Advanced 9 minutes
Planning with tables: the arithmetic your computer hides from you
Pressure group, residual nitrogen time, and how to enter an air table when you're breathing nitrox
The article on repetitive diving already explained what residual nitrogen is and why the second dive of the day has less available time. What it didn't explain is how that number was worked out before there was a computer to do it for you, and that calculation is still the backup you need on the day yours fails between dives.
In this article you'll see how the three charts of a recreational decompression table chain together, a complete worked example of a three-dive day calculated by hand, the rules the table imposes and the computer quietly smooths over, and how you enter that same table when the mix is nitrox instead of air.
The article on decompression already noted that tables are still taught because they force you to understand the logic your computer runs silently. This is the article that delivers on that promise: the full mechanism, not just a mention that it exists.
The three charts and how they chain together
A recreational decompression table isn't a single chart: there are three, and each one feeds the next. Understanding the chain matters more than memorizing any particular number, because the method is the same in whichever table you use, whether it's the one from your training or the one your dive center keeps on the wall.
The first chart crosses depth with bottom time and returns a letter. That letter (from A to Z, generally) is shorthand for how much nitrogen you have dissolved in the reference tissue when you leave the water: the deeper and longer the bottom, the further along the letter. It isn't a grade for the dive, it's a loading state.
The second chart crosses that exit letter with the time you spend on the surface and returns a new letter, always earlier in the alphabet. It's the direct translation of the surface interval you already saw in the article on repetitive diving: the more time on the surface, the more nitrogen eliminated, the further back the letter.
The third chart crosses that reduced letter with the depth of your next dive and returns a number of minutes: the residual nitrogen time. It represents how much bottom time, at that new depth, it would have taken you to build up the load you're already carrying before you put your head back under the water.
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