actual NDL calculations

[scc135]

I recently got padi nitrox certified and they introduced how to more accurately calculate equivalent depths and such, and was wondering, how come they don't give formulas for figuring out actual NDL, pressure groups, RNT and the like? And why don't they teach multi-level diving as almost all are? The square dive planning was drilled into my head well, but I did a deep dive and stayed at 105 for 3 minutes before coming up to 80 then 60 then 30 for a 35 min dive, and that square profile just doesn't work with an rdp.

I figure a multilevel dive would be basically if you did 3 square dives without surface intervals? Wouldn't that round out a lot from your ndl?

 

[Blackwood]

The NDL computations are fairly complicated. It's not just f(x)=NDL. For dissolved gas models, you can have 16 (or more) compartments tracking exponential growth and decay based on variable ambient and dissolved gas pressures, and generating time limits for an assumed max ascent rate such that no compartment ever exceeds its theoretical maximum gradient.

I wrote a buhlmann excel sheet once, but it's not something I'd want to do for every dive.



I suspect they teach square profiles BECAUSE most people do multilevel, and treating a highly-multilevel profile as a square (i.e. max depth for the entire duration) is mega conservative.

I figure a multilevel dive would be basically if you did 3 square dives without surface intervals? Wouldn't that round out a lot from your ndl?

NDL tables assume you ascend at the end of the dive (often at 60fpm). Simply adding two more levels with zero surface interval would neglect that in-water deco. Problematic on average? I doubt it given the level of conservatism already built in. Something to consider before experimenting, though.

 

[muddiver]

I'm sorry, but where do you keep coming up with 60fpm?

 

[Peter Guy]

OP -- PADI teaches "square profiles" as part of the basic Open Water class. However, if you want, you can use the "new" eRDPml (the RDP planning computer) to plan multilevel dives using the DSAT RDP model (hence teh "ml" added to the eRDP). And, of course, you can always go away from using the tables and just use a Dive Computer which is what most recreational divers do (as far as I can tell).

 

[LeadTurn_SD]

I'm sorry, but where do you keep coming up with 60fpm?

PADI RDP are (Were?? I'm not sure if this has recently changed) based on 60' per minute ascent rate. USN tables used to be 60' per minute, but I believe this changed to 30' minute with the new USN tables? I'd need to check.

Best wishes.

 

[scc135]

I was taught with 60fpm, but I believe it's 30 fpm now.

I bring this up as I got myself a dive comp but always back up myself with charts and putting all that info into my dive log. I never had a problem with square dive profiles until I started to become more comfortable in the water and took my AOW. Now, I did deep dives and they blow past my NDLs. Sure my dive comp says I'm good but when I log it, it shows a max depth of 105 (for instance) with a BT of 35 min, which doesn't make sense on paper.

 

[Blackwood]

I was taught with 60fpm, but I believe it's 30 fpm now.

I don't make a habit of looking at many dive tables, but while they may be recommending 30fpm now, I doubt they changed the math to assume 30fpm in the computations. If anything, they'd have gone the other direction.

It would be easy enough to check, though. Compare NDLs for a given depth between "new" and "old" tables. If the "new" tables have longer allowables, they've changed the math.

 

[muddiver]

I only asked because the PADI Dive Planner, and all tables PADI, as is mentioned by the OP has a 30fpm ascent rate. And no I don't know when that changed, but it was definately before 2003 when I finished my Divemaster course, and the U.S. Navy table for No-Decompression dives, Revision 5 (latest revision is 6) is also 30fpm.

I just want to make sure we are all taking the same certification agency here, and not mixing in ratio-deco or some other theory.

Thanks all.

 

[captain]

Navy tables revision 6 is still 30 fpm, only change appears to be adding the option of in water decompression stops 30 feet and above on 100% O2

 

[RTee]

Rate of ascent...interesting. I would need to check but I think PADI still teaches 60fpm. I will also need to recheck te user manual for my suunto but I think it is based on 60 fpm up to 60 ft and then 30 fpm afterward...