Using the dive planner (new user)

I don't have a huge issue with the NDL for the first dive.  As you say, this is most likely the conservatism. The problem I see is the NDL is not dropping enough on the subsequent dive.  It can't just be the conservatism.

Basically what the dive planner is saying is that if you stay right to your NDL and then ascend with a safety stop, after a 15 minute surface interval you have very little residual nitrogen.  With a 1 hour surface interval it is suggesting you have no residual nitrogen as the NDL for the second dive is the same as the first.

Further, if the Mk1 is more conservative than the tables (and it mostly likely is), you shouldn't be able to do two 30 metre dives with a 15 minute surface interval and still end up on the cusp of NDL on the second dive with both the table and the Mk1.

Interestingly, I just tried the dive planer on my Perdix for the first time.  Same algorithm same conservatism and it tells me NDL for 30m is 14 minutes (not 12).  Unfortunately, it only does one dive so I can't check what happens on the second dive.  One thing it does have that the Mk1 doesn't have, is you enter when you will be diving - i.e. it takes your current residual nitrogen into account and any surface interval between now and when you dive.

It would be interesting to get some input from Garmin how the dive planner is intende to be used and why we get these values for NDL...

Input from Garmin? That would be an interesting change, last time we got "input" Garmin deleted the entire thread. 

Here's something else...

Using the Dive Planner, I could do 4 dives to 30m each with a 1 hour surface interval between each dive and still stay within NDL for each dive.

If I plan the same 4 dives with the DSAT tables, the NDLs for dives 3 and 4 are 11 minutes for both dives.  i.e. according to DSAT I'm into deco.

Perhaps this is a "feature" of the Bühlmann algorithm: be conservative for the first dive and liberal for subsequent dives.  Seems odd.

Yes, I think I´ve seen something abut that the  Bühlmann is conservative from the start.

Would be interresting to get this confirmed and also if the Garmin planner is correct according to the Bühlmann...

Just had to look at the NDL times from different computers. My wife has an Oceanic where you can choose DSAT or Bühlmann. So I stepped through the different options for different depts, first dive and 21%O2. I tested Conservatism Low (L) and Medium (M) on Garmin. On Oceanic you can just choose Conservatism (C) or none. And here are the NDL timses:

Depth: Oceanic DSAT -  DSAT (C) - Bül - Bül (C) <>  Garmin (M) - (L)

9: 283 - 190 - 217 - 143 <> 390 - 557

12: 144 - 97 - 116 - 77 <> 115 - 149

15: 85 - 60 - 69 - 51 <> 66 - 78

18: 59 - 40 - 50 - 35 <> 41 - 52

21: 41 - 30 - 37 - 24 <> 27 - 35

24: 32 - 23 - 28 - 18 <> 19 - 25

27: 25 - 17 - 20 - 13 <> 15 - 18

30: 20 - 14 - 16 - 10 <> 12 - 14 

33: 17 - 11 - 13 - 08 <> 09 - 12

36: 14 - 09 - 10 - 07 <> 08 - 10

39: 11 - 08 - 09 - 06 <> 07 - 08

42: 09 - 07 - 08 - 05 <> 06 - 07

Wow, quite different values. And this is only considering the first dive. What would you instructors advice here...?

I don´t know if any of those are close, but perhaps the Oceanic Bühlmann (both C and no C) isare close to Garmin (M and L), at least from 24 meters ?

And Bühlmann is more conservative that DSAT on the Oceanic, at least for this forst dive...

This has been a very interesting thread to follow. Very appreciative for all the input everyone has been providing, especially as a new diver. Based on all the data that Milosherman provided, it seems like the Garmin's diver planner is drastically higher on NDL compared to the Oceanic at the shallower depths (9 and 12 meters), with both set for the conservative Bul algorithm, and is even higher than the DSAT/Bul low conservative algorithms. I'm wondering if this disparity would also exist on the NDL it tracks during actual diving, rather than just using the planner. I guess the times are so long at that time anyway, that I'll run out of air first. But it does make me wonder about cumulative NDL over multiple dives at shallower depths...

Here is a video showing some tests of this...
https://www.youtube.com/watch?v=gvfykrC83Jc

Here is an interesting twist. If I go to Dive Planner and tell it I want to dive 40% and go to 100', the Descent tells me I can go to that depth for 29:22, but has the MOD as 81'. It should tell me that that depth is below the MOD for that EANx % and never give me an NDL.