Using the dive planner (new user)

The Descent is telling you the maximum NDL for your next dive at the selected depth taking your previous dives into account. So it is factoring in the residual nitrogen into the NDL it is giving you. 

Thank you for the reply. So, when I enter a surface interval and the dive depth again, is the new data that it shows for the dive that is after my next dive (after the next surface interval)? If so, then it must be using my current residual nitrogen from dives that I actually completed that day, plus the full NDL from the next dive, followed by the reduction in nitrogen from that surface interval following my next dive (?).

Yes, it is showing you the NDL for the dive you are planning given the estimated residual Nitrogen in your body. If you plan the same dive for Dive 1 and Dive 2, you should get different NDLs. 

Mark_H20 said:

If you plan the same dive for Dive 1 and Dive 2, you should get different NDLs. 

You definitely should!

Unfortunately, the dive planner is clearly broken and it gives you the same NDL for both dives regardless of the surface interval.

Note that this was the first time I tried to use the dive planner on the Mk1.

I put in air (21%) and a depth of 30m.  It gave me an NDL of 12:10.  I then selected "Add Repeat Dive", entered a surface interval of 1:00, 21% again, and 30m. I got the same NDL of 12:10.  This is clearly wrong.

I did the same thing for 20m and go 32:35 and 31:59.  It changed it but the difference can't be that small.  

If the dive planner is going to use your current residual nitrogen at the time you are using it, it is missing an extra "surface interval".

What I mean is, let's say I have just done a dive.  I get back to the boat and use the dive planner for my next dive.  It will know my residual nitrogen right now, but how do I tell it I'm not planning on starting the next dive for another 90 minutes?  My residual nitrogen will decrease in that time.

Or do Garmin assume you will use the dive planner moments before you jump back in?

Why am I not surprised. I did say it "should" give you different NDLs. This is just another example of the long running software issues with the Descent. Not sure if this was a software specifications error, or a coding errror, but it is a safety issue that Garmin needs to address. They should either fix it, or remove the dive planner until/if it is fixed. 

I will admit I have only played with the dive planner, and have never actually used it while diving. 

I also tested the planner. If I enter 30m it gives me 11:44 NDL. If I continue and ad a second dive with 15 min surface time and again 30m, it now gives me 9:59 NDL. (Both times 21% O2). Isn’t that correct,,,?

Using my DSAT tables....

NDL for a 30m dive is 20 minutes.  If I did that, I end up in group N.

15 minute surface interval puts me in group K.  

In Group K my NDL for 30m is 4 minutes.

But let's try 30m for 12 minutes.

That's group G.  15 minute surface interval takes me to group E. NDL at 30m is now 10 minutes. 

That mostly matches what you got but the original NDL was wrong.

Yes, I have discovered when diving with others that Garmin gives shorter NDL than others get.

Seems that the BühlmanN (is the name, right?) algoritm is more conservative on NDL.

My wife have Oceanic and there one can choose algorithm. When she chooses the Bühlmann, she gets about the same NDL time as I do on my Garmin.

By the way, if I set my Garnin to low conservatism (previously I had medium), I get 14:21 NDL on 30m, and after 15 min surface time I get 11:31 NDL on the next dive...

So perhaps the Garmin is correct, but The Bühlmann is more conservative on the first dive?

Because if you add the two dive times you get about 25 minutes on 30m with the 15 mins surface time in between, about the same as if you add the two dive times with the DSAT tables...