DPJ:
About the tables, I noticed that SSI tables give you less credit for surface intervals. I know this has to do with the two different "decompresson models" used by the agencies.
SIMPLE ANSWER: The decompression models are actually quite similar. The difference is that the USN derived tables track the only 120 minute compartment while the PADI RDP & wheel track the only 60 minute compartment. So PADI will reduce the assumed N2 load by half in a 1 hour SI, while it takes 2 hours to do the same in the USN/NAUI/SSI/YMCA tables.
The USN tables were made for both scuba and surface supplied use, and needed to track the longer compartments. In open circuit scuba the 60 minute and faster compartments are usually the limiting ones, and PADI avoids exceeding the limit in the 120 minute compartment and slower compartments through the use of the ad hoc X,Y, and Z rules in the the fine print of the tables.
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More Detailed Answer:
Both PADI RDP and the USN tables and their derivatives (SSI, NAUI, YMCA, etc.) are simple dissolved gas models. The limits (aka M-values) of the USN/Workman and the PADI/DSAT/Spencer models are similar, with the USN limits being more liberal. That is why, as Rich Lockyer noted, that the NAUI tables give more time than PADI on
FIRST dives of the day.
The PADI RDP tracks only the 60 minute compartment. Pressure Group Z means that the 60 minute compartment is at its M limit (actually, per my reverse engineering of the tables, it appears that PG Z is really only 95% of M0, but that's another story).
For dives deeper than 40', a faster compartment will hit the limits before the 60 minute compartment. That's why, for example, an 80' to the 30 minute limit leaves you in pressure group R. The 20 minute halftime compartment has hit the limit, and both the 10 and 30 minute compartments are near their limits after 80' 30 minutes, but the 60 minute compartment is only at 70% of the limit. (The time constant of the limiting compartment for any given square profile depth can estimated as NDLtime/1.25)
The SI tracks only the decay of the 60 minute compartment. You can easily verify that tracking only the slower compartment adds in a level of conservatism.
The USN table works in a similar way, but tracks the 120 minute compartment. This is a less accurate model since the limiting compartment is much faster than 120 minutes.
By tracking a compartment much, much slower than the actual controlling compartment, you end up adding a very significant, but rather strangely distributed safety factor when using the USN table.
I prefer to use a table that better models that type of diving I am doing.
