Here is our dive profile.
An advanced dive team is planning two wreck dives. Dive #1 is to 70 feet for 28 minutes. After the dive, the team plans to start dive #2 90 minutes later. The second dive is to a depth of 55 feet for 25 minutes. The team may plan to do a third dive later, so we want to determine what repetitive group they will be in after dive #2.
To answer this question we must:
- Determine the repetitive group designation after dive #1
- Determine the new group designation after the surface interval
- Determine the residual nitrogen remaining after our surface interval
- Determine the repetitive group designation after dive #2.
Fill out a dive profile with all the information you are given. In this case, the first dive is to 70 feet for 28 minutes. The surface interval is planned for 90 minutes. Dive #2 will be to 55 feet for 25 minutes.
Determine the repetitive group designation after dive #1.
Starting on the top table, Table 1, we find 70 feet in the Depth column at the far left. Go across this row to the right to find the no-decompression limit for 70 feet, which is 40 minutes - so a 30 minute dive is safe.
We find our Repetitive Group Designation letter by moving to the right until we find the column that has our bottom time - in this case 30 minutes, rounded up from 28 minutes (always round "in between" depths and times up to the next higher number).
From this position of 30 minutes, we follow this column down to find our dive team is now in repetitive group designation letter "F".
Determine the repetitive group designation after our surface interval.
Taking the Group Designation letter "F" to the lower table, Table 2, we find letter "F" on the diagonal line. We then work our way across the row to the left until we find the box that contains the time that the dive team spent on the surface.
The first box goes from 10 minutes to 45 minutes, the second box goes from 46 minutes to 1 hour 29 minutes but the third box goes from 1 hour 30 minutes to 2 hours 28 minutes.
Since our dive team was on the surface for 90 minutes (1 hour 30 minutes) we stop at the third box and then go straight down the column to find our New Group Designation letter. In this case, our new group letter is "D".
Determine the residual nitrogen remaining after our surface interval.
We now know that our dive team is in group "D" before the start of dive #2. Since the dive team is planning a third dive, we need to use ABT+RNT=TBT to find the repetitive group after dive #2.
ABT is the Actual Bottom Time from the previous dive, which is 25 minutes and is plugged into our profile. RNT is the Residual Nitrogen Time. It is found on the other side of the tables on Table 3.
Find group "D" in the New Group Designation row across the top of Table 3. Next, find the depth of the second dive in the Repetitive Dive Depth column along the far left side of Table 3. In this case, it is 60 feet rounded up from 55 feet. Then, find the box where the New Group Designation column (column "D") and the Repetitive Dive Depth row (60 feet ) intersect and you will see two numbers. The top number represents the Residual Nitrogen Time, which is 24 minutes and is also plugged into our profile. The bottom number shaded in green represents the maximum Adjusted No-Decompression Time Limit for the next dive, which is 26 minutes. Therefore, our planned bottom time of 25 minutes makes it a safe dive.
We next do the math to find our Total Bottom Time for the second dive, which is the Actual Bottom Time plus our Residual Nitrogen Time. Our TBT becomes 49 minutes.
Determine the repetitive group designation after dive #2.
At this point we refer back to Table 1 and determine what our new repetitive group letter will be after a dive to 55 feet for 49 minutes. (If you forgot how to do this refer back to Step #2).
Follow across the 60 feet row to the 50 minutes column and then down to find the Repetitive Group Designation letter. If done correctly you should end up in group designation "H". Got it?!