First dive
Using the PADI Recreational Dive Planner tables, let's say you plan on visiting a reef that's 60 feet down. The No Decompression Limits and Group Designation Table, shows that the absolute maximum time 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 Pressure Group N; more on that later). You do that dive and come back up. Can you now just use that same table, get a fresh tank of air, and go right back down for a second look? 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 Credit Table of the PADI dive planner is all about. It essentially tells you how much nitrogen leaves your body over time.That is where the "Pressure Groups" come in. After your first dive you are in a certain Pressure Group, as shown in Table 1. That 35 minute dive to 60 feet put you in Pressure Group N. Table 2 shows what Pressure 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 Planner does 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 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 PADI table, we find that the first dive put us into pressure group N. We follow "N" into table 2 and then find the column for a 30 minute surface interval. That would be column I.So now we flip the table (told you it was a bit cumbersome) and look at column I. Then we look at the cell where column I intersects with the 50 feet row. There will be two numbers: 31 on top (white background) and 49 on the bottom (blue background). The top number is your residual nitrogen time. So on your second dive to 50 feet, you still have as much nitrogen in your system as you'd absorb in 31 minutes down there. The second number, 49, is your adjusted no-decompression limit, the time you cannot exceed on your next dive. In other words, your actual bottom time can be no more than that. Let's say you decide to stay down for only 30 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 31 minute dive, and you now added another 30 minutes of actual bottom time on your second dive. So your total bottom time is now 61 minutes. Flip the chart around and look at Table 1. Yikes. At the end of your second dive to 50 feet, you're now in pressure group T.
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 30 minutes at 50 feet on the second dive, you need to add the 31 minutes of residual nitrogen you still had in your system, i.e. 31 minutes. So the total nitrogen is as if you'd stayed down there for 61 minutes, making 61 minutes your total bottom time and you are in Group T. 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 Pressure Group E. Flip the chart once again so you can see Table 3. Find where column E intersects with the 50 foot depth row and you find that your residual nitrogen time is now 21 minutes and your new adjusted no-decompression limit is now 59 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 Pressure Group N, 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 row intersects with an adjusted no-decompression time of 40 minutes, and you find it's column L. Flip the card and see where your current Pressure Group, row N,intersects with the Pressure Group you'll be in after that second dive, L. You find that your surface interval needs to be just 9-13 minutes. 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 Pressure Group D. Flip the chart and intersect column D with row N. Oops. Now you have to wait 1:00 to 1:08 hours between dives. See what a big impact the extra bottom time has on surface interval time?PADI Dive Table "Fine Print"
The PADI recreational dive planner tables have some "fine print" (literally) on the backside. That's important stuff and not to be ignored, ever. Safety stops -- the optional 3-minute safety stop at 15 feet becomes mandatory if you get within three pressure groups of a no-decompression limit, or for any dive deeper than a hundred feet.Emergency Decompression -- If for some reason you exceed a no decompression limit by up to five minutes, you must make an 8-minute decompression stop at 15 feet, and then not dive for six hours. If the no decompression limit is exceeded by more than five minutes, you must make a 15-minute stop at 15 feet and then not dive for at least 24 hours.
Air travel after dives -- After a single dive, wait 12 hours. After multiple dives, or several days of diving, wait 18 hours. If decompression stops were necessary, wait more than 18 hours.
Altitude diving -- Special procedures are required if you dive in altitudes in excess of 1,000 feet. (You add two PADI pressure groups per 1000 feet, so if you drove from zero up to mountain pass at 8000 feet, it'd be 16 pressure groups once you reach the summit, and you'd be the equivalent of a PADI "P" diver.)
Multiple dive rules -- Anytime the ending pressure group on the PADI table is W or X, all following surface intervals must be at least an hour. Anytime an ending pressure group is Y or Z, all following surface intervals must be three hours. Cold water dives -- if diving in cold water, add ten feet to the actual depth.
