Is this Bühlmann or do Garmin have it wrong?

[ATJ]

I was playing with the Dive Planner feature on my Garmin Descent Mk1. I'm using GF40/85.

If I put in a 30m dive on 21% it tells me the NDL is 12 minutes. Seems low, but I can accept that as I have medium conservatism.

I can then plan a second dive. If I put in a 60 minute surface interval and then dive to 30m on 21% again, the NDL is still 12 minutes. How can that be? Surely I still have quite a bit of residual nitrogen and the NDL should be significantly less.

Do Garmin have it wrong, or does Bühlmann shorten the NDL on the first dive to allow for a longer NDL on a second dive.

Interestingly, if I was to plan with DSAT tables and did a 12 minute dive to 30m on air, had a 60 minute surface interval the NDL for my second dive on air to 30m would be 14 minutes. The end result is similar even though DSAT has an NDL of 20 minutes for the first dive.

 

[dmaziuk]

DSAT will have NDLs similar to ZH-L16 at 95+ GFHi. ZH-L is strictly symmetrical, as in: you on-gas for 30 minutes, then you off-gas for 30 minutes, you're back at square 1. Which is how it normally works in a dive computer "plan" mode (it assumes square profile). I think DSAT is the same, with the caveat that you shouldn't do 6 tanks/day.

60-minute SI is the key: I don't get much NDL reduction even on my scary conservative RGBM computer with those.

Edit: Interesting: subsurface planner gives about 14 minutes at 40/85 w/o a mandatory stop @ 3m, then 18 minutes after the 3/3 stop and 60-minute SI. I'd have to think about why that is.

Or ask someone: @atdotde? @KenGordon?

 

[CanBelieveTheStuffOnHere]

I was playing with the Dive Planner feature on my Garmin Descent Mk1. I'm using GF40/85.

If I put in a 30m dive on 21% it tells me the NDL is 12 minutes. Seems low, but I can accept that as I have medium conservatism.

I can then plan a second dive. If I put in a 60 minute surface interval and then dive to 30m on 21% again, the NDL is still 12 minutes. How can that be? Surely I still have quite a bit of residual nitrogen and the NDL should be significantly less.

Do Garmin have it wrong, or does Bühlmann shorten the NDL on the first dive to allow for a longer NDL on a second dive.

Interestingly, if I was to plan with DSAT tables and did a 12 minute dive to 30m on air, had a 60 minute surface interval the NDL for my second dive on air to 30m would be 14 minutes. The end result is similar even though DSAT has an NDL of 20 minutes for the first dive.

I have the same issue, I use a Perdix, which is pretty close to Baltic, but when I use the planning function on my backup (Garmin MK2s) I get a very different plan. I bought the Garmin as a backup computer but the idea was that as both computers use Buhlman ZHL 16, if I were to have a problem with the primary, I would still have a backup before going to tables. But am now very nervous as the planner is not giving me anything like the Shearwater. Also the deco planner displays times in seconds! I am not aware of any other computer that does this. I am worried that I have bought a very expensive depth timer with a sports watch function.

 

[inquis]

Edit: Interesting: subsurface planner gives about 14 minutes at 40/85 w/o a mandatory stop @ 3m, then 18 minutes after the 3/3 stop and 60-minute SI. I'd have to think about why that is.

[Edit: the Garmin's 12 min seems right to me.]
Subsurface gives 12 minutes for a teleport/bolt to the surface (no ceiling) and 14 mins for a controlled ascent (ceiling clears before reaching it). I don't get an increase on a second dive, though. Did you possibly bump one of the settings on the second plan?

 

[scubaozy]

Could it be descent and ascent are calculated differently?

 

[lowwall]

Answering the 2-year-old original question. It makes sense. The NDL time on your 30m dive is controlled by the fastest tissue. This will clear almost completely in a 60 minute SI.

On your next dive, the slower tissues will only be starting with a slightly higher than surface level nitrogen load. It's not high enough for them to become the controlling tissue on the 2nd dive.

If you keep doing these dives, you'll get to a point where the residual nitrogen in slower tissues builds up enough to become controlling.

 

[dmaziuk]

Subsurface gives 12 minutes for a teleport/bolt to the surface (no ceiling) and 14 mins for a controlled ascent (ceiling clears before reaching it). I don't get an increase on a second dive, though. Did you possibly bump one of the settings on the second plan?

I don't think so. I never got that round tuit though, and now I don't recall ever remembering having that conversation.

 

[scubaozy]

I have the same issue, I use a Perdix, which is pretty close to Baltic, but when I use the planning function on my backup (Garmin MK2s) I get a very different plan. I bought the Garmin as a backup computer but the idea was that as both computers use Buhlman ZHL 16, if I were to have a problem with the primary, I would still have a backup before going to tables. But am now very nervous as the planner is not giving me anything like the Shearwater. Also the deco planner displays times in seconds! I am not aware of any other computer that does this. I am worried that I have bought a very expensive depth timer with a sports watch function.

I am setting the garmin deco planner to 100/100 and my favorite profile 51m 18mins gives, 3min(4) at 9m, 6min(5) at 6m and 15min(13) at 3m. In brackets same profile on actual Bühlmann table. Give or take they are comparable. Garmin also gives the travel times but not sure whether they also add them into the calculation.