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You are searching on the subject of the Recreational Dive Planner also known as the RDP. You found a good source for that information here at Divetalking.
In this session, various ways to use the Diving Science & Technology Corp recreational dive tables supported by PADI will be displayed. From here on the Diving Science & Technology Corp recreational dive tables sanctioned by PADI will be called, RDP.
Let us first understand that what you are about to learn here is NOT a substitute for proper training. Proper training may only be acquired through an accredited organization, with reputable Instructors. Divetalking strongly advises you seek professional guidance and instruction before attempting to use what you learn here.
What you are learning here is an introduction to the ways the RDP may be used. You will begin by learning the purpose and use of the three tables found on the RDP, both front and back. The RDP contains legends and instructions to follow under certain circumstances. Your goal is to become familiar with the card, terminology and its use, to better prepare yourself for your Open Water Class or to help reinforce what you have already been taught, serving as reminders on how to perform steps to figure out, time.
The RDP is based on many years of research and factors, averaging the best numbers, with weight towards safety to calculate times. The RDP basic function is to provide a tool to calculate time. Whether it be surface interval, bottom times, No Fly Times, etc… it is about calculating time.
What you will learn:
What is the RDP
What an RDP is comprised of
What each of the three tables of an RDP perform
How to use the RDP
What is the RDP?
The RDP is a tool, whether it be in the form of a card or electronic device, assisting in the calculation of No Decompression Time, NDT, your Surface Interval Time, SI and Adjusted No Decompression Time, ANDT. Please note the use of Time in all of the previous descriptions.
Putting it simply, the RDP assist you in determining, time.
Let us use the PADI Imperial RDP as a reference.
Let’s identify the features of the card, both front and back. We will then go on to learn how to use each table.
What does an RDP is comprised of?
The Recreational Dive Planner, RDP comprises of 3 tables, No Decompression Limits and Pressure Group Designation Table, Surface Interval Time Credits, and Repetitive Time Dive tables, Depths and Times and of course legends.
What do each of the three tables of an RDP perform?
Each of the tables performs a number of things. Before we go into the details of each table, let’s identify each table and understanding of what you can get from the tables.
Beginning with the front of the RDP
Let us start with the legend. On the left side, boxed in green is a legend. Three descriptions fill that box. The first description is Pressure Group with an arrow pointing to the Alpha Characters to its right.
The Alpha characters boxed in blue represent Nitrogen load (We’ll cover Nitrogen load in more detail later) and is known as the Group Designation Table
The second description is a Black box with a number in it. The number in the box is an example for the actual number found in each of the black boxes located at the bottom of each column inside the yellow boxed area.
The number represents the MAXIMUM time, in minutes one may spend at a given depth without going into decompression. Exceed the number in the black box and you just entered into Decompression, something we want to
avoid in ‘recreational’ diving.
The third description is a grey box which represents whenever you reach a given time at a given depth, where that time falls within a grey box, a safety stop is required. (More on safety stop later).
Next, the large box in yellow is one of two tables found on the front side of the card. It is used to determine your No Decompression Limits and your Group Designation.
The second table is boxed in red on the right. This table is used to determine your Surface Interval Credit. Dive time given back to you for spending time at the surface.
On the front side of the card, (Lets call this side the front) you will find two tables. (just described above)
Table 1 – No Decompression Limits and Group Designation Table(Surrounded by the yellow box), on the left
Table 2 – Surface Interval Credit Table (Surrounded by the red box), on the right.
What do each of the tables perform
Let’s focus on the table surrounded by the yellow lines
Table 1 consists of a series of columns and rows.
The numbers found cross the top of each column represent feet*.
*Note: we are using the Imperial version of the RDP for this example. There is a Metric version and those numbers would represent meters.
The numbers running down each column represent time, in minutes.
How to use the RDP
If you have the plastic RDP available, take it out now. Also recommended is a straight edge, like a piece of paper or ruler.
Let us walk though a scenario. Let’s presume this is the first dive of the day. We will also presume our Group Designation is A.
Starting with Table 1, the No Decompression Limits and Group Designation Table
Look at the top row, where you see 35 40 50 60 70 80. Looking down the row with the box with the 60 in it, you notice a series of blue/white boxes. In each of those boxes is a number. The number represent minutes.
If you were preforming a dive to 60 feet for 36 minutes, you would scan down the column under the 60 until you find 36. Scanning down we find boxes with Note there is not a box with 36 minutes under the 60 foot column. When ever you are looking up time on this table and the time you are looking for is not there, you must select the next highest time.
In this case must select the higher number, Now follow to the Group Designation to the right and you will come to This is where having a straight edge helps, like a ruler.
What this means is, after spending 36 minutes at a depth of 60 feet, you went from a Group Designation of A to a Group Designation of O.
Group Designations represent the load of Nitrogen within the bodies tissues as time.
The closer the Group Designation is to Z, the more load of nitrogen you have in your body and that correlates to a penalty of time taken
away from your next dive. Your goal to to try to move the Alpha designation back towards A. You do this through a Surface Interval.
Surface Intervals are time spend out of the water, between dives. Doing so facilitates Nitrogen escaping from your body, ie through normal respiration.
You may hear this referred to as Off Gassing. This leads you to table #2, the Surface Interval Credit. Note the word, Credit.. You are credited with time
when spending time out of the water. Time that you may use for your next dive.
The more time you spend out of the water, the more credit, or time will be given back for your next dive.
Once you are done with Table #1 and you determined which Group Designation you are placed in after spending a specific amount of time at a given depth,
you hope back onto the boat, or exit the water for a period of time before re-entering for your next dive. Spending time out of the water between dives is
known as Surface Interval, where Interval is time. The more time you spend out of the water on your Surface Interval, the more time is given back to your
next dives bottom time. Let’s look at Table #2, the Surface Interval Credit Table.
Table two, Surface Interval Credit Table surrounded by the red line will tell you what Group Designation you end up in after a period of time out
of the water, Your Surface Interval, SI. The more time you spend out of the water on your Surface Interval, the more time you are credited for your next dive.
Continuing with the example from Table #1, where you performed a dive to 60 feet for 36 minutes, exit the water in Pressure Group Designation O,
on the line with the Group Designation of O, move across to the right and into the Surface Interval Credit Table, Table 2.
There are two numbers in each box. They represent a range of time spent on the surface. For example the box with is the range from 29 minutes (0:29) to 34 minutes (0:34). Another example
is the range from 56 minutes (0:56) to 1 hour and 3 minutes (1:03).
You exit the water for 1 hour and 10 minutes. Spending 1 hour and 10 minutes on your Surface Interval places you in Pressure Group, D.
This is where a straight edge comes in handy. A ruler or a piece of paper will suffice. You would scan to the right from Table 1 on the line for Group Designation O until you can to the box containing the range of times containing the time you spent out of the water. In this case, you spend 1 hour and 10 minutes and locate the box containing
represents the range from 1 hour and 4 minutes to 1 hour and 12 minutes. What you do next is scan DOWN and you will find the letter D at the bottom. This means exiting the water as an O, spending 70 minutes on the surface, you off gassed enough Nitrogen to move you from an O to a D. You are now a Group Designation of a D after spending 70 minutes (1 hour and 10 minutes) on the surface. Because you have spent time of the surface off gassing, you were credited with time. Time which you may apply towards your next dives bottom time. How much time were you credited may be seen on the next table, table #3, the Repetitive Dive Table
So far so good, right?
Well, you have a second dive to do and you will be entering the water with a Pressure Group of D. Remember, you entered your first dive with a Pressure Group of A. Your second dive as a D. This means your body still contains some residual nitrogen from your previous dive in your body. This is OK because you will take this into consideration for your second dive. You will essentially remove some time from your second dive so not to go into decompression. Let’s see how this works…
Turn the RDP over. You should be looking at Table 3. The table is intended to be flipped upwards, vs. left or right.
Locate Pressure Group D. The pressure group you entered after spending 1 hour and 10 minutes at the surface after your previous dive.
You will also need to locate the row representing the depth of your next dive. Remember, if you do not see the exact number for the depth along the left hand side, select the next higher depth. For example, if you next dive is to a depth of 44 feet, you would select 50 feet. Let’s use 44 feet for the second dive. This means you must use the row at 50 feet, NOT 40 feet.
Follow down from the Pressure Group, D and across from the depth 50 where the two intersect. You will come to a box containing two numbers.
In this example, the box you arrive to 19 and 61.
You ask, what do the two numbers mean? Look down at the legend for
Let me explain what the two numbers mean and represent. Let’s start with the top number, the one in the white. Remember the comment earlier about residual nitrogen? After spending only 1 hour and 10 minutes on the surface, after your previous dive, your pressure group was a D. The D represents a residual nitrogen still within your body tissues. Here the Pressure Group Designation is represented with time, RNT and that time will need to be add to the Actual Bottom Time, ABT of your next dive. The number in the white section, 19 is added to your actual bottom time, where your actual bottom time cannot exceed 61 minutes, the number in the blue section. The sum of the numbers, in the white added to your actual bottom time becomes your Total Bottom Time, TBT and can not exceed 80 minutes. Hence the Actual Bottom Time cannot exceed 61 minutes, (80 minus 19 = 61).
You may be asking, where did the 80 minutes come from? Simple, using 50 foot in our example, the maximum bottom time, without going into decompression at 50 feet is 80 minutes (19+61) or flip to Table #1, find the 50 foot column and what number is found in the black box, 80? Right? Yet this is your second dive, in this example and you are entering the water with a Pressure Group Designation of D which means you have Nitrogen in your system. Because you have residual nitrogen in your system, the D represents the load of nitrogen as time. In this example the time is 19 minutes. Hence 80 minus 19 leaves 61, which is the number in the blue area found on table #3 and the legend reads the blue area represents the ADJUSTED no decompression limit. Your Actual Bottom Time to 50 feet can be no longer than 80 minutes, but now your Actual Bottom Time is limited to 61 minutes. This is due to the reason you have 19 minutes of penalty against the 80 due to the residual nitrogen left over from the 1 hour and 10 minute surface interval between your first and second dive still in your body. Make sense?
So far so good? Read it again if you need to…
To continue the example of making that second dive, the question you are asking is, how long will we spend at 44 feet? It can’t be more that 61 minutes. Let’s agree on 45 minutes..
You make the dive to 44 feet for 45 minutes, using table 1, what pressure group do you end the dive in?
If you figured O because you found the 47 minutes under the 50 foot column you would be correct if this was your first dive BUT that is not correct. Remember the 19 minutes found in the white box in table 3? The 19 represents residual nitrogen still in the body in the form of time. This means you start your dive at 19 minutes. You then stayed at 44 feet for 45 minutes of actual bottom time, making the total bottom time 64 minutes (19 + 45). Knowing now you must add the number from the white box, table 3 to your actual bottom time to determine your total bottom time. Looking at the 50 foot column for 64 minutes, we find a 63 followed by a 67 minute box. We must use the higher number, 67 making our pressure group at the end of the second dive a U. Look at the legend again. The White Area indicated what?
RNT + ABT = TBT
19 + 45 = 64
Now, you may be asking, what if I am planning a 3rd and/or a 4th dive on this day, how do I calculate those dives? Simple, you repeat the cycle of finding your pressure group (RNT+ABT=TBT), follow that Group Designation to table #2 (Surface Interval Credit), follow that column down to Table #3, locating the pair of numbers (white/blue) where the Pressure Group Designation after your surface Interval and the depth for the next dive intersect to find your new ABT and RNT, etc… continuing this cycle for each dive that day.
Question for you. When scanning down the column found on table #1 for a planned dive to 64 feet, which column would you use? The 60 foot column or the 70 foot column?
What you have learned:
What is the RDP
What is an RDP comprised of
What does each of the tables perform
How to use the RDP
Should you have any questions, comments on this post, please feel free to contact the author.
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