How to Use Dive Tables
Dive Tables look really complicated at first glance, but they're actually very simple - and the tables themselves give you the warnings, explanations, and definitions you need. To keep this as simple as possible, I'm going to give a quick explanation of each table (1-3), and then walk you through one example.
Table 1: End-of-Dive Letter Group
Obviously, you start out with Table 1. The depths are listed down the left side (meters and feet) and possible dive times are listed to the left of each depth. As you can see, the deeper you go, the less time you get to spend down there - safely, anyway, and that is the goal of SCUBA diving.
This table gives you the letter group designation you will be at by the finish of the dive (listed along the bottom), as well as providing the Maximum Dive Time (MDT) for depths between 40 and 130 feet. MDTs are the circled for each depth - these are the times that you should not exceed. Then why are there numbers afterward? People do exceed the MDT. The split squares provide (on the top) the dive time, and (on the bottom) the amount of time you must spend on your decompression stop. (Decompression stops are all done at 15 feet.) If you were, to say, dive to 80 feet and stay for 40 minutes (MDT for 80 feet is 35 minutes), then you need to spend 5 minutes at 15 feet to decompress.
Decompression stops allow extra time to remove the nitrogen from your system. The more nitrogen you can get out of your system, the better off you are - and the lower the risk of decompression sickness (again, wait for Lesson #3). MDTs are designed to keep you from needing those extra minutes at 15 feet (which would be why they're in bold, red, and circled! *-*)
Table 2: Surface Interval Time (SIT) Table
You enter Table 2 from your letter designation. Your SIT time is the time you spend out of the water (on the boat or on land). As you can see, the minimum amount of time you're allowed is 10 minutes, and the maximum is 24 hours. After 24 hours, you are considered "free" or residual nitrogen (the nitrogen that builds up in your system while you dive), and you can start over fresh. So, even if you made 7 dives in one day, if you wait 24 hours, it's like they never happened. Of course, what's the fun of waiting 24 hours between each dive? NAUI recommends a SIT minimum of 1 hour. This is sufficient to move you from one letter group to the one above it (from H to G), for the most part (the higher letters - A through D - are the exception, as you can see).
So you have your letter, you know you're going to sit on the boat and drink water for 1 hour, and then you're going to get back in the water. You use this table to find your new letter and move on to Table 3.
NOTE: If you are not going back in the water, you have no need to go on to Table 3. Table 3 is only used when you have intentions of going back for another dive.
Table 3: Repetitive Dive Timetable
Here's where things get a little confusing/complicated. You rested, and you have your new letter group. You know the depth you want to dive to next, but how much time can spend down there and keep yourself safe? This is the table that will tell you. The blue numbers represent your Residual Nitrogen Time (RNT) which you need when calculating a dive table (you'll see below). The red numbers represent the new MDT for each depth (listed across the top) - your Adjusted Maximum Dive Time (AMDT). Even looking across the letter group A, compare the AMDT with the original MDT - it's always less. Due to the build-up of nitrogen, your dive time will always shrink. Once again, you should never go over your AMDT if you want to be safe.
From here, you go back around to Table 1, using the AMDT rather than the circled MDT - and you start all over. Easy enough, right? *-* Dive plans (created with the table) are best done before you ever step foot on the boat. You plan your dive and dive your plan. The back of the Dive Table has a worksheet with space for three dives (which is typical). You can write right on the worksheet with pencil, clean it off, and be ready for next time.
So, ready for an example to (hopefully) clear things up? *-*
I'm going to do a three-dive plan, and I will fill in the spaces as we go along. (You can refer to the above tables to follow along) If you'd like, go ahead and draw out the lines below and follow along yourself:
Okay, some quick definitions. You already know MDT, RNT, and AMDT. ADT is your Actual Dive Time - the amount of time you plan to spend on the dive. TNT is your Total Nitrogen Time - the sum of your RNT (from Table 3) and your ADT. The TNT may NEVER exceed the MDT/AMDT.
This is a fresh day, been 24 hours since you last went in the water - you're clear. Therefore, your RNT is 0 (as you can see). It's nice and early in the day, you're fresh, and you've got a lot of reef you want to see. The Dive Master has decided you'll be visiting reefs at 80 feet, 60 feet, and 90 feet. Since you always dive your deepest first, you're going to go to 90 feet first, and you've decided to dive for 20 minutes. Go to Table 1 and fill in that information. The worksheet should now look like this:
What letter group are you in? Use Table 1 to figure that out, then go down to Table 2. 20 minutes at 90 feet is letter group F. You're going to spend 2.5 hours on the boat before your next dive. 2.5 hours of SIT time puts you at what new letter group? Worksheet should now look like this:
On to Table 3! *-* Okay, you're now a C diver, and you need to dive to 80 feet next as it's the deeper of the two remaining dives. Follow the C row over to the 80 column. Your RNT is 13 (can't be 0 anymore because you made a dive) and your AMDT is now 22. Let's say you want to dive 20 minutes again. 13+20=33...that's 11 minutes over your AMDT. You can't go over your AMDT. So 20 minutes is out. How about 7 minutes, though? That puts you under your AMDT. So now your worksheet should look like this:
Back to Table 1. Well, you're only diving for 7 minutes, but now you're also carrying residual nitrogen. So instead of looking for 7 minutes (and rounding UP to 10), you're going to go to 20 minutes (your TNT). You always go to your TNT to determine your dive time to get your end-of-dive letter group. You will still only dive for 7 minutes, though! Remember, you plan your dive and DIVE YOUR PLAN. 7 minutes means 7 minutes - the 20 just keeps you from decompression. Your diving time is important, but so is keeping track of the amount of nitrogen you're amassing - which is what the Dive Table is designed to do. Simple explanation: always use TNT as your dive time when figuring out the letter group, but ONLY DIVE THE ADT.
Well, 7 minutes wasn't very fun, was it? What can you really see in 7 minutes? Only one dive left for the day, and you want to make it worth it. Besides, it's getting to be lunch time, it's getting hot outside...you want a nice long break. So, this time your SIT time is going to be 4 hours - find your new letter group. Your worksheet should now look like this:
Think you've got it now? Follow B across to 60 feet. What's your RNT? Your AMDT? Fill those in on your worksheet. No 7 minute dive this time - you're going to make a nice 30-minute dive. "Drown the RAT" (fill in the RNT, ADT, and TNT) and find your final letter group. Your worksheet (now completed) should look like this:
Hopefully, this wasn't too complicated. *-* (Though, to give you an example of how complicated it can be, I had to redo my numbers several times to make it work with no decompression! *-*) This is what you would do the night before you made these dives. You'd write it all down on your Table, and take it with you the next day. Follow this, and you never go into decompression times. Avoiding decompression is a really good thing.
You can also see, though, how important it is to work this out ahead of time. I picked times at random - and ended up with a 7 minute dive. 7 minutes isn't worth it. If I were actually going to plan this, I would go backwards around the table to make sure I gave myself more time (by increasing my SIT time).
If this is completely incomprehensible, I made zero sense, by all means, ask me your questions. If not, feel free to make up a few exercises on your own - or let me know, and I'll pull some out of my dive books for you to try. I do know this may not help with writing at all, but, depending how in-depth you want your potential diver character to be, it can be. Maybe he dies and the detective needs to examine his dive plan for clues. Otherwise...well, just gives you a little more insight. *-* And it gives a good example why you should take an offical course before ever attempting SCUBA diving yourself!