What is the NDL pressure group designation at the end of the first dive? Based on the information from the dive profile after the first dive, you know the depth was 15 meters/49 feet. Look at Table 1 again. There is no entry for 15 meters/49 feet, so you need to go to the next greatest depth listed on the table. Look at the entry for 16 meters/53 feet. Go across the row until you find 40 minutes (under the heading “Dive Time With End of Dive Group Letter”). Then, go down the column to locate the row “Group Designations.” The NDL pressure group designation for the first dive is F.
The group designation is a simple method of accounting for a certain level of nitrogen loading in the body. The longer you stay out of the water after a dive the less nitrogen you will have in your body. After 12 hours, nitrogen levels will be the same as before your first dive.