Help with Buhlmann ZHL-16c GF

[uncfnp]

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?

Interesting question and I will take a stab at it and see just how wrong I am. If it is NDL dive, I don't think the low GF will apply no matter how deep the dive. The high GF will shorten your NDL time as you go more conservative in your setting. Then if you do exceed the NDL limit, even in the recreational mode, the low GF may then apply.

 

[scubadada]

Interesting question and I will take a stab at it and see just how wrong I am. If it is NDL dive, I don't think the low GF will apply no matter how deep the dive. The high GF will shorten your NDL time as you go more conservative in your setting. Then if you do exceed the NDL limit, even in the recreational mode, the low GF may then apply.

I believe this is correct. Only GF high pertains to no stop dives. This is your surfacing GF. Higher GF high is more liberal, lower GF high is more conservative. GF low comes into play when you exceed NDL and go into deco. GF low will dictate the depth of your 1st stop. A low GF low will give deeper stops than a higher GF low. Recreational mode also takes into account that you may go into deco, and nicely deals with that possibility.

---------- Post added September 30th, 2015 at 03:04 AM ----------

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 Craig,

The PADI and NOAA NDLs are slightly different, probably the basis for tursiops statement about matching both. In fact, DSAT, basis for PADI, also has minor differences from PADI. Oceanic's implementation of Buhlmann ZHL-16c, PZ+, has considerably shorter NDLs. I asked Oceanic for details such as GF high, etc, but never received a reply.

Air NDLs 1st dive (min)
Depth (ft)	PZ+	DSAT	PADI RDP	NOAA
60	48	57	55	60
70	35	40	40	48
80	26	30	30	39
90	19	24	25	30
100	16	19	20	25
110	12	16	16	20
120	10	13	13	15
130	8	11	10	10

Good diving,

Doctor Craig

 

[tursiops]

The PADI and NOAA NDLs are slightly different, probably the basis for tursiops statement about matching both. In fact, DSAT, basis for PADI, also has minor differences from PADI. Oceanic's implementation of Buhlmann ZHL-16c, PZ+, has considerably shorter NDLs. I asked Oceanic for details such as GF high, etc, but never received a reply.

Air NDLs 1st dive (min)
Depth (ft)	PZ+	DSAT	PADI RDP	NOAA
60	48	57	55	60
70	35	40	40	48
80	26	30	30	39
90	19	24	25	30
100	16	19	20	25
110	12	16	16	20
120	10	13	13	15
130	8	11	10	10

Good diving,

Doctor Craig

The NOAA numbers are not quite right....the first three shold be 60, 50, 40; not sure wher you got the 60, 48, 39.
I think the DSAT and PADI numbers are different only because of the round-off error in the algorithm's implementation. I assume you got the PADI numbers from the table, and the DSAT numbers from your computer?

 

[Scubanoobi]

For what it's worth, you mentioned you currently dive oceanic, I have a Geo 2.0 and Petrel for recreational dives. PZ+ and 40/85 (medium on Petrel rec mode) are very close in NDL time on single and repetitive dives. Within about 2 minutes of each other.


Sent from my iPhone using Tapatalk

 

[Lorenzoid]

Interesting question and I will take a stab at it and see just how wrong I am. If it is NDL dive, I don't think the low GF will apply no matter how deep the dive. The high GF will shorten your NDL time as you go more conservative in your setting. Then if you do exceed the NDL limit, even in the recreational mode, the low GF may then apply.

I believe this is correct. Only GF high pertains to no stop dives. This is your surfacing GF. Higher GF high is more liberal, lower GF high is more conservative. GF low comes into play when you exceed NDL and go into deco. GF low will dictate the depth of your 1st stop. A low GF low will give deeper stops than a higher GF low. Recreational mode also takes into account that you may go into deco, and nicely deals with that possibility.

From the little that I have found explicitly addressing gradient factors in no-stop diving, that is exactly what I have found. So when, for example, Shearwater sets the Petrel Recreational mode setting for Low conservatism to 45/95, the GF-low number of 45 will never come into play unless the diver exceeds the NDL? You mean the 45 is a total red herring? It's a little misleading to anyone not well versed in gradient factors, since Shearwater sets the GF to 45/95 in Low conservatism, 40/85 in Medium conservatism, and 37/75 in High conservatism. Whether the GF-low number is 45, 40, 37 or anything else is totally irrelevant unless the diver exceeds an NDL--which would presumably be inadvertent and due to diver error in what was planned as a no-stop or "recreational" dive.

 

[scubadada]

The NOAA numbers are not quite right....the first three shold be 60, 50, 40; not sure wher you got the 60, 48, 39.
I think the DSAT and PADI numbers are different only because of the round-off error in the algorithm's implementation. I assume you got the PADI numbers from the table, and the DSAT numbers from your computer?

Hi turisops,

I got the NOAA NDLs from tables off the internet: http://www.ndc.noaa.gov/forms/New_NOAA_Air_Deco_Tables_2008.pdf maybe yours are older?
DSAT and PZ+ came off my Geo2, same as Oceanic owner's manual
PADI also came from tables off the internet

Best, Craig

---------- Post added September 30th, 2015 at 08:16 AM ----------

From the little that I have found explicitly addressing gradient factors in no-stop diving, that is exactly what I have found. So when, for example, Shearwater sets the Petrel Recreational mode setting for Low conservatism to 45/95, the GF-low number of 45 will never come into play unless the diver exceeds the NDL? You mean the 45 is a total red herring? It's a little misleading to anyone not well versed in gradient factors, since Shearwater sets the GF to 45/95 in Low conservatism, 40/85 in Medium conservatism, and 37/75 in High conservatism. Whether the GF-low number is 45, 40, 37 or anything else is totally irrelevant unless the diver exceeds an NDL--which would presumably be inadvertent and due to diver error in what was planned as a no-stop or "recreational" dive.

Hi Lorenzoid,

Shearwater rec mode high conservatism is 35/75. I believe you are correct, GF low only comes into play is you "accidentally" go into deco, or, like me, you choose to do light deco by plan. The exact same principal applies when you are using tec mode and set your own GFs.

Good diving, Craig

 

[Divin'Papaw]

From the little that I have found explicitly addressing gradient factors in no-stop diving, that is exactly what I have found. So when, for example, Shearwater sets the Petrel Recreational mode setting for Low conservatism to 45/95, the GF-low number of 45 will never come into play unless the diver exceeds the NDL? You mean the 45 is a total red herring? It's a little misleading to anyone not well versed in gradient factors, since Shearwater sets the GF to 45/95 in Low conservatism, 40/85 in Medium conservatism, and 37/75 in High conservatism. Whether the GF-low number is 45, 40, 37 or anything else is totally irrelevant unless the diver exceeds an NDL--which would presumably be inadvertent and due to diver error in what was planned as a no-stop or "recreational" dive.

How so?

Shearwater has simplified it by giving you three levels of conservativeness. They've then set the Lo and Hi GF numbers to translate into low/mid/high levels of conservativeness. It appears to me that Shearwater has done exactly what they should have done. I see no red herring or anything misleading.

If a diver goes into deco, accidentally or on purpose, the GF settings will handle that to the desired level of conservativeness.

Personally, I prefer an algorithm that has been designed as a bubble model from the get go. I therefore use VPM on my Petrels and set the Conservatism accordingly without a need to bother with GFs.

 

[scubadada]

For what it's worth, you mentioned you currently dive oceanic, I have a Geo 2.0 and Petrel for recreational dives. PZ+ and 40/85 (medium on Petrel rec mode) are very close in NDL time on single and repetitive dives. Within about 2 minutes of each other.


Sent from my iPhone using Tapatalk

Scubanoobi,

I'm having a blast playing around with my newly acquired MultiDeco. I've only had a chance to run 1st dives and have not looked into multiple dives yet. Buhlmann ZHL16-C 40/85 runs close to Oceanic PZ+, 45/95 runs pretty close to DSAT/PADI. I don't own a Petrel (yet) so can't compare the NDLs directly. Someone with a Petrel could easily fill in the table I posted earlier.

Thanks, Craig

---------- Post added September 30th, 2015 at 08:45 AM ----------

How so?

Shearwater has simplified it by giving you three levels of conservativeness. They've then set the Lo and Hi GF numbers to translate into low/mid/high levels of conservativeness. It appears to me that Shearwater has done exactly what they should have done. I see no red herring or anything misleading.

If a diver goes into deco, accidentally or on purpose, the GF settings will handle that to the desired level of conservativeness.

Personally, I prefer an algorithm that has been designed as a bubble model from the get go. I therefore use VPM on my Petrels and set the Conservatism accordingly without a need to bother with GFs.

I've been wondering, why does a lower GF hi necessarily go with a lower GF lo in the conservativeness ratings? Are the surfacing and the deeper stops GFs directly related, , linear, of the same magnitude of risk, etc.? I've talked to individuals who set custom GF levels and choose to go with a more conservative (lower) GF hi and a more liberal (higher) GF lo. The most common explanation is that they choose not to do the deeper stops or do not "believe" in them. Does this general principle apply to conservative settings in VPM?

 

[Divin'Papaw]

I've been wondering, why does a lower GF hi necessarily go with a lower GF lo in the conservativeness ratings? Are the surfacing and the deeper stops GFs directly related, , linear, of the same magnitude of risk, etc.? I've talked to individuals who set custom GF levels and choose to go with a more conservative (lower) GF hi and a more liberal (higher) GF lo. The most common explanation is that they choose not to do the deeper stops or do not "believe" in them. Does this general principle apply to conservative settings in VPM?

Did you have a chance to read Steve Lewis' blog that I posted earlier in the thread? That is one of the best explanations I've seen on how Lo and Hi GFs are related to each other and what they mean. If you haven't read it, please do. It's not very long and I found it very helpful.

Full disclosure though, I don't dive Buhlmann with GFs, so my interest is purely intellectual. I learned technical diving using VPM and V-Planner and still dive it today. That is the algorithm I run on my Petrels and use on all of my dives, rec and tec.

 

[Divetech99]

Is your Multideco an iOS app?