rdp tables metric version

The RDP has three tables...

Table 1 shows how depths and times interact to yield a certain nitrogen load. Depths are in metere and times are in minutes; Nitrogen levels are expressed with letter designations A-Z. A is little. Z is a lot. We call these letter designations pressure groups, abbreviated "pg".

Table 2 shows time intervals spent on the surface following a dive. When you surface after a dive your body begins losing the nitrogen built up during the dive. This table lets you know how much. The period of time spent on the surface in between dives is called the surface interval, abbreviated "si".

Table 3 is on the flip side of the RDP. (It is not labeled "Table 3" but that's what we'll call it from now on.) Use it to plan repetitive dives. If you do only one dive in a day, you won't need this table.

We've worked out some sample dive profiles to give you practice using the RDP. Read the instruction manual that came with your RDP and then work through these problems. If you get stuck, review the instruction manual and read the step-by-step solution we've given you after each example.

Get into the habit of using a dive planning profile for every dive. It looks like this.

diveprofile.jpg

Example 1

Finding a No-decompression Limit

What is the no-decompression limit for a dive to 14 meters?

To find this answer use Table 1. Note that depths in meters are given in the top row of the table. Find 14 meters. Follow down the column under 14 meters until you come to the last box. It is highlighted in black. It says 98 minutes. This is the no-decompression limit in minutes for a dive to 14 meters. (The no-decompression limit for each depth is in the last box in the column for that depth and is highlighted in black.)

Example 2

Finding the Pressure Group After a Single Dive

You dive to 14 meters and stay down for 32 minutes. What is your pressure group at the end of the dive?

Draw the diagram filling in all the information you have.

diveprofile2a.jpg

Next, find the depth in the top row of table 1. Follow down in the column under 14 meters until you find 32 minutes. Now, follow the row with 32 minutes to the left until you find a letter, in this case, the letter "H." The table is telling you that after a dive to 14 meters for 32 minutes, you are in pressure group "H." Write the "H" in your dive profile diagram.

diveprofile2B.jpg

ALWAYS Round Up!

ALWAYS Round Up!

Pop Quiz: When using the tables, if the exact depth or time you're working with is not on the table, should you round UP or DOWN?)

Do this next profile. It demonstrates how to calculate a dive when the exact depth and/or time you're using is not on the table.

diveprofile2c.jpg

Find the dive depth in table 1. Fifteen meters is not on the table so round up to the next greatest depth, in this case 16 meters. (ALWAYS round up. It makes your dive planning more conservative and therefore safer.) Follow down the 16 meter column to find 33 minutes. It's not on the table either. Again, ROUND UP, in this case, to 35 minutes. To find your ending pressure group, follow the 35-minute row to the right until you meet the letter "K." "K" is your ending pressure group after a dive to 15 meters for 33 minutes.

Example 3

Finding the Pressure Group After a Surface Interval

In Problem 2 you ended a dive in pressure group K. What will the new pressure group be after you stay on the surface for 28 minutes?

diveprofile3a.jpg

To find this answer starting from the pressure group designation K on the table, continue following in that row until you find 28 minutes (:28). In this case you'd choose the box with :23 to :29 minutes since 28 minutes is between the two. From that box, follow the column straight down to the bottom to the letter G. Gis the new pressure group after a 28-minute surface interval.

diveprofile3b.jpg

Example 4

Finding the Maximum Allowable Bottom Time for a Repetitive Dive

diveprofile4a.jpg

In problem 3 you ended in pressure group G after your first dive and a surface interval. Now you want to make another dive to 14 meters. What is the maximum allowable bottom time for this dive?

This is a repetitive dive so you will use the table on the flip side of the RDP. The depths in meters run down the left side of the table. Pressure groups run across the top of the table. Since you are in pressure group G, find G on the top row of the table. Now find the depth you plan to dive to, 14 meters. Find the square where 14 meters and pressure group G intersect. There are two numbers in the box, one blue and one white. The blue number is the maximum allowable bottom time. The answer to this problem is 69 minutes. This tells you that if you are in pressure group G at the start of a dive you can safely dive to 14 meters for no more than 69 minutes.

diveprofile4b.jpg

Example 5

Finding the Pressure Group After a Repetitive Dive

You and your dive buddy dive to 21 meters and stay down for 28 minutes. Following the dive you get back on the boat and stay on the surface for one hour and five minutes. On your second dive you go to 18 meters and stay for 30 minutes (your Actual Bottom Time (ABT). What is your pressure group after the second dive?

diveprofile5a.jpg

Start by finding your pressure group after the first dive. Twenty-one meterrs rounded up to 22 for 28 minutes puts you in pressure group N. After a one hour and five minute surface interval your pressure group has dropped to D. Now flip the chart over to the repetitive dive table. Find pressure group D at the top of the table. Find the depth of the dive, 18 meters, in the column at the left edge of the table. Now find the square where pressure group D and 18 meters intersect. There are two numbers in the square, one blue highlighted, the other white highlighted. The white number is the residual nitrogen time (RNT) in minutes, 16. This number tells you how much nitrogen is left over in your body tissues from your first dive. What do you do with this number? Remember the word "RAT."

R = Residual Nitrogen Time

+ A = Actual Bottom Time

_____________________

T = Total Bottom Time

In our problem:

16

+ 30

________

46

R, the RNT is 16. A, ABT, is 30. Add them together to get what we call the Total Bottom Time (T). Forty-six is the sum of the RNT and the ABT. This time, the Total Bottom Time is what you will use to complete the dive profile.

diveprofile5b.jpg

To get the final pressure group just flip back to table 1. You are diving to 18 meters for 46 minutes (not the actual bottom time of 36 minutes). The result is pressure group R.

diveprofile5c.jpg

Example 6

Finding a Minimum Surface Interval

You may know the exact profiles of dives you'd like to do. For example, if you want to do two dives on a wreck one after another, you would have to figure out how long a surface interval you'll need between the two dives in order to stay safely within no-decompression limits. In other words, you must find the minimum surface interval.

You want to dive the wreck of the Hesperus in 14 meters of water. Based on what you know to be your usual rate of air consumption you estimate you'll be able to stay down for 68 minutes. Since there's so much to see on the wreck that won't be enough time so you want to do a second dive. Again, you'll be in 14 meters of water and will stay down for 68 minutes. How long a surface interval must you have?

Start by finding the pressure group after the first dive. You know how to do that!

diveprofile6a.jpg

After the first dive, you are in pressure group S. Now you have to find the pressure group that you must be in before you start your second dive. Since it's a repetitive dive, use table 3.

Find the depth of the second dive, 14 meters, in the depth column of table 3. Next, follow across the 14 meters row in the blue numbers until you come to a number that is greater than or equal to the time of the second dive, 68 minutes. Sixty-eight minutes is not in the table, so, as always, ROUND UP to 69. Follow up in the column where you find the 69 to the top row of the chart. There you'll find the letter G telling you that you must have a surface interval sufficient to get you down to pressure group G before you start your second dive. Fill it in on your profile.

diveprofile6b.jpg

There's only one more step: find the minimum surface interval needed to reduce your residual nitrogen from pressure group S to pressure group G. Flip to the surface interval table. Find the higher pressure group, S, in the diagonal line of pressure groups bordering the left side of the table. Find the lower pressure group, G, in the row of pressure groups along the bottom of Table 2. Follow across the one and up the other until the S row and G column intersect. There are two times in the box, :57 and 1:03. You want to know the minimum surface interval so the lower of the two numbers is the correct answer. Fifty-seven minutes is your minimum surface interval. Fill it in on your diagram.

diveprofile6c.jpg

Now you've thoroughly learned the RDP. If you ever forget how to use it, read the instruction manual, come back to our website or contact us for help. It's important that you know the RDP even if you have a computer and rely on it. Suppose you spend lots of money to go on a dive vacation and your computer malfunctions before you start your dives. You can whip out your RDP and track your dives with it. Even worse, suppose your computer failed during a series of dives. If you'd kept track of your dives with the RDP as back up for your computer, you could fall back on that and continue your dives. If you don't know where you are on the RDP when your computer fails, the only safe thing to do would be to end your diving for the day. Always using the RDP can be a dive saver.