How to use the dive tables
Examples using NAUI Dive Tables
First diveYou're on a dive boat and ready for diving. Using the NAUI Dive Tables shown to the right, let's say you plan on visiting a reef that's located at a depth of 60 feet. NAUI Table 1, the End-of-Dive Letter Group Table on the upper right of the plastic dive table, shows that the Maximum Dive Time (or MDT) you can stay at that depth without having to make a decompression stop is 55 minutes (if you have enough air, that is).
It's never a good idea to dive to the limits, so you decide to stay down 35 minutes (which places you in Letter Group G, but more on that later).
Residual nitrogen after a diveSee, the problem is that while science has determined that it is safe to ascend from 60 feet after 35 minutes -- "safe" meaning that nitrogen gets released at a sufficiently slow rate so as not to pose a danger -- it does NOT mean ALL the nitrogen that was absorbed into your body down there was released during ascent. Think of the soda bottle example again: even if you leave the bottle cap off, the soda doesn't go all flat immediately. Some of the fizz stays in, and only after a few hours or even a day or two does it go all "flat." Same with the nitrogen in the human body. After you're back up, there's still some nitrogen left in your tissues, and it takes time for that to be released. What that means is that if you dive again, you still have some extra nitrogen in your body, and therefore reach the maximum safe time limit of nitrogen absorption sooner. Which means if you go down to the same depth, you can't stay as long as the first time. And that is what Table 2, the Surface Interval Table, or SIT, of the NAUI Dive Tables is all about. It essentially tells you how much nitrogen leaves your body over the time you spend on the surface, sitting on the deck of the dive boat.
That is where the "Letter Groups" come in. After your first dive you are in a certain Letter Group, as shown in Table 1. That 35 minute dive to 60 feet put you in Letter Group G (you always round up). Table 2 shows what Letter Group you will be in after a certain "surface interval," i.e. the time between the end of your first dive and the start of your second dive. Obviously, the longer you wait, the more of the extra nitrogen your body absorbed during the first dive gets released.
How to plan for the second diveSo let's see how we use the NAUI tables to plan the second dive after our 35 minute stay at 60 feet. We plan on a surface interval of half an hour, and then go see another reef that's 50 feet down. Using the NAUI table, we find that the first dive put us into Letter Group G. We follow "G" down into Table 2 and then find the new Letter Group for a 30 minute surface interval. That would also be Letter Group "G".
So now we follow the Group G row left into Table 3, the Repetitive Dive Timetable. Then we look at the cell where row G intersects with the 50 feet column in Table 3. There will be two numbers: 56 on top (number in blue) and 24 on the bottom (bold number in red). The top number is your residual nitrogen time (RNT). So on your second dive to 50 feet, you still have as much nitrogen in your system as you'd absorb in 56 minutes down there. The second number, 24, is your adjusted maximum dive time (AMDT), the time you cannot exceed on the dive.
In other words, your actual bottom time can be no more than that. Let's say you decide to stay down for only 20 minutes.
And a third
How to back into surface interval time using the dive tables
Backside of the NAUI Dive TableThe NAUI Dive Tables have some explanations and additional rules on the backside. That's important stuff and not to be ignored, ever.
- Dives to less than 40 feet depth are treated as 40 foot dives
- Do not ascent faster than 30 feet per minute
- To maximize dive time, start with the deepest dive, and then make each repetitive dive shallower than the prior one.