How to use the dive tables
Now what do Dive Tables do? They tell you how long you can stay, maximum, at certain depths and then come straight back up, without any decompression stops.
Examples using NAUI Dive Tables
First dive
You'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).
You do that dive, enjoy the scenery, and then come back up. Can you now just use that same dive table, strap on a fresh tank of air, and go right back down for a second look at that reef? No way.
Residual nitrogen after a dive
See, 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.
So what the Dive Tables do is determine how much extra nitrogen is still in your body after a dive, and then convert that into "Residual Nitrogen Time," or "RNT." That sounds intimidating, and they really should come up with a simpler term and explanation. As is, "Residual Nitrogen Time" tells you how much time at a certain depth it would take to absorb the amount of nitrogen you already have in your body from the previous dive, and you can find that in Table 3, the Repetitive Dive Timetable. What does that mean? Well, if the table says your residual nitrogen time is 20 minutes for a given depth, then you can stay at that depth 20 minutes less than on your first dive because, after all, you already absorbed that much nitrogen and it still is in your system.
How to plan for the second dive
So 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.
So where do you stand after your second dive? Well, You still had enough residual nitrogen in your system as you'd get from a 56 minute dive, and you now added another 20 minutes of actual bottom time on your second dive. So your total bottom time is now 76 minutes. Now go back to Table 1 and see what Letter Group 76 minutes at 50 feet puts you in. Yikes. At the end of your second dive to 50 feet, you're now in Letter Group "J".
And a third
But let's say you're still not done after your second dive and you plan a third, again to 50 feet. As stated above, although you only stayed for 20 minutes at 50 feet on the second dive, you need to add the 56 minutes of residual nitrogen you still had in your system, i.e. 56 minutes. So the total nitrogen is as if you'd stayed down there for 76 minutes, making 76 minutes your Total Nitrogen Time and you are in End-of-Dive Letter Group J.
If you now plan on waiting an hour and 15 minutes, and then go see that reef at 50 feet again, you find yourself in the new Letter Group H. Now move left to Table 3. Find where row H intersects with the 50 foot depth column and you find that your residual nitrogen time is now 66 minutes and your new adjusted no-decompression limit is now 14 minutes. And so on.
How to back into surface interval time using the dive tables
In real life, reality often interferes with the best laid plans and time is an issue. Let's say you do that first 35 minute 60 foot dive, ending up in End-of-Dive Letter Group G, and then want to see another dive site that's 50 feet down and you'd like to explore for 40 minutes. How long would you have to wait on the surface? Well, start with Table 3, find where the 50 foot column intersects with an Adjusted Maximum Dive Time of at least 40 minutes, and you find it's row E. Move right to Surface Interval Table 2 and see where your current Letter Group, column G, intersects with the Letter Group row you'll be in after that second dive, E.
You find that your surface interval needs to be between 1:16 and 1:59 hours. Just to practice a bit more, assume the second dive goes down to 50 feet again but you'd like to stay for an hour. Now Table 3 shows you're in New Group B. Follow row B to the right to Table 2 where it intersects with column G. Oops. Now you have to wait 4:26 to 7:35 hours between dives. See what a big impact the extra bottom time has on surface interval time?
Backside of the NAUI Dive Table
The NAUI Dive Tables have some explanations and additional rules on the backside. That's important stuff and not to be ignored, ever.Repetitive Dive -- The term "repetitive dive" refers to any dive made less than 24 hours after a prior dive.
Actual Dive Time (ADT) -- By NAUI definition, that is the time from the start of descent to the time you are back up at the surface.
Letter Group -- As explained above, for repetitive dive planning purposes it represents the "Letter Group" of the amount of nitrogen that remains in your body after a dive.
Surface Interval Time (SIT) -- That is the time you spend on the surface, between two dives.
Residual Nitrogen Time (RNT) -- Represents, for repetitive dive planning purposes, the amount of nitrogen remaining in your body from a dive, or dives, made within the prior 24 hours.
Adjusted Maximum Dive Time (AMDT) -- That is how long you can stay at a certain depth in a repetitive dive. It is the depth you could stay there if it were your first dive minus the residual nitrogen time.
Total Nitrogen Time (TNT) -- Add up your actual dive time (ADT) and your residual nitrogen time (RNT). That number of minutes is used to find the letter Group after your next dive.
The NAUI Dive Tables also remind that:
- 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.
