Dive tables or dive charts are tables used to determine the amount of nitrogen that is absorbed by your body. The charts provide a guide for the amount of nitrogen which can be safely absorbed with no adverse health effects. The nitrogen content of your body is designated by a Letter Group; from A to L with A being the lowest and L being the highest.
A familiarity with dive tables is required for certification even though there are dive computers which automate the entire process of nitrogen management. The justification is that tables require only your brain. Computers won’t always work. Proper planning and a sensible conservative approach to nitrogen management is always safer than relying on a machine.
Dive tables give you a recommended depths and times for a single dive. The deeper you go the less time you may spend submerged. If you dive shallowly you may spend more time underwater before you reach your nitrogen limit.
Dive tables were first developed in 1915 and the numbers were based on awful inhumane decompression experiments on unsuspecting volunteers. Later, the U.S. Navy developed better tables in the 1930’s using far more humane tests with military volunteers. It is worth pointing out that the U.S. Navy tables are based on experiments on fit young men in the peak of athletic health. Without being derogatory it is simply true that most recreational scuba divers are not buff young athletes with zero body fat. The 1990 NAUI tables are more conservative and are thus appropriate for the “rest of us”.
When you dive underwater the increased air pressure forces more nitrogen to be absorbed into your body. This is called ingassing – forcing gas in to your system. When you return to the surface you will have more nitrogen in your body than before you dove.
As you spend time on the surface your body will expel gases, mostly through normal exhalation. This is called offgassing and is analogous to a tire or balloon with a slow leak; as you spend more time on the surface your nitrogen level will become closer to normal. The period between repetitive dives is called the Surface Interval Time (SIT).
If you decide to dive again before your nitrogen level has returned to normal it is called repetitive diving. If your body already has residual nitrogen then your second dive of the day may not be as long or deep as the first. The dive tables can be used to find your letter group after the first dive, calculate your letter group after the SIT (Surface Interval Time) and start your second dive knowing the depth and/or time limit for the second dive.
A diver goes to a depth of 60 feet in the morning. According to the chart the maximum dive time for 60 feet is 55 minutes, but you stay submerged for only 50 minutes. You would emerge from the water in Letter Group H. If you stayed on the surface for 8 hours, your Letter Group would drop right back to an A. Instead, you go to shore, have some lunch, browse the dive shop and head back out to the water a few hours later. After 3 hours, you have offgassed a lot of nitrogen, but you are still in Letter Group D. As a “D”, your nitrogen level is the same as if your first dive had been 20 minutes; this is known as your Residual Nitrogen Time (RNT). On the second dive, you want to visit the same reef – at 60 feet – but this time because you have 20 minutes-worth of residual nitrogen you can only stay for 25 minutes.
Adjusted Maximum Dive Time = Maximum Dive Time – Residual Nitrogen Time
AMDT = MDT – RNT
NAUI dive tables are copyright protected and may not be reproduced on this website. A diver should carry the tables with them in their gear bag and for this a laminated copy is ideal. Every dive shop will carry a selection of laminated charts and since you may be consulting them before and after every dive it is a good idea to get a good one. Laminated paper copies are good and very inexpensive. A laminated paper print is especially good if it has a sealed key ring hole punched so it can be tied to your bag (so it won’t blow away in windy weather). Some charts are printed on hard plastic – do not buy these. They may seem very sturdy in the store, but after a few weeks of abuse they become brittle and crack. The best and most permanent charts are stamped onto pliable rubber.