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The No Decompression Limits and Group Designation table (PADI Table 1) indicates the maximum no-decompression dive time associated with a particular depth and the Pressure Group — a measure of residual nitrogen — based upon a dive profile.
Table 1 is arranged as a set of columns, each corresponding to the depths listed across the top. The last number in each column, denoted in red, is the Maximum Bottom Time (MBT) associated with that depth. (As an example, the MBT for a dive to 70 ft. [21 m] is 40 minutes.) The goal of safe dive planning is to avoid going over your MBT which would require decompression stops.
The letters on the left indicate the Pressure Group based upon the duration of the dive. To determine the Pressure Group:
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Find the column 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 column down to find the time corresponding to the Total Bottom Time (TBT) of your dive; the TBT 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 TBT after your dive to 52 ft. [16 m] is 26 min., find the row for 27 min.).
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Follow the row across to the left to find the Pressure Group (this 52 ft. [16 m] dive for 26 min. puts the diver in Pressure Group J).
The student should be able to verify:
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The MBT for a 70 ft. (21 m) dive is 40 minutes.
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A 58 ft. (17 m) dive for 42 min. puts a diver in Pressure Group Q.
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 Bottom Time (ABT).