Why you can't use the PADI RDP table for Multi-Level dives

Please register or login

Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

Benefits of registering include

  • Ability to post and comment on topics and discussions.
  • A Free photo gallery to share your dive photos with the world.
  • You can make this box go away

Joining is quick and easy. Log in or Register now!

tursiops

Marine Scientist and Master Instructor
ScubaBoard Supporter
Scuba Instructor
Messages
14,213
Reaction score
12,945
Location
U.S. East Coast
# of dives
2500 - 4999
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?
  • 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.
 

rhwestfall

Woof!
ScubaBoard Sponsor
Messages
19,783
Reaction score
26,490
Location
"La Grande Ile"
# of dives
200 - 499
The Wheel had "max time" for ML dives that were shorter than the NDL times, and also did not allow for planning of a ML dive that exceeded 80 feet.
 
  • Like
Reactions: EFX

EFX

ScubaBoard Supporter
ScubaBoard Supporter
Messages
1,094
Reaction score
737
Location
North Central Florida
# of dives
200 - 499
I ran my spreadsheet using the 80/20, 60/20, 35/30 ft/minutes profile above. I ran two profiles on those depths: a sawtooth profile ascending to a depth of 1 ft for 0 min, and a multilevel profile with a 15 ft 3 min stop at the end. Both profiles used a 60 fpm descent, 30 fpm ascent, GF 95/95, and DSAT tables which I believe is what the RDP is based on. In both profiles the limit in NDL was achieved after spending 20 minutes at 60 ft.

The sawtooth profile allows the ss to use offgassing on ascents which presumably the RDP is incorporating in the table. The difference in NDL's was only 3 minutes. The sawtooth being 7 minutes and the multilevel coming in at 4 minutes. As you say you will always have less NDL than what you think using the RDP for multilevel dives. I was puzzled why the wheel wouldn't allow the ML dive over the RDP until rhwestfall posted above. Part of the issue could be that the RDP & wheel use the 60 minute ht compartment from the DSAT table which is TC6. The sawtooth ss profile at minimum NDL shows TC4 as the controlling TC. The multilevel ss profile shows TC3. Both of these TC's are significantly faster (30 and 20 minute ht resp.) than the RDP/wheel's TC6 therefore more offgassing.
 
OP
tursiops

tursiops

Marine Scientist and Master Instructor
ScubaBoard Supporter
Scuba Instructor
Messages
14,213
Reaction score
12,945
Location
U.S. East Coast
# of dives
2500 - 4999
The Wheel had "max time" for ML dives that were shorter than the NDL times, and also did not allow for planning of a ML dive that exceeded 80 feet.
The Wheel allowed the first level to be as deep as 130 ft; but whether the first level was 130 or 120, the second level had to be at least 80 or shallower. The paper by Duis (1991) explains:
upload_2020-5-8_15-54-19.png


Part of the issue could be that the RDP & wheel use the 60 minute ht compartment from the DSAT table which is TC6. The sawtooth ss profile at minimum NDL shows TC4 as the controlling TC. The multilevel ss profile shows TC3. Both of these TC's are significantly faster (20 and 30 minute ht resp.) than the RDP/wheel's TC6 therefore more offgassing.
I think Duis refers to this here:
upload_2020-5-8_15-57-13.png
 
OP
tursiops

tursiops

Marine Scientist and Master Instructor
ScubaBoard Supporter
Scuba Instructor
Messages
14,213
Reaction score
12,945
Location
U.S. East Coast
# of dives
2500 - 4999
The Duis report contains this example:
120 ft for 10 mins, ascend to 70 ft for 10 mins, ascend to 40 ft for 35 mins, and end as a U diver.
The figure shows the sample dive, but also shows the depth limitations at the top, and the reduced NDLs marked in black.
upload_2020-5-8_16-9-35.png
 

boat sju

Contributor
Messages
1,574
Reaction score
440
Location
Haslett, Michigan
# of dives
200 - 499
What? Why to any of this? I don't understand - but it may be I'm trying to perfect my margarita recipe so I've had a couple experiments so far tonight.
 

Top Bottom