Chapter 5- Decompression, Dive Tables, and Dive Computers Decompression, Dive Tables, and Dive Computers 127 DIVE TABLE OVERVIEW The NAUI Dive Tables use a Letter Group designation to express the amount of residual nitrogen in your body. The letters range in sequence from A to L. The letter A represents a small amount of nitrogen and the amount of nitrogen increases as the letters progress towards L. When you dive, a Letter Group from the tables designates the amount of nitrogen you have absorbed during the dive. As you spend time on the surface between dives, you are assigned to “lower” Letter Groups as you offgas nitrogen. When you start a dive again to a given depth, your Letter Group at that time determines the time representing the residual nitrogen in your body. The tables show you how to subtract this time from the normal dive time limits, which results in a shorter time limit for your repetitive dive. You must add your residual nitrogen time to the time you actually spent diving to determine your total nitrogen time at the end of the dive. You use the total time to determine a new end-of-dive Letter Group. The NAUI dive time calculator is based on the NAUI dive tables, but it eliminates the calculations required to determine Letter Groups when you make more than one dive. The dive time calculator also reduces the errors that are often made when reading dive tables. It is easy to learn how to use a dive time calculator, but you should do this only after you are familiar with the procedures for planning dive time limits using the dive tables. A dive time calculator might not always be available, but dive tables usually are (figure 5-4). A variety of dive tables, dive time calculators, and dive computers exist. The information they provide varies and some are more conservative than others. Numbers and Letter Groups are not always interchangeable between tables. Always use the type of table, calculator, or computer with which you are familiar. If your dive buddy is using a different type, you should agree to use the most conservative dive planning information. Dive Table Terms Different dive tables and dive computers might use terms different from the ones used in the NAUI Dive Tables (figure 5-5). Also, the same term might have a different meaning. The following terms apply to the NAUI Dive Tables: • Dive Schedule. This is an abbreviated statement giving the depth and time of the dive. It is expressed as depth/time (for example, 21 meters/40 minutes or 70 feet/40 minutes). It is also called a dive profile, especially when it is one dive in a sequence of dives. • Maximum Dive Time (MDT). This is the time you can spend at a given depth without having to do a required decompression stop during ascent. This time is also known as the Maximum Allowable Dive Time or no-decompression limit. • Decompression Stop. This is a point in a dive where you stop at a specified depth for a specified No Decompression Limit System Nitrogen Residual Nitrogen Normal Surface ADT SIT Letter 30 Min. 30 Min. Decompression Stop (3 min.) Repetitive Dive E Group E I 50' 50' 30 Min. FIGURE 5-5. THE DIVE TABLE TERMS FOR THE NAUI DIVE TABLES MIGHT FIGURE 5-4. D IVE TABLES ARE USED TO PLAN YOUR DIVE. HAVE DIFFERENT MEANINGS WITH OTHER DIVE TABLES.
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