Help with Buhlmann ZHL-16c GF

[Divin'Papaw]

Another great blog article on GFs. I found it very helpful.

https://decodoppler.wordpress.com/2015/09/17/gradient-factors-a-simplified-primer/

 

[doctormike]

I'm confused and could use help

I have read that Buhlmann intended using no higher than 90/90. Liquivision Lynx offers a default option of 100/100. Shearwater offers a rec setting of 45/95 and states that it matches PADI/NOAA NDLs. Though I cannot confirm, I've heard Oceanic PZ+ without conservatism runs something like 45/95. I've read that many choose to dive something like 30-40/85. I've also read that some, wishing to avoid deep stops, dive something like 75/85.

Can someone help shed light on this topic? More and more good computers appear to be running Buhlmann ZHL-16c with GF (Shearwater, Liquivison, Seabear, Dive Rite, Divesoft, xDeep...)

I'm still not sure exactly what the question is. Is it why do people chose different gradient factors to modify the algorithm that controls their calculated ascent profile? That's because people have different risk tolerances and medical histories, and there are situations where a given diver might choose a more aggressive algorithm. I don't know much about the history of Buhlman's research, why he might have recommended what at any given point, or how things have changed since the early 1980s when this research was first published.

But for example, 100/100 (which is the "man on wire" approach of riding Buhlmann's M value line) will get you out of the water a lot faster than a standard conservative 30/70, while still controlling decompression stress to some degree. So that might be useful in cases where the risk of DCS is not as bas as the risk of staying in the water (e.g. combat situation, massive gas loss, medical emergency).

Or, in other cases, where you still want to minimize DCS risk but can't tolerate deep stops (say if you were planning to do a lot of your decompression on an O2 regulator hanging from a dive boat at 20 feet), you could have a high LoGF setting but a more conservative HiGF setting.

 

[scubadada]

Thanks for all the quick replies. I apologize to those I've offended. This is not a simple topic, I will try to be more specific. I have successfully dived the DSAT decompression algorithm for 14 years using Oceanic Pro Plus, Pro Plus 2 and currently, a VT3/Geo2. As you know, the DSAT algorithm is probably the most liberal, widely used algorithm, though more conservative of that from Cochran (which I used prior to DSAT). I do mostly recreational dives but a little more than 5% of my dives include a short deco obligation. The DSAT algorithm was not designed for deco and is suboptimal. I see transitioning to a computer with the Buhlmann algorithm and am wrestling with how this work out. I'm looking for information from others regarding implementation of Buhlmann with GF

Thanks in advance for your time and consideration.

 

[doctormike]

Thanks for all the quick replies. I apologize to those I've offended. This is not a simple topic, I will try to be more specific. I have successfully dived the DSAT decompression algorithm for 14 years using Oceanic Pro Plus, Pro Plus 2 and currently, a VT3/Geo2. As you know, the DSAT algorithm is probably the most liberal, widely used algorithm, though more conservative of that from Cochran (which I used prior to DSAT). I do mostly recreational dives but a little more than 5% of my dives include a short deco obligation. The DSAT algorithm was not designed for deco and is suboptimal. I see transitioning to a computer with the Buhlmann algorithm and am wrestling with how this work out. I'm looking for information from others regarding implementation of Buhlmann with GF

Thanks in advance for your time and consideration.

There are people here with a lot more experience than me, but my understanding is that most of the dive computer implementations of GF (modified Buhlmann) and the bubble models (RGBM, VPM) have track records in preventing DCS that are similar - given the difficulty of picking up statistically significant differences with the small potential study population of divers doing staged decompression.

As someone who has taken a chamber ride, I tend to pick conservative gradient factors. I occasionally will switch my Petrel to VPM with high conservatism if I am diving with someone who uses that model.

The Petrel is an amazing computer, even leaving aside the algorithms. Fantastic customer support, very readable, AA batter that you can replace yourself, extremely well built, and not that expensive compared to some of the bling out there...

 

[tursiops]

Thanks for all the quick replies. I apologize to those I've offended. This is not a simple topic, I will try to be more specific. I have successfully dived the DSAT decompression algorithm for 14 years using Oceanic Pro Plus, Pro Plus 2 and currently, a VT3/Geo2. As you know, the DSAT algorithm is probably the most liberal, widely used algorithm, though more conservative of that from Cochran (which I used prior to DSAT). I do mostly recreational dives but a little more than 5% of my dives include a short deco obligation. The DSAT algorithm was not designed for deco and is suboptimal. I see transitioning to a computer with the Buhlmann algorithm and am wrestling with how this work out. I'm looking for information from others regarding implementation of Buhlmann with GF

Thanks in advance for your time and consideration.

I suggest you get one of the desktop planning programs that calculates GF profiles, and play with them in comparison to what you are used to with DSAT (which *is* PADI). You'll find nearly any of the mid-range GFs will do you fine, even with light deco. (There is another whole thread on which software will allow you to play like this.)

 

[uncfnp]

Choice of GF really is a very personal decision based on your health, risk factors for DCS and risk tolerance. If you use the Petrel in rec mode you have 3 fixed choices. If you have traditionally dove "liberal" computers and been satisfied with this, choose the lowest setting. If you decide you want a little more safety margin, pick the middle or for even more the highest setting. Like doctormike, while no chamber ride, I am older and have a higher risk. I choose to dive ultraconservative so I dive in the tech mode even for recreational dives an set the GF manually.

 

[scubadada]

I suggest you get one of the desktop planning programs that calculates GF profiles, and play with them in comparison to what you are used to with DSAT (which *is* PADI). You'll find nearly any of the mid-range GFs will do you fine, even with light deco. (There is another whole thread on which software will allow you to play like this.)

I just purchased MultiDeco and am doing exactly that, thanks

 

[C Dub]

Really? PADI RDP does not match NOAA, so how can 45/95 (or *any* GF) match them both?

Cut and paste from page 19 of the Petrel Nitrox Rec Mode manual:
"The Low setting is similar to PADI and NOAA no-stop time
tables for air and nitrox diving."

On same page it states that the low setting is 45/95.

To be clear, their low conservatism setting means most aggressive, longer bottom times.

Craig - I switched to low when diving with you and your son, and it seemed like the NDL's matches your computer pretty well.

 

[tursiops]

Cut and paste from page 19 of the Petrel Nitrox Rec Mode manual:
"The Low setting is similar to PADI and NOAA no-stop time
tables for air and nitrox diving."

On same page it states that the low setting is 45/95.

To be clear, their low conservatism setting means most aggressive, longer bottom times.

Craig - I switched to low when diving with you and your son, and it seemed like the NDL's matches your computer pretty well.

Thanks, this was also answered in posts #11 and #18. I'd missed it because I installed the Recreational Mode to try it out and never got around to reading the manual, either Rev A or B. The best part to me was the 3-minute times, but now that there is a timer in the tech mode, I just leave it on tech.

 

[Lorenzoid]

I'm not sure this is a good place to ask this question, but I have been meaning to ask if anyone knows of a source that explains gradient factors specifically as they would apply to no-stop diving, such as in the case of the Petrel's Recreational Nitrox mode. I understand the explanations in Deco For Divers and other sources as they pertain to making deco stops, but I can't seem to figure out how that translates to no-stop diving. For example, for a setting of "45/95," the GF-Low number of 45 is commonly said to determine how deep the first stop will be. So what happens when one sets the Petrel to the Low Conservatism setting of 45/95 in Recreational mode with the presumption there will be no stops on ascent? Can someone point me to a straightforward explanation?