The PADI RDP table is designed for square profiles, not multi-level dives. The pressure groups after a dive ASSUME you are going to the surface, and that off-gassing time at a shallower depth is included in the computations.
Many, however, have invented a procedure that they think allows them to do multi-level dives with the RDP. What they do is (say) go to 80 ft for (say) 20 mins, then rise to (say) 60 ft for (say) 20 mins, then rise to (say) 35 ft for 30 mins, and then ascend. They have plenty of gas to do this dive, they are happy. They cannot use the RDP with a square profile for this dive: 20+20+30 mins = 70 min, which at 80 ft is WAY over the NDL of 30 mins for a square profile, all the time having been spent at 80 ft.
So they use the RDP for this dive as follows: First the "first" dive go to 80 ft for 20 min; end in PG K. The table shows for PG K you stay in PG K for a "SI" of 0-4 min. So they say they are starting their 60-ft second level as a "second dive" in PG K, and after 20 min they are now in PG T. The RDP says they stay in PG T for a SI of 0-2 mins. So they now go to 35 ft as a "T" diver, and the table says they have 88 mins before they hit their NDL, so their 30 mins at 35 ft is fine. they spend 30 min, and the RDP puts them in PG W, with a mandatory safety stop; no big deal, they were going to do one anyway?
What's wrong with this?
Let's calculate the very same dive with the eRDPml, which allows one to do multi-level dives within the constraints (including testing) of the RDP.
First level: 80 ft for 20 mins; end in PG K.
Second level: 60 ft for 20 mins; the maximum time allowed is 17 mins! 20 min is not allowed, it exceeds the RDP limits!
The dive as planned cannot be performed within the RDP limits. The use of the RDP table for this dive -- designed for a square profile, not a multilevel dive -- is a misuse of the table.
By using the RDP table for planning a multi-level dive, you will always be closer to your allowable NDL than you think (because you are not getting as much off-gassing as the table assumes), and perhaps exceed it, as this example shows.
The good new is you CAN use the eRDPml (or the Wheel, if you really courageous) to plan this 3-level dive of 80, 60, and 35 ft.
First dive: as above, 80 ft for 20 mins, end in PG K.
Second level, 60 ft, but don't exceed the 17 min limit.....end in PG S.
Third level, 35 ft, the multi-level limit is 82 mins, so the 30 min planned time at 35 ft is doable.
Conclusion:
Misuse of the RDP to do a multi-level dive can easily put you into a "exceeding NDL" condition and require a deco stop (not just a safety stop). But there is no way to know if your dive stayed within the limits or not, unless you can also calculate the correct multi-level dive for comparison.
What to do?
For the really adventurous:
A procedure to validly use the RDP table for multi-level dives was published by Duis (1991), after the Wheel came out but long before the eRDPml. The procedure was developed to be consistent with the Wheel. It has two constraints: (1) each level must be significantly shallower than the preceding level (60 ft to 35 ft is OK, but 60 ft to 40 ft is not, for example), and (2) at each new level the allowable adjusted NDL is less than the table shows. One could mark up their RDP table to include these adjustments, and have a satisfactory multi-level table planning too.
Many, however, have invented a procedure that they think allows them to do multi-level dives with the RDP. What they do is (say) go to 80 ft for (say) 20 mins, then rise to (say) 60 ft for (say) 20 mins, then rise to (say) 35 ft for 30 mins, and then ascend. They have plenty of gas to do this dive, they are happy. They cannot use the RDP with a square profile for this dive: 20+20+30 mins = 70 min, which at 80 ft is WAY over the NDL of 30 mins for a square profile, all the time having been spent at 80 ft.
So they use the RDP for this dive as follows: First the "first" dive go to 80 ft for 20 min; end in PG K. The table shows for PG K you stay in PG K for a "SI" of 0-4 min. So they say they are starting their 60-ft second level as a "second dive" in PG K, and after 20 min they are now in PG T. The RDP says they stay in PG T for a SI of 0-2 mins. So they now go to 35 ft as a "T" diver, and the table says they have 88 mins before they hit their NDL, so their 30 mins at 35 ft is fine. they spend 30 min, and the RDP puts them in PG W, with a mandatory safety stop; no big deal, they were going to do one anyway?
What's wrong with this?
Let's calculate the very same dive with the eRDPml, which allows one to do multi-level dives within the constraints (including testing) of the RDP.
First level: 80 ft for 20 mins; end in PG K.
Second level: 60 ft for 20 mins; the maximum time allowed is 17 mins! 20 min is not allowed, it exceeds the RDP limits!
The dive as planned cannot be performed within the RDP limits. The use of the RDP table for this dive -- designed for a square profile, not a multilevel dive -- is a misuse of the table.
By using the RDP table for planning a multi-level dive, you will always be closer to your allowable NDL than you think (because you are not getting as much off-gassing as the table assumes), and perhaps exceed it, as this example shows.
The good new is you CAN use the eRDPml (or the Wheel, if you really courageous) to plan this 3-level dive of 80, 60, and 35 ft.
First dive: as above, 80 ft for 20 mins, end in PG K.
Second level, 60 ft, but don't exceed the 17 min limit.....end in PG S.
Third level, 35 ft, the multi-level limit is 82 mins, so the 30 min planned time at 35 ft is doable.
Conclusion:
Misuse of the RDP to do a multi-level dive can easily put you into a "exceeding NDL" condition and require a deco stop (not just a safety stop). But there is no way to know if your dive stayed within the limits or not, unless you can also calculate the correct multi-level dive for comparison.
What to do?
- Don't use the RDP table to do multi-level dives.
- Do consider the eRDPml for planning a multi-level dive, or buy a computer....although most of them do not have a planning function powerful enough to allow planning a multi-level dive! Most of them only allow planning a square-profile dive.
For the really adventurous:
A procedure to validly use the RDP table for multi-level dives was published by Duis (1991), after the Wheel came out but long before the eRDPml. The procedure was developed to be consistent with the Wheel. It has two constraints: (1) each level must be significantly shallower than the preceding level (60 ft to 35 ft is OK, but 60 ft to 40 ft is not, for example), and (2) at each new level the allowable adjusted NDL is less than the table shows. One could mark up their RDP table to include these adjustments, and have a satisfactory multi-level table planning too.
