PADI RDP Table Puzzling Question!

[XS-NRG]

Hi there,

I have been looking over these PADI tables again just to keep fresh but I have one burning issue and I know this came up during class but there was a bit of controversy as to what the answer was so I'm bringing it to the table here.

Let's assume I'm repetitive diving. After a dive I have a SIT and end up as a "K" pressure group. I'm planning my next dive to 50 feet. On table 3 it shows that a "K" diver @ 50 feet has an adj. NDL of 44 mins and 36 min RNT. Now does this mean that in order to stay under 44 mins I can only dive for 8 mins (8 + 36 RNT) so as not to exceed the 44 Adj NDL? OR Can I truly dive for up to 44 mins no problem? It seemed like during class our planned ABT + RNT was not to exceed the adj NDL but that didn't seem right. We did it anyway but everyone including the instructors all got confused. I think too many questions were asked.

If I did the dive for 44 mins, then my RNT + ABT would equal 80 mins (TBT) which is greater than the adj NDL of 44. Is that a problem? I can't see a dive only being 8 mins for this pressure group. Also the resulting 80 mins (TBT) is the NDL at 50 ft on table 1.

My thoughts are I can dive for 44 mins max because an 8 min dive is ridiculous for only being a "K" at 50 ft. Please someone confirm that I'm correct here. This has to be right because a "P" diver at 70 ft has a residual greater than the adj NDL which means you wouldn't be able to dive at all if your RNT + ABT couldn't be greater than the adj NDL.

Thanks alot.

 

[Thanshin]

If you translate that to how deep and for how long you are in first dive, how much surface interval you do and how deep you plan to go on second, I'll give you the result as per the NOAA tables (those I have in front of me now).

 

[Ste Wart]

It's 44min, you add RNT onto that figure. The adjusted bottom and the rnt should equal the maximum NDL for any pressure and depth.

For example. All the ABT's and NDL's for repetitive dives to 18m will add to 56.

 

[DevonDiver]

After surface interval pressure group is K
Plan dive for 50ft.

On the RDP:




From the table (above) you can see that:

The Adjusted No-Decompression Limit (A/NDL) is 44 Minutes. Your Actual Bottom Time (ABT) should not exceed this limit. You should not physically be diving for longer than 44 minutes. You just have to remember this number - as it dictates your absolute maximum dive time on the planned repetitive dive.

If you don't want/need to know your end pressure group after the dive, then you don't need to go any further. Just dive within the A/NDL. Simple.

However, if you want/need to calculate your End Pressure Group after the dive, then you'll need to calculate your Total Bottom Time (TBT) so you can track your end nitrogen on the table.

To calculate the TBT you simply add your RNT and ABT. Explained:

The Residual Nitrogen Time (RNT) is 36 minutes. This is the theoretical time that you use for planning purposes. It represents the 'amount' of nitrogen you have in your body from the previous dive/s.

To calculate the dive, accounting for residual nitrogen and 'new' nitrogen, you need to add the RNT to your Actual Bottom Time (ABT). The ABT is your desired bottom time for the planned dive. You can dive for as long as you want, providing the ABT doesn't exceed your A/NDL.


Now, turn over the table;



From this you can see the relationship between ABT, TBT, RNT and A/NDL.

If you wanted to do the maximum permissible duration on the second dive - you would dive to the A/NDL. We know that is 44 minutes.

The (unadjusted) NDL for 50ft is 80 minutes. Can you see how the A/NDL (44mins) + RNT (36mins) = 80 minutes? As a scuba diver, not going into Deco... you are permitted to dive for 80 minutes at 50ft. However, you have already filled your body with 36minutes worth of theoretical nitrogen. 36 minutes of 'old' nitrogen, from a total permissible of 80mins, leaves 44 mins of 'new' nitrogen for the next dive..

That's enough to plan our dive.... but not enough to calculate our Ending Pressure Group. That is where we actually use the RNT and ABT calculation. We use the accumulated old (RNT) and new (ABT) to create a theoretical dive time, that allows us to determine the pressure group:

We add 'old' nitrogen to 'new' nitrogen, giving total nitrogen absorbed. Total nitrogen is our Pressure Group.

So.... 36 minutes RNT + 44 minutes ABT = 80 minutes TBT.

We use the TBT to identify our End Pressure Group. That pressure group is X.




So, to conclude:

You plan a repetitive dive to 50ft and you are PG 'K'.
Your A/NDL is 44 minutes. This is the 'real world' limit of your dive time. Important!
To (only) calculate your End PG, you add your 'real world' dive time (ABT) to your 'left over' nitrogen (RNT).
ABT 44mins + RNT 36mins = TBT 80mins
On Side 1 of the tables, a TBT(theoretical) of 80mins & ABT (real) of 44mins at 50ft gives you End PG of 'X'.

 

[Jim Lapenta]

Gotta find the rat after the first dive.

 

[DivemasterDennis]

What Devon Diver said. Clear, concise. Good teacher. I'll have to read you other posts more closely, as I am sure I will learn from them too.
DivemasterDennis

 

[XS-NRG]

Thanks Andy and Stewart!. That's what I figured. I've been making my own DIY log book as I altitude dive and there is no section on that to account for actual depth, cold water, theoretcial, and then even nitrox EAD after that. I've laid it out in such a way that info is together and co-relates to each section and one is an 02 NDL, and a N2 NDL (oxygen toxicity, and nitrogen no stop time) and then you can clearly see which is smaller for your max NDL. Now that I have my answer it will help me with this layout.

 

[C P]

The number in blue is your adjusted no decompression limit ( how long you CAN stay on the current dive) The number in the white box is how much nitrogen you have ( in minutes at that depth) you still have in your body from the previous dives. If you add the two numbers together, they equal the number in the black on table 1. ( maximum no decompression limits) I said CAN for the blue box because only you can decide to push your limits to the maximun. Air consumption allowing.

 

[DiveNav]

XS-NRG, for the sake of the exercise, what is the profile of dive 1?

 

[Karibelle]

I know this came up during class but there was a bit of controversy as to what the answer was

That's not a controversy. That is either you didn't understand the table discussion, or your instructor didn't know how to explain the tables.

It sounds like the second is more likely.