NDLs

[scubadada]

Oceanic PZ+ and Shearwater Petrel rec mode 45/95 seem very close, at least for 1st dive NDLs. I would be interested to know if anyone has dived these repetitively and how closely they match

 

[tbone1004]

I would certainly hope so as they are basically the same algorithm.... the Gradient factors have more to do with decompression stop times than basic NDL profiles, so since PZ+ is based on the Buhlman ZHL-16C, they should have pretty similar NDL profiles, for both single and repetitive diving.

 

[Scubanoobi]

I just picked up a petrel and have an oceanic geo. The GF high number will affect the NDL times on the petrel (it's not just for deco). Setting it to low conservative (45/95) will be slightly less conservative under 70-80 feet and slightly more conservative deeper. But overall within a few minutes of PZ+.

Conservative levels of medium or high will be much more conservative than the oceanic.

 

[tbone1004]

GF low is the depth of the first deco stop as a function of total depth and tissue saturation, GF high is the time of the last, function of what your tissue loading is. Max is 100%, minimum is whatever you tell it you want it to be. Changing the bottom number won't change your NDL unless you are going quite deep fo a long time, but the top number will change your significantly NDL as it is the amount of tissue loading you are allowing yourself to leave the water with. Lower number is shorter NDL's and longer decompression stops because you are only allowing your tissues to load to whatever percent the GF high is. If 95 is giving you the same NDL's as the PZ+, then if you are staying within the NDL's the repetitive dives will be the same.

The GF low set that high shouldn't be a factor for NDL diving as it is just telling you to sit at a depth until your tissues have offgassed enough to go up. A lower GF low will just tell you to start cranking out deep stops, but in NDL diving it shouldn't factor in. Gradient factors aren't intended to be used by NDL divers which is why in the rec mode of the petrel you can't change them and for NDL diving to recreational depths, you shouldn't see any change in NDL's if you change low conservatism to 40/95, or 50/95 unless you are 100ft plus, then it will be a minute or two different.

GF 10/95, lots of deep stops, really short final deco stop and short NDL's
GF 45/70 gets you up to the shallow stops quicker, but you'll be on deco longer, short NDL's, about 30% shorter than a GFhigh of 95 or the PZ+

If the PZ+ is following pure Buhlmann then GF is 99/99, pure linear curve, it's modified somewhat, so is likely following 45/95 or something close to that to give actual deep stops for true technical diving. Will try to find out what the curve actually is. I'll run some of the gradient factors later, but for recreational profiles you shouldn't see any meaningful difference between the two, and if you're diving below 100ft, you should really have ANDP cert because it never hurts to have your safety stop on O2....

 

[scubadada]

I just picked up a petrel and have an oceanic geo. The GF high number will affect the NDL times on the petrel (it's not just for deco). Setting it to low conservative (45/95) will be slightly less conservative under 70-80 feet and slightly more conservative deeper. But overall within a few minutes of PZ+.

Conservative levels of medium or high will be much more conservative than the oceanic.

Exactly, thanks.

---------- Post added January 15th, 2015 at 04:07 PM ----------

If the PZ+ is following pure Buhlmann then GF is 99/99, pure linear curve, it's modified somewhat, so is likely following 45/95 or something close to that to give actual deep stops for true technical diving. Will try to find out what the curve actually is. I'll run some of the gradient factors later, but for recreational profiles you shouldn't see any meaningful difference between the two, and if you're diving below 100ft, you should really have ANDP cert because it never hurts to have your safety stop on O2....

I posted the question to Oceanic in this forum