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October 03, 2007

Less dive time for NAUI divers?

Two friends, a NAUI diver and a PADI diver, decide to go on a dive trip. Though they have dive computers, they decide to play it by the book and use dive tables. They also decide to take it easy and do just two dives each day, though some are fairly deep. They know the dive sites, decide on surface intervals between each day's dives and begin working out their dive plans. When they are done, they compare their plans and find something very peculiar. For their second dives of each day, the PADI diver shows a total permissible dive time of 443 minutes whereas the NAUI diver arrived at only 301 minutes. They recheck their calculations. They are correct, yet for each dive except one, the PADI diver finds longer allowable dive times for the second dive, sometimes by a lot. For example, after a 60 foot dive and one hour surface interval, the PADI diver can do a 61 minute dive to 50 feet. The NAUI diver can stay only 42 minutes. After a 40 minute dive to 70 feet, the NAUI diver can hang around at 40 feet for 69 minutes, whereas the PADI diver can stay for 115 minutes (if he has that much air). After a deep dive to 133 feet, the PADI diver can, after three hours on the surface, make another deep one to 90 feet for 21 minutes whereas the NAUI diver has only 18 minutes.

What gives? Aren't all those dive tables based on the same principles? Should they not yield approximately the same results? True, the PADI and NAUI tables are different, with PADI breaking things down into 26 "Pressure Groups" while NAUI has less granularity with just 12 "End-of-Dive Letter Groups." So you'd expect the tables to occasionally produce slightly different results, but not by much. Sometimes PADI would show more bottom time and sometimes NAUI. But that does not appear to be the case. The NAUI tables seem to consistently yield more residual nitrogen time and less maximum dive time for repetitive dives.

Does that mean NAUI is more conservative? I don't know the answer just yet. Based on my own experience, I've come to view PADI as more tourist and recreation oriented, and NAUI as more technical and detailed. If that were indeed so, then one would expect PADI to be more conservative so that its broader and perhaps less experienced diver base stays within safe limits at all times. Instead, the respective dive tables almost always allow less repetitive dive bottom time to the assumedly more experienced average NAUI diver.

Could definitions have something to do with it? Just like the PADI and NAUI dive tables are different, so are the two competing certification entities' terminologies and definitions. As a result, as if dive tables weren't confusing enough, those trained by different agencies must also figure out if "Actual Bottom Time" is the same as "Adjusted Maximum Dive TIme," and "Total Bottom Time" the same as "Total Nitrogen Time." That's just not good.

Looking at my notes and instruction materials, I find that NAUI defines "actual dive time" as "the time from the moment of descent until returning to the surface." Breaking the surface or starting to return to the surface? For PADI, on the other hand, "bottom time" is "the total time in minutes from the beginning of descent until the beginning of final ascent to the surface." So the NAUI "total nitrogen time" which adds "actual dive time" and "residual nitrogen time" would yield a larger number than PADI's "total bottom time" that adds "actual bottom time" (which does not include the time it takes to ascend) and "residual nitrogen time." Confusing for sure.

And there's another difference. PADI states that "if you don't plan to dive for at least six hours, the residual nitrogen has little consequence. True enough, if you look at the PADI Recreational Dive Planner, you find that after a six hour surface interval you are no longer in any pressure group at all and a second dive, even if the same day, would apparently not be treated as a repetitive dive. NAUI, on the other hand, categorically states that "any dive made less than 24 hours after a previous dive" is a repetitive dive. Which means that no matter how long the surface interval on any given day, for the second dive you'll always start out at least in End-of-Dive Letter Group A.

What does it all mean? Nothing for most divers because very few will ever agonize whether to use the PADI or the NAUI dive tables. And most divers simply rely on their dive computers anyway. But those large discrepancies between the tables are still amazing after being taught in class that even a couple of minutes of extra bottom time can be the difference between a safe dive and the prospect of getting bent.

Posted by conradb212 at October 3, 2007 11:20 PM