multilevel table question

[wedivebc]

OK I am probably going to regret asking this but here goes.
When I was taught to use tables we planned multilevel dives on tables using zero surface interval time and calculating the residual nitrogen time into thenext level. When looking at the PADI RDP wheel it basically represents the same thing except each level has a minimum depth (the inner circle) that the diver must ascend to for the next level to plan for.
Is there a reason for requiring the minimum depth? I should be able to go up in 10ft increments if I decide to. The minimum depth seems arbitrary unless I am missing something.

 

[dschulte]

You have found one of the limitations to the Wheel that I noticed as well. It was explained to me that to get a smaller increment would require more data points on the Wheel, I do not think it would be usable/readable.
:monkeydan

 

[boat sju]

http://scubaboard.com/showthread.php?t=18253

This old thread might interest you.

Maybe just the last couple of pages.

 

[Charlie99]

The minimum depth seems arbitrary unless I am missing something.

The problem is that repetitive pressure groups are based solely upon the 60 minute halftime compartment, but NDLs are based upon all compartments.


It's a bit complicated, but that forces both restrictions on depth combinations and the use of reduced NDLs on multlevel dives.


First you need to understand how the RDP actually works. All of the numbers in the body of Table 1 are based solely upon the 60 minute halftime compartment, except the final NDL number. In other words, the pressure groups are based only upon the 60 minute compartment, nothing else. Surface interval offgassing only tracks the 60 minute haltime compartment. Everything else is ignored.

The NDL number is based upon whichever compartment first hits the limit at that particuar square profile depth. For example, at 90', the 20 minute haltime compartment will hit the limit first. At 60' the 30 minute compartment hits the limit first, well before the 60 minute compartment hits its limit. (This is why dives to NDL limits at depths greater than 40' don't reach pressure group Z)

What happens if you first get close to the limit at 90', and then go up just 10' and hang out a while more, is that you may exceed the limit for the 20 or 30 minute compartments.

In addition to the limitations on allowable combinations of depths, the PADI Wheel has additional restriction called "ML" s or "Multilevel Limits". These are the reduced NDLs that are used on the wheel for repetitive dives. Both the depth level combinations and the reduced NDLs are needed to keep ALL compartments within NDL limits at all times.

Charlie Allen

 

[Diver0001]

Have you looked at how much you can go over, Charlie?

Brilliant post, btw.

R..

 

[Charlie99]

Have you looked at how much you can go over, Charlie? .

Short answer: I haven't run an exhaustive search, but it is pretty easy to find combination valid per "zero SI method" that push compartments to about 104% of limits.



Long Answer & Examples:

I just looked at a few possible combinations and found some that would push compartments to 104% of the M-value. For example, 10@120' + 8@100' puts the 10 min compartment at 104%, as does 17@100'+6@90'.

25@80' + 11@70' puts both the 30 and 40 minute compartments to 101%, but it appears that PADI backed off the DSAT model by 5% for the 40 minute and slower compartments and this combo has the same loading as 44 minutes at 70' -- 4 minutes beyond NDL.

47 @ 60' + 20 @ 50' takes the 30 minute compartment to 98% -- a bit more than than the "95% of unpadded DSAT model" that is used for depths of 50' and shallower.

While I was able to reverse engineer the RDP and make a spreadsheet that generates nearly the same table, I was never able to figure out the exact logic behind the depth combinations and ML limits. The ML limits seem to be a bit more conservative than needed.

In real life, I use tables to roughly plan multilevel repetitive dives, but then execute them by computer and seat-of-the-pants. In that situation I just go ahead and use the RDP with 10' increments, NDL numbers rather than ML, and no restrictions on depth combinations. After all, there isn't any qualitative difference between being at 95% of M value and 105% of M-value. In real life, what is more important is which compartments are fully loaded and how long of ascent and stops is it going to take me to get everything back down to the approxmately 85% of M-value that I prefer (this also about where the yellow to green transition occurs on Oceanic/Aeris/Pelagic computers).

Charlie Allen

 

[wedivebc]

Charlie, thanks for posting that excellent explaination.

 

[MatWill]

awesome post charlie