picked up the ball and run with it. Though I have only contributed questions and no answers, I have been closely following the discussion. The fact that the Petrel tells you what gradient factors are selected to correspond to "Low," "Medium" and "High" conservatism in Recreational Mode is a testament to the fact that the Petrel is a computer for the "thinking diver," not the passive diver, even if the diver is doing purely recrea ... er, I mean, no-stop diving.

 

[CptTightPants21]

Finally cracked open my Deco for Divers. Gradient Factors are covered on pages 145-147 in the Deep Stop and Bubble Models section for those that have the book and/or are interested in getting "all the info".

Also found a good link
Gradient Factors

 

[uncfnp]

Finally cracked open my Deco for Divers. Gradient Factors are covered on pages 145-147 in the Deep Stop and Bubble Models section for those that have the book and/or are interested in getting "all the info".

Also found a good link
Gradient Factors

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/

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.

 

[doctormike]

The lower GF is a percentage of the M value for each compartment. Depending on the dive, some are on gassing, some are already saturated, some may be already off gassing.

Well, there aren't separate GFLs for each compartment - this is a mathematical model in which you pick a GFL which then is overpressure of the leading compartment, but which compartment that is varies. So whatever your GF is, by definition if it is greater than 0 (that is, to the left of the ambient pressure line on that diagram), then the leading compartment is supersaturated and therefore offgassing. But you are correct that some of the slower compartments may be ongassing even at fairly shallow depths. Again, by definition, if they aren't saturated, they have to ongas.

Your NDL time using Bulhmann with GFs is the point in the dive where your tissue loading is such that you can ascent at 30ft per minute and never breach the lower GF in any of the 16 compartments and have all 16 compartments end with equal or less than nitrogen loading of the High GF. If you would breach the lower GF in ANY compartment, you will do a deco stop even if your overall nitrogen loading is still pretty benign..

I think that you mean never breach the higher GF, right? The GFL is continually "breached", your leading TC overpressure gradient is the GFL on the first stop, but at each subsequent stop it is higher, until at the last stop it is up to the GFH.

Also, GF isn't a measure of total N2 loading, it is a measure of how close a TC is to Buhlman's M value (theoretically the point of clinically significant bubble formation). So with a given amount of N2 in your tissues, if you ascend, your overpressure GF immediately goes up (before any change in N2 load), if you descend, it goes down.

 

[CptTightPants21]

Well, there aren't separate GFLs for each compartment - this is a mathematical model in which you pick a GFL which then is overpressure of the leading compartment, but which compartment that is varies. So whatever your GF is, by definition if it is greater than 0 (that is, to the left of the ambient pressure line on that diagram), then the leading compartment is supersaturated and therefore offgassing. But you are correct that some of the slower compartments may be ongassing even at fairly shallow depths. Again, by definition, if they aren't saturated, they have to ongas.



I think that you mean never breach the higher GF, right? The GFL is continually "breached", your leading TC overpressure gradient is the GFL on the first stop, but at each subsequent stop it is higher, until at the last stop it is up to the GFH.

Also, GF isn't a measure of total N2 loading, it is a measure of how close a TC is to Buhlman's M value (theoretically the point of clinically significant bubble formation). So with a given amount of N2 in your tissues, if you ascend, your overpressure GF immediately goes up (before any change in N2 load), if you descend, it goes down.

Nevermind. You are correct about GFL.

 

[doctormike]

Nope. I was correct in saying never breach the GFL. I am looking at the diagram in deco for divers right now. Your stops will be determined when you hit up against the GFL line, the only time you breach the GFL line is when you are making your final ascent the surface and you will end where your GR-High line intersects 0 depth.

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.

You are misspeaking slightly and it might be causing some confusion. Gradient Factors are percentages and static, but because they are percentages of the M-value which gets larger with depth the "Gradient Factor created M value" will be a larger or smaller number.

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.

 

[CptTightPants21]

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.

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.