Is PZ+ really aggressive?

[anchochile]

I use an Oceanic Geo with the PZ+ algorithm, which is considered more conservative than the other option, DSAT. I've read through a number of old threads on the board and there seems to be a consensus that PZ+ is equivalent to a GF Hi of about 85%.

Subsurface lets you set any GF Hi that you want in the settings and it will look at your dive profiles and calculate your NDL and any deco time based on the settings you've chosen. I've noticed that when setting Subsurface to use a GF Hi of 85%, it will still show a deco ceiling on some dive profiles where my Oceanic running PZ+ definitely did NOT put me into deco.

Here's an example, using the most aggressive dive profile I've done, at Maracaibo in Cozumel. The green shaded area at the top of the graph shows the deco ceiling calculated by Subsurface (as you can see, it clears around the time my safety stop begins so Subsurface says I never actually violated a deco ceiling), and gray inset shows detailed data at 31 minutes into the dive: a calculated ceiling of 6.3 feet and notably a Surface GF of 91%.

This is the most extreme example, but there are plenty of other dives in my log where Subsurface shows a calculated deco ceiling and a Surface GF above 85% at some point during the dive.

Does this suggest that the estimates of PZ+ using a GF Hi of 85% are wrong, and it is actually a much more aggressive algorithm? If so, I am inclined to dive much more conservatively vs what the computer is telling me - maybe just not letting my displayed NDL go below 10 min, or turning on the added conservatism factor in the computer settings.

 

[Belzelbub]

No, I don't think so. PZ+ is generally considered moderate. I wouldn't worry at all about this if the above is your most extreme example. DSAT is more liberal. PZ+ is reportedly based on Buhlmann, but is tweaked somewhat, so won't be exact.

Also, it's not uncommon to have the DC and Subsurface disagree somewhat, even if the algorithm is the same. Sampling rates, etc. all play into that.

All that said, steering farther from NDL is not a bad idea, but completely a personal choice based on your risk tolerance.

 

[dmaziuk]

...
Does this suggest that the estimates of PZ+ using a GF Hi of 85% are wrong, and it is actually a much more aggressive algorithm? If so, I am inclined to dive much more conservatively vs what the computer is telling me - maybe just not letting my displayed NDL go below 10 min, or turning on the added conservatism factor in the computer settings.

No, it means that if on the 35th minute of your dive you teleported from 80 fsw straight to the surface, you'd've been at somewhat higher risk of clinical DCS. A calculated ceiling means very little if it'll clear by the time you get there, which is normally the case on no-stop dives: the fast tissue compartment(s) are the first to have it, and they're also the fastest to off-gas during ascent.

That is one of the programming differences between a deco planner and a dive computer.

 

[scubadada]

Both DSAT and PZ+ are more liberal than a GF high of 95 or 85 on the 1st clean dive of the day, especially if deeper. The Buhlmann settings become more liberal on the 2nd dive, especially if shallower. The algorithms do not act similarly on 1st dive and repetitive dives, just the way they are.

I have over 2100 dives on DSAT, only 1000 on Buhlmann. If you want your dives to match exactly, use the same deco algorithm, I did that your years with 2 computers running DSAT.

1st dive on DSAT is more like a GF high of 99, for PZ+ it is more like a GF high of 90-95.

Sorry, it's just not a simple conversion. That would be just too easy. DSAT with conservatism starts off at a GF high of 80-85, but would be more conservative on repetitive dives.