How about Dive tables. My cert. is NAUI and I dive with mostly PADI cert. divers. I noticed some differences in both tables while reviewing my skills today. Then I noticed that SSI uses the same table as NAUI however one can understand SSI tables better then the NAUI one.
Anyways whats the best to use? Whats the safest?
Others have answered the agency/instructor questions, my comments are on the tables. As long as you have learned to properly use the table or dive computer or other decompression tracking method, it really doesn't matter, outside of class, which table or method you use. Newer divers are often limited more by air than by NDL limits. If you are doing >80' dives or dives where much of the time is spent at depths much shallower than the deepest depth, then the tables may start to be the limiting factor. Repetitive dives will also often cause the tables to be the limiting factor, particularly when using the USN/NAUI/YMCA/SSI type tables.
The NAUI and SSI tables are similar since they are both derived from the US Navy tables. The PADI tables are based upon a slightly different decompression model (same basic concepts, but some limits made more conservative).
The biggest difference you will find in the tables is on repetitive dives. Tables are rather crude methods of tracking decompression, and there are some severe tradeoffs that need to be made to keep the tables simple. One very important simplification is that only 1 compartment is tracked for N2 loading calculations on repetitive dives. The USN tables upon which the NAUI/SSI/YMCA tables are based had to handle both decompression dives and the extraordinarily long dives in the 30 to 50' range that take place while working on the hulls of ships while breathing air supplied by hoses from topside. For this reason, the USN-derived tables base repetitive dive calculations upon the 2 hour compartment.
PADI commissioned tests specifically to test repetitive diving of recreational dive profiles. Using the results of those tests, and knowledge of the limitations of scuba tank sizes, PADI/DSAT created a table where the repetitive diving calculations were controlled by the 60 minute compartment. This allows the table to much better track the acual decompression status of a dive, and greatly increases the allowable bottom times on repetitive dives as compared to the USN-derived tables. For those rare cases where the 120 minute compartment would be the limit for repetitive dives, there are some special rules that require a minimum 1 hour or 3 hour SI. These rules are rarely needed since they only apply when doing 3 or more dives per day, and the dives to trigger the rules need to be unusually long to reach the limit.
A dive computer tracks all of the compartments and will generally allow even more time on repetitive dives, even when the underlying computer algorithm is more conservative than the table. Dive computers will also allow dramatically longer dive within NDL if the dive is multilevel. Tables treat all bottom time as if it is at the max depth (or the next multiple of 10' below the max depth). Computers monitors the actual dive depth and a dive that starts with a short deep section, and then moves shallower for later portions of the dive will result in greatly increased NDL times compared to a table that assumes a square profile dive.
When done properly, recreational diving has such a low incidence of DCS that it is nearly impossible to compare the safety statistics of different tables and computers. In real life, the easiest way to add significant safety margin is to add a couple of deep stops during the ascent, and spend a bit of extra time at the safety stop.
Charlie Allen
