Using the dive planner (new user)

Yes, Oceanic has DSAT and PZ+. But the PZ+ is a version of Bühlmann?
Here is good site on this:
https://dipndive.com/blog/dive-computer-algorithms-for-dummies/

"The Pelagic Z+ is based on the popular Bühlmann ZHL-16C algorithm and was developed to safely maximize dive times at depth without penalizing the diver for performing repetitive deeper dives."

"Based on" not "version of".  It has been modified so it is effectively a different algorithm.

Here's a graph showing the NDLs from a dive (on air) with my Perdix (Bülhmann ZHL-16C with 40/85 same as my Mk1) and my Oceanic Atom 3.0 (PZ+ with conservatism off).  While they track pretty closely in the early stages of the dive, with the Atom being slightly more liberal, after 60 minutes the Atom becomes significantly more conservative.

The other thing I find with PZ+ is it is not at all forgiving with Deco.  I had a dive where I went into deco but was right on the cusp. I stayed in the cusp for quite some time before ascending slowly. The Mk1 cleared deco before I hit 5m and I just suggested a 5 minute safety stop.  The Perdix had me do 3 minutes of Deco and then my safety stop.  The Atom wanted 35 minutes of deco!

When I get some time I'll graph up NDLs from second dives comparing the Perdix to the Atom with PZ+ but it is quite different.

Note that I'm comparing to the Perdix rather then the Mk1 as it is much easier for me to pull the NDL data from the Perdix.  When I compare the two they are almost identical.

Not having dived with a computer before, I was wondering something...if I want to maximize my dive and finish it with little or no NDL left, at about what NDL remaining would I typically start my ascent? I realize that the NDL time remaining starts to increment as I ascend to shallower depths, but I also need to leave time for a three minute safety stop. Can anybody advise me on a good rule of thumb on that? I will of course be using the Garmin Descent (from the start of this thread). Thanks again...

Basically, the moment you start to ascend (even a couple of metres) the NDL will increase.  The more you ascend, the more the NDL will increase.  By the time you get to 5m for your safety stop your NDL will be back to 99+ so you will always have time for a safety stop from an NDL perspective.

Of course, you need to factor in some safety, especially if you don't have a second air source.  Probably best to stay until no less than 5 minutes, but 10 minutes would be a good safety factor.

Thanks for the detailed answer, Nate! Very informative.

I would have assumed that at the end of each dive to NDL that the N2% would be the same.  i.e. you have gone for as long as you can for each dive.

The data you present suggests that the NDL is very conservative for the first dive and becomes more liberal (less conservative) with each successive dive.

Can you tell us what the N2% actually represents or how it should be used?  It is an interesting number but without a definition it is somewhat meaningless.

Does N2% represent the controlling tissue compartment supersaturation? i.e. 100% would mean that even a small drop in pressure could result in DCS symptoms?  Is it also what is displayed graphically on the left side of the dive screen and represents what the N2% would be if surfacing immediately?

The Buhlmann algorithm the Descent uses doesn't penalize the diver much over repetitive dives that are under deco limits. The N2% is based on the most saturated of the 16 compartments. This is also what is displayed graphically on the left side of the dive screen. It is showing your N2 for the most saturated compartment. The "Approaching NDL" dive alert displays when that compartment is at 80% of the tissue load.

The screenshots below are from the surface interval widget after each dive I simulated earlier. You can see the reason the N2% varies for each dive is the tissue compartments are loading at different rates. The tissue load for the first dive is primarily in the fast tissues which off gas quicker. After each successive dive the tissue load increases in the slower tissues which take longer to off gas.

Thanks Garmin-Nate, that makes a lot of sense. And to just complete the picture, I assume that the Oxygen Toxicity Unit at the bottom of the surface interval display relates to the right hand (CNS) side of the dive screen? If so, I believe 80% is the safe upper limit for that?

Hi Milosherman,

Very disappointed with the dive planning feature, it appears to be in seconds for a start, is there any way this can be changed? No other dive planning software that I am aware of shows run time in seconds. I have compared the plans generated with my Perdix, and Baltic using ZHL16 with GF45/70 and the Baltic and Perdix plans are very close, not so the MK2 which is. much more conservative. Do Garmin not understand how important this is for Open Circuit, with no reliable dive plan, you do not have a reliable gas plan. This is not acceptable and needs urgent attention. As all the reviews say that the computer behaves very close to the Perdix in water (due to injury, I have not yet got mine wet). I am concerned that I may have bought a very, very expensive sport watch and depth timer if this is not the case, as the reviewers all mention dive planning, but not that it is in seconds etc. makes you wonder if they have just read the literature. Just in case you think I am a Garmin basher, yes they do exist, I also own a Garmin 1030, VIRB, and vector pedals very happy with these. The planner needs a lot of work, and is important if they wish to sell to techies. It is just not a tech computer without a reliable planner, it would be safer and more responsible to remove the planner and ask people to use another method for planning dives, but that would not tick the box.