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The End-of-Dive Letter Group table (NAUI Table 1) indicates the maximum no-decompression dive time associated with a particular depth and the Letter Group — a measure of residual nitrogen — based upon a dive profile.
Table 1 is arranged as a set of rows, each corresponding to the depths listed on the left. The circled number is each row is the Maximum Dive Time (MDT) associated with that depth. The small red numbers indicate the dive times for which a decompression stop is required at 15 ft. (5 m); the small black numbers indicate the length of the required decompression stop. (As an example, a 50 minute dive at 70 ft. [21 m] would require a 5-minute decompression stop; 60 minutes at the same depth would require an 8-minute decompression stop.) The goal of safe dive planning is to avoid the necessity of required decompression stops.
The other large numbers are used by the diver to determine their Letter Group based upon the duration of the dive. To determine the Letter Group:
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Find the row corresponding to the greatest depth of the dive; use the next greater depth if your actual depth is not listed on the table (e.g., if your actual depth is 52 ft. [16 m], use the row corresponding to 60 ft. [18 m]).
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Carefully follow the row across to find the time corresponding to the Total Nitrogen Time (TNT) of your dive; the TNT is the actual duration of this dive plus any Residual Nitrogen Time (RNT) from previous dives. Round up to the next greater time if your actual time is not listed on the table (e.g., if the TNT after your dive to 52 ft. [16 m] is 27 min., find the column for 30 min.).
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Follow the column down to find the Letter Group (this 52 ft. [16 m] dive for 27 min. puts the diver in Letter Group F).
The student should be able to verify:
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The MDT for a 70 ft. (21 m) dive is 45 minutes.
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A 58 ft. (17 m) dive for 42 min. puts a diver in Letter Group H.
NOTE: Although the tables guide a diver to plan no-decompression dives, divers are advised to always make a 3-minute safety stop at 15 ft. (5 m) and to never ascend at a rate faster than 30 ft. (9 m) per minute. The time spent on the safety stop does not count towards the Actual Dive Time (ADT).