Use these pages to re-familiarize
yourself with your PADI RDP.
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Understanding
the RDP
Example #1
| Example #2 | Example
#3 | Example #4 | Problems
for Home
This section will introduce (or re-familiarize) you with the PADI Recreation Dive Planner (RDP). The dive table is used to gauge how much excess nitrogen you have in your body, so you can determine your maximum safe time and depth limits.
The RDP has three parts, Tables 1, 2 and 3. Table 1 tells you the maximum amount of time you can stay at a certain depth on your first dive, and it tells you how much nitrogen you have in your body after a dive (letters A through Z, Z meaning you have plenty of it, A very little).
To use the dive table, on your first dive, determine your depth. Anything under 35 feet is considered 35, and anything above 35 should be rounded UP to the nearest 10th, ie: 46 feet would round UP to the 50 foot column. Go down the selected depth column until you find your bottom time (again, rounding up), ie: if you dived for 38 minutes, under the 50 foot column, you would end up an 'L' diver. Note, that since there was no 38 minute row, we had to round up to the next (39 minutes). Looking to the right on the bottom time row, you get your end of dive designation. In this case, an 'L'. The letter 'L' means you have a quantity of nitrogen in your system. Turn the table over, and note on table 3, on the L column how much residual nitrogen you have at the different depths.
Okay, back to the front of the table. Table 2 (on right) is concerned with the surface interval time (SIT). It determines the time it takes to move from letter to letter. For example, if you were an L diver (as in the example above), and waited 1 hour (move right on the L line until you get to the column within your SIT), you would be a C diver.
Now, knowing your ending designation, you can determine your safe diving limits on the next dive. As an example, we plan the second dive to 40 feet and 40 minutes. On table 3, going down the C column until we find the 40 foot row, we can see our maximum safe bottom time (in blue) for 40 feet is 118, so we know we safely dive our 40 minutes. We still need to determine our ending letter designation, like the first dive, but now we have to add the residual nitrogen to our minutes. This number is in the white part of the 40 foot row on table 3. In this example the residual nitrogen is 22. 22 added to 40 is 62. Returning to table 1, we compute our ending designation on the 40 foot column, moving down to the 62 minute row (rounding up to 64). We are now an O diver. Repeat the above steps for subsequent dives.
It is recommended that you draw a diagram (Figure 1), just like the one below to record, compute, and to determine you dive profile.
Figure 1 |
Example
#1
Finding your ending dive letter
designations
Now we will work a 3 dive profile together (Figure 2). The first is to 70 feet for 30 minutes, then a surface interval (SIT) of 1 hour and 10 minutes, then a dive to 60 feet for 35 minutes, then a surface interval of 1 hour and 35 minutes, then a final dive to 50 feet for 40 minutes.
Figure 2 |
Referring to Figure 3, to finish we have to find out our ending dive, letter designations (in blue), whether a safety stop is required (the brown dots), and the residual and total bottom times (in pink)
Figure 3 |
With the RDP in hand, go to the 70 foot column and work your way down to 30 minutes. You have to round up to the 31 minute row which corresponds to a letter designation of 'O'. Since this was the first dive, there was no residual nitrogen factors to consider, and a safety stop was not required since we did not end our dive in the gray or black (near the no decompression limits - NDL).
Move right on the 'O' line until you reach a column that includes 1 hour and 10 minutes. It is the 'D' column. 1 hour and 10 minutes falls between the 1 hour and 4 minutes and 1 hour and 12 minutes.
Now follow the 'D' column down and over to the next side (table 3), then continue down the 'D' column until you get to the 60 foot row. There are 2 numbers here, one in white, one in blue. The blue number is the maximum allowed bottom time you could stay down at this depth and letter designation, but we need the white number because it is our residual nitrogen time (RNT). The RNT is 16 and added to the actual bottom time (ABT) of 35 gives us a total bottom time (TBT) of 51 minutes.
Turning the table back to the front, we can now compute our ending dive letter designation by going down the 60 foot column until we get to the 51 minute row (rounding up to 52), which is the letter 'U'. Note that we ended our dive in the gray area of the table. That means we are required to make a 3 minute safety stop.
Computing the last dive is done just like our dive 2. After waiting on a SIT of 1 hour and 35 minutes, our ending dive letter designation is a 'C'. On table 3 we find the RNT of 17 on the 50 foot row, and adding that to the ABT we get a TBT of 57 minutes. On the front of the RDP, table 1, going down the 50 foot column until we get to the 57 minute row, we end up with a letter designation of 'R', and no required safety stop.
That completes this dive profile. The only other total you might want to compute is the total bottom time. The total bottom time is the sum of the ACTUAL bottom times, including required safety stops, ie: 30+35+3+40=108 minutes.
Figure 4 |
Example
#2
Finding safe bottom times
Draw the dive profile diagram like Figure 1, and fill the following, given data:
First dive to 90 feet, surface interval of 30 minutes, then a dive to 80 feet. What is a safe bottom time for the 1st and 2nd dive that does NOT required a safety stop after the first dive? With the given information we can create a profile like the following Figure 5:
Figure 5 |
Using Table 1 on the RDP, we simply run down the 90 foot column until we get to the gray area and we find that we can dive without a safety stop for 21 minutes and we end up with a letter designation of 'M'. Using Table 2, after 30 minutes we are a 'H'. Turning to the back, Table 3, we go down the 'H' column until we get to the blue number on the 80 foot row. That number is 13 minutes (see Figure 6)
Figure 6 |
Example
#3
Exceeding the no decompression limit
First dive to 80 feet for 20 minutes, surface interval for 1 hour, then 70 feet for 29 minutes, then you determine the surface interval before the 3rd dive to 33 feet for 15 minutes (Figure 7). This is a tricky problem and the right answer would be that it was unsafe, but do the profile using the Emergency Decompression rules on the back of the RDP.
Figure 7 |
Using the front of the RDP, Table 1 you find you are a 'K' diver by moving down the 80 foot column until you get to the 21 minute row (rounding up) and the ending dive designation of 'K'. After 1 hour, moving right on the 'K' line, using Table 2, you stop at column 'C', 1 hour.
Moving to the back of the RDP, Table 3, and down the 'C' column until you get to the 70 foot row, you find the RNT is 12 minutes. You also note a maximum bottom time of 28 (the number in blue). This means that this dive was unsafe, but we are going to continue the profile. You were certainly not plan a dive like this.
On Table 1, you run down the 70 foot column until you get to 41 minutes and find it exceeds the no decompression limit by 1 minute. Reading the special circumstances rules on the back of the RDP, we find we must do a safety stop for 8 minutes, and a required surface interval of 6 hours before diving again (Figure 8).
Figure 8 |
Example
#4
Finding your minimum surface interval
Your dive plans include a 120 foot dive for 12 minutes, then a 90 foot dive for 18 minutes, then end the day with a 60 foot dive for 41 minutes (Figure 9).
Figure 9 |
Running down the 120 foot column, on Table 1, you stop at 12 minutes and ending letter 'J' row. Since you are in the gray area, you are required to do a 3 minute safety stop. Looking on the back of the RDP, Table 3, you move right on the 90 foot row until you find a column where it is safe to dive for 18 minutes (in blue) and find it in column 'B'.
Moving back to the front of the RDP, Table 2, you move right across the 'J' row until you get to the 'B' column. The top number there, 1 hour and 12 minutes is the minimum surface interval.
Back to Table 3 on the RDP, the 'B' column and 90 foot row, you find the RNT is 7 minutes. This added to the actual bottom time of 18 minutes gives you a total bottom time of 25 minutes at 90 foot.
Running down the 90 foot column on Table 1, to the 25 minutes row, you find your ending dive letter as 'Q'.
Now, like between dives 1 and 2, you move to the back of the RDP, Table 3, and move right, across the 60 foot column until you find a column where the blue number is greater than or equal to 41 minutes. You find this column to be 'C'.
Back on the front of the RDP, on Table 2, move right across the 'Q' line until you get to the 'C' line and you find the minimum SIT to be 1 hour and 21 minutes.
Figure 10 |
Problems for you to work at home
Draw a diagram like Figure 1, for many of the following problems.
1) First dive to 80 feet for 10 minutes, then a surface interval of 1 hour, then a dive to 50 feet for 40 minutes. What is your ending letter designation? Answer
2) First dive to 110 feet for 10 minutes, then a 1 hour and 20 minute SIT, then a dive to 88 feet for 15 minutes, with a SIT of 1 hour, then a final dive to 37 feet for 35 minutes. What was your actual bottom time and ending letter designation? Answer
3)
You are a 'L' diver. How long do you need to wait to be able to dive 110 feet
for 10 minutes?
Answer
4) You are doing 3 dives today. They are to 39 feet for 30 minutes, 55 feet for 30 minutes and 91 feet for 10 minutes. Order your dives in the recommended order (deepest first), then compute your ending letter designation AND the minimum surface intervals between each dive. Answer
5)
Fill in the maximum bottom times for the following depths and ending letter
designations:
Ending Dive Letter |
Depth | Maximum Bottom Time |
K | 100 | |
L | 60 | |
A | 50 | |
Y | 50 | |
M | 70 |
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