@tursiops , I can't find the post right now, but IIRC you claimed that tables couldn't be used for multilevel dives.
A little time ago,
you yourself posted a link to a
method for planning multilevel dives using only the PADI RDP table. I've been using that method for much of my diving career since most of my dives are multilevel or triangular profile. Granted, you have to do some simplifications, but IME it works pretty well. At least for planning.
I've looked a bit more into that method, and it seems that what it does is basically treating a multilevel dive as a series of square dives with zero SI. It has its limitations, but not more than that I routinely use it for my dive planning. With pretty good results.
Sorry to be confusing about this.
Tables cannot be used for multilevel dives
unless you modify something.
The trick of assuming zero SI as you move up from one level to the next shallower is OK, but you still have to have a way to take into account the nitrogen you did
not off-gas because you did not spend the time (and decreased ambient pressure) to go all the way to the surface. That is what the attached article takes into account, and what the Wheel and eRDPML do.
There are some levels and times at each where the nitrogen differences get lost in the coarseness of the tables quantizing. But you cannot tell in advance which depths/times these are, so the rules of the attached article take that into account to help you:
What is clear is that you do NOT get the same nitrogen status after a multilevel dive using these correct methods as you would if you use a time-depth averaged "equivalent depth" (as Jale called it) for the total time of the dive. In fact, you may not even be able to execute the dive without exceeding NDL by using an equivalent depth, namely the average depth of the DIVE (not of the BOTTOM). Worse, you do not know if you exceeded NDL because there may be nothing in the dive calculation to tell you. An example, using the air eRDPML because it is easy to use:
30m for 18 mins (NDL is 20 min); PG is M
10m for 100 mins (reduced NDL is 130 mins); PG is now X. (SS required)
Average ("equivalent") depth is 13m.
13m for 118 min (NDL is 98 min). I can't do this dive without exceeding NDL.
If the second segment of my dive were just 80 min instead of 100 mins, the calculation would go like this:
30m for 18 mins (NDL is 20 min); PG is M
10m for 80 mins (reduced NDL is 130 mins); PG is now W. (SS required)
Average ("equivalent") depth is 14m.
14m for 98 min (NDL is 98 min). PG is now Y. (SS required)
So this dive is possible, but my ending pressure group is TWO groups higher than the actual multi-level calculation.
Consequences: longer SI than actually needed, or else less surface of-gassing than expected if this is not taken into account.
Summary: the table are not designed for use with any kind of depth-averaged dive profile, but you can concoct situations where is can still work. But is is not clear what these situations are without actually doing the multio-level dive calc, in which case you did not need to do the depth averaging.
One additional comment:
by "average depth" some folks clearly mean the average depth OF THE BOTTOM, not the average depth of the dive. Those are very different numbers, calculated differently.
