Nitrox and computers

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My Petrel 2 in Rec Mode uses Low, Medium, High GFs of 45/95, 40/85, and 35/75. What is interesting is if I set the computer in Tec Mode, at 35/75, I do NOT get the same NDLs, by 1-2f minutes (more) at 6-=110 ft.

I checked my Perdix when I got home from work and the low, med, and high conservative settings are the same as what you state above. I compared an 80 ft first dive on air for both rec and tec mode using med and 40/85 conservative settings and got 21 and 22 minutes for NDL respectively. Stuartv thought this discrepency could be due to the anticipation of a safety stop in rec mode. I thought the same thing but realized the definition of an NDL is time remaining before a stop is required. A safety stop is just a non-mandatory stop, so a safety stop would be excluded in the calculation.

The difference is probably due to rounding in the calculations. The calculations are done using real numbers but need to be rounded up or down because the NDL display is an integer. This would account for a 1 minute difference but not two. Also, the difference could be due to using different methods to calculate NDL. Rec mode may be using the Haldane equation while tec mode is calculating the deco ceiling using the time remaining to get the ceiling down just past the surface (ceil > 0 ft.) as the NDL. The difference in methods may account for the 2 minutes.

I too am looking forward to an explanation from Shearwater. Anyone from Shearwater care to comment?
 
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I checked my Perdix when I got home from work and the low, med, and high conservative settings are the same as what you state above. I compared an 80 ft first dive on air for both rec and tec mode and got 21 and 22 minutes for NDL respectively. Stuartv thought this discepency could be due to the anticipation of a safety stop in rec mode. I thought the same thing but realized the definition of an NDL is time remaining before a stop is required. A safety stop is just a non-mandatory stop, so any kind of stop is excluded in the calculation. The difference is probably due to rounding in the calculations. The calculations are done using real numbers but need to be rounded up or down because the NDL display is an integer. This would account for a 1 minute difference but not two. Also, the difference could be due to using different methods to calculate NDL. Rec mode may be using the Haldane equation where tec mode is calculating the deco ceiling using the time remaining to get the ceiling to 0 ft as the NDL. The difference in methods may account for the 2 minutes.

I too am looking forward to an explanation from Shearwater. Anyone from Shearwater care to comment?
@lynnpartridge can probably answer the question raised in the last few posts.
 
I couldn't resist comparing all my computers in the 60-110 foot range shown earlier.
Assumptions are AIR for all, first dive, no residual N2. Three tables are shown as well.
View attachment 378311

What did I learn?
SW Rec Mode Low conservatism is even worse than the USN tables.
The Dive Rites and the Zeagle (all made by Seiko) are nearly identical to the PZ+ in the OC1.
Nothing else jumps out at me.

The Shearwater NDLs seem unbelievable to me and different from what I've seen before. There's no way that the Navy NDL at 60 ft is 60 minutes, DSAT is 57 minutes, and Buhlmann ZHL-16C 45/95 is 89 min! I've run this on Multideco and 89 min gives 6:20 deco at 20 ft and 15:00 at 10 ft. Something is quite wrong here, maybe nitrox setting? This is wrong!!
 
Exactly. That's the kind of thing I was referring to when I mentioned being a tech diver and knowing what you are doing by skipping the last minute. If I'm diving GFhi of 70, I might skip my last minute of deco, if circumstances called for it. Sure.

None of that applies to a Rec diver using a Rec computer. Or really even a Tech diver using a Rec computer (with its proprietary algorithm). We're talking about a computer locking you out for a deco violation. Doesn't that pretty much mean, by definition, it's a Recreational computer? And don't they all (meaning all the ones that could/would lock you out) have proprietary algorithms? I don't have any sympathy for a Rec diver who causes his computer to lock him out and then complains about not being able to dive again within 24 hours. And I really don't have any sympathy for a tech diver who does the same. If you're qualified to second guess and override a computer with a proprietary algorithm, then you should be okay to use that computer in gauge mode for your next dive.

instructors.... sometimes you can't help it if you have to go back after a student. This computer doesn't revert to gauge mode which is the problem, it just bricks, which some of the others do as well
 
The Shearwater NDLs seem unbelievable to me and different from what I've seen before. There's no way that the Navy NDL at 60 ft is 60 minutes, DSAT is 57 minutes, and Buhlmann ZHL-16C 45/95 is 89 min! I've run this on Multideco and 89 min gives 6:20 deco at 20 ft and 15:00 at 10 ft. Something is quite wrong here, maybe nitrox setting? This is wrong!!

I checked the NDL's on my Perdix for a 60 ft dive on air. For med conservatism I got 42 min. At the low setting I got 53 minutes. The PADI (DSAT) table tops out at 55 min. In my spreadsheet using the DSAT table I got 57 minutes. Switching to Buhlmann gave me 48 minutes (GF-Hi of 95%). Buhlmann at GF-Hi of 100% gave 58 minutes. The difference between these tables is that for all the compartments except for the first the HT's are shorter for Buhlmann than DSAT. Therefore, for a given time the N2 loads will be higher for Buhlmann.

Using Buhlmann with GF-Hi of 95% I got 89 minutes using EAN31.
 
My bad. My VERY bad. I made up my table by combining some shorter tables for individual computers....but I had NOTdouble-checked that all were actually on air! So I redid all the Shearwater stuff. It all looks better now. Note that this table is slightly different: the Tec Mode 35/75 is gone, replaced by a 49/99 column, which is the most aggressive setting for my SW. I did. in fact, recalculate the Tec 35/75 to compare with the Rec-High, and they are NOT quite the same....the Tec version is 1-2 minutes longer NDLs at each depth.
upload_2016-7-26_21-53-5.png

So, what have I learned?
Don't stick together piece-meal calculations without a LOT of checking.
SW 49/99 is almost the same as PADI RDP.
SW Rec-Low is almost the same as the DCIEM tables.
I apologize to everybody for wasting their time earlier.
 
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I did. in fact, recalculate the Tec 35/75 to compare with the Rec-Medium, and they are NOT quite the same....the Tec version is 1-2 minutes longer NDLs at each depth.
View attachment 378317

Thank you for checking your data and re-posting with the correcter stuff. :)

If you really compared Tec 35/75 to Rec Medium, maybe that's why they didn't match? Since Rec Medium in your chart says 40/85. I'm guessing you really meant that you compared Tec 35/75 to Rec High.

And I'm still hoping someone from SW will explain that to us.
 
Dang, you are right. The comparison was to 35/75, so Rec High. I went back and changed the post. I also went back and put a caveat on the earlier version of the table.
 
My bad. My VERY bad. I made up my table by combining some shorter tables for individual computers....but I had NOTdouble-checked that all were actually on air! So I redid all the Shearwater stuff. It all looks better now. Note that this table is slightly different: the Tec Mode 35/75 is gone, replaced by a 49/99 column, which is the most aggressive setting for my SW. I did. in fact, recalculate the Tec 35/75 to compare with the Rec-High, and they are NOT quite the same....the Tec version is 1-2 minutes longer NDLs at each depth.
View attachment 378317
So, what have I learned?
Don't stick together piece-meal calculations without a LOT of checking.
SW 49/99 is almost the same as PADI RDP.
SW Rec-Low is almost the same as the DCIEM tables.
I apologize to everybody for wasting their time earlier.

Nice table, thanks for all the work and the corrections. The USN NDLs from Revision 6 of the US Navy Diving Manual, 2008, are actually slightly different than the values in your table. This was the first revision of the tables since 1959.

The NDL for 70 feet is 48 minutes, the NDL for 80 feet is 39 minutes, the other depths are unchanged.
 
i use the mares puck pro and it has the capability for air and nitrox -- maybe you should look into that

but then again -- it depends on what you are using it for -- the puck pro is more for recreational diving i guess.
 
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https://www.shearwater.com/products/peregrine/

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