Another "Am I Reading the PADI Dive Table Correctly?" post... ???

[AC26XP]

Per the NOAA/NAUI NDL air table: http://www.ndc.noaa.gov/pdfs/NoDecoAirTable.pdf
A dive to 60ft for 45min has me in pressure group H, and after a 3 hour interval I am in group D.

However, per the PADI Dive Table: http://www.scubatoronto.com/images/rdp1.jpg
A dive to 60ft for 45min has me in pressure group S, and after a 3 hour interval I am in group A.

Group A...???

That doesn't seem too conservative in relation to the NOAA table.
Could someone offer a detailed explanation.
There are plenty of threads out there that talk about the "general differences", but in this case my question has to do with why these "short" invervals to Group A exist on the PADI table.
And to be more specfic: I am looking for the "programming" that goes into the table's design and mathematical differences are between the tables.

And with all due respect, I am not looking for anecdotal/tongue-in-cheek "That's PADI for ya..." or "My table can beat-up your table..." type answers.
I have already read plenty of those while searching...

Thanks.

Happy diving,
AC

 

[Colliam7]

However, per the PADI Dive Table: http://www.scubatoronto.com/images/rdp1.jpg
A dive to 60ft for 45min has me in pressure group S, and after a 3 hour interval I am in group A. . . . Group A...???

Yes, you are reading the PADI table correctly.

That doesn't seem too conservative in relation to the NOAA table. Could someone offer a detailed explanation.

You are right, it does not seem conservative, specifically compared to the NOAA table. However, tables, in general, are conservative, at least compared to the use of dive computers. And, atables, there may be relative degrees of 'conservatism', and this is an example of where at least two tables diverge. While it is not appropriate to compare 'letter groups' across tables from different agencies, the effects on adjusted NDL are what count and there is a difference in this case.

I suspect your question relates to the fact that, using the PADI tables, you appear to be back to 'baseline' / 'normal / whatever after three hours, but according to the NOAA tables you have a remaining nitrogen load. In fact, in the PADI nomenclature, Group 'A' still has a residual nitrogen load, albeit it modest, and therefore is required to pay a penalty on a subsequent dive, in terms of a reduced adjusted no-decompression limit. It is not 'baseline'.

In the specific example you cited, after a 45 minute dive to 60 feet, you would be in Group S according to the PADI tables and in Group H according to the NOAA tables. After three hours, you are in Group A according to PADI tables. My interpretation of the NOAA tables (and maybe I ended up looking at a different set than you used - I used the 2010 table), is that you would be in Group E (not D) after a 3 hours surface interval.

Irrespective of letter group, There IS a difference in the adjusted NDL between the two. If you were to repeat the dive, and go back to 60 feet, your aNDL would be 31' with the NOAA table and 49' with the PADI table.

I suspect the models used in development of the two tables vary enough to produce the difference. Which is 'right'? Probably both are conservative enough to allow for safe diving, and both are more conservative than a computer, tracking your actual depths, would be.

 

[AC26XP]

I suspect your question relates to the fact that, using the PADI tables, you appear to be back to 'baselne' / 'normal / whatever after three hours, but according to the NOAA tabnles you have a remaining nitrogen load. In fact, in the PADI nomenclature, Group 'A' still has a residual nitrogen load, and therefore is required to pay a penalty on a subsequent dive, in terms of a reduced adjusted no-decompression limit. It is not 'baseline'.

Exactly.

However, I would like more info on PADI Group A residual nitrogen, since planning-out multiple dives the Group A residual nitrogen/time seems negligible and definitely not the NOAA group D differential...

Happy diving,
AC

P.S. I use the NOAA charts as my planning guideline, however this post has nothing to do with what table to use... it's about understand the mathematical difference between tables.

 

[--tom--]

I am not smart enough to go into great detail, but some things might become obvious if we just look at the information posted.

When adopting a chart that is converting what the body is doing to a mathematical formula, some compromises must be made to adopt a conservative model. NAUI (form what I remember) emphasizes a longer single dive & shows a higher charge (more residual nitrogen time) for repetitive dives. PADI uses shorter dive times and a faster off gas formula so repetitive dives are available more quickly.

What does that all mean? In coarse terms, a single chart can only track a single rate of nitrogen absorption (& its corresponding rate of off-gassing). NAUI chose a (theoretical) tissue (called compartments) that takes on N2 slower and off gasses slower than the compartment that PADI chose. You can see that on the charts for maximum time of off gassing. On PADI you are off the chart at 6 hours, on NAUI it takes 12 hours--how else would you explain that?

NAUI used to print an encyclopedia for its Master Scuba Diver course (I don't know if it is still in print), NOAA also prints one, as well as PADI, that all go into some detail about this subject.

One of the reasons I like diving with a computer. Where a table can only track one rate of absorption, a computer can easily track multiple rates and choose which rate is likely to reach saturation first. I know the tables well enough to see a major anomaly, but the computer helps a lot.

 

[boulderjohn]

On PADI you are off the chart at 6 hours, on NAUI it takes 12 hours--how else would you explain that?

Before the PADI table was created, the U.S. Navy created the 120 minute compartment and made that the controlling compartment for dives and surface intervals. PADI reasoned that recreational divers were doing very different dives than the U.S. Navy, and it conducted its own research. This research was published in peer reviewed journals. Michael Powell, AKA Dr. Decompression on ScubaBoard, was part of that research. They determined that for the vast majority of recreational dives, the 40 minute compartment, not the 120 minute compartment, should be the controlling compartment. In order to add a level of conservatism to the tables it created, it used the 60 minute compartment as the controlling compartment.

A theoretical compartment is considered to be washed out in 6 multiples of its half time. The Navy tables (and apparently the NOAA tables) wash out at 6 X 120 minutes = 12 hours. The PADI tables wash out at 6 X 60 minutes = 6 hours.

This is the cause for the differences in surface intervals.

The exception to this is when doing multiple dives to the limits of the NDLs. That is why you have the WX and YZ rules on the PADI tables. These extend the surface intervals for those kinds of dives.

The PADI research and the subsequent history of the use of the tables shows that they are plenty safe and conservative. The NOAA tables are even more conservative.

 

[DuboisP]

Where a table can only track one rate of absorption, a computer can easily track multiple rates and choose which rate is likely to reach saturation first.

depending of the duration of the dive, the table has been computed with different compartments

i can post an excel sheet where you can change the duration and depth to see which compartment will be the "director" one.
it's based on the tables used in France to teach decompression dives. some divers use them really, or to calculate a plan "on the fly" to have some direction.
we call them MN90 (Marine Nationale 90 "French Navy 90"), derived from US Navy Tables.
I can translate some words, but you will have to use metrics system

 

[AC26XP]

A theoretical compartment is considered to be washed out in 6 multiples of its half time. The Navy tables (and apparently the NOAA tables) wash out at 6 X 120 minutes = 12 hours. The PADI tables wash out at 6 X 60 minutes = 6 hours.

This is the cause for the differences in surface intervals.

Thanks for the math !!!

Happy diving,
AC

 

[redacted]

First off, the NOAA and PADI PG values are not the same so "D" in one is not necessarily the same as "D" in the other. NOAA only uses 14 pressure groups while PADI uses 26 pressure groups. So comparing PG letters directly should not be expected to yield the same results. A better compaision would be to compare RNTs and ABTs for the same subsequent dive. In this particular example PADI seems to be more liberal than NOAA. Some of that is simply the higher resolution of the PADI tables and the fact that, in both cases, you round up (safe side) to the next available point thus you should expect more round up error (safe sided) in the NOAA results. If you pick another example like 60 ft for 60 minutes you will see a case where PADI is more conservative, probably due to somewhat different models. Or consider 130 ft for 8 minutes, a 1:30 surface interval followed by another dive to 130ft. In this case (if I got it right) NOAA gives you 8 minutes ABT while PADI gives you only 5 minutes.

You have to remember that tables are not designed to tell you exactly where the edge is, rather to keep you a safe distance from that edge where ever it is.

 

[AC26XP]

First off, the NOAA and PADI PG values are not the same so "D" in one is not necessarily the same as "D" in the other. NOAA only uses 14 pressure groups while PADI uses 26 pressure groups. So comparing PG letters directly should not be expected to yield the same results.

No doubt that D on the PADI table is not D on the NOAA table, however I am looking for specific mathematical corollaries, ratios, etc., much like BoulderJohn expressed above.
My intent of this thread was really to ask more of a math-related story problem than to question the practical application of the tables.

Happy diving,
AC

 

[Max Speed]

I believe the cause is lack of research because political correctness has caused an unwillingness to explode goats for testing purposes. I mean would you really want to use a shampoo that hadn't been tested on animals? Your head might melt!