RDP Plus – Calculating Surface Interval

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You may have read the post on Calculating your No Decompression Limit, NDL
and are here to learn more of what the RDP can do. Better stated, what you can
do with your RDP!
Note, even though electronic versions of the RDP are available, they are too
simplified when compared to walking through the tables. The results of the two
when compared should be very similar. Besides, they are electronic and didn’t
I mention something about electronics going out and reverting back to the
tables in my previous post? Ah huh.. So let’s move on.
RDP Plus take the basics of calculating your No Decompressiong Limits and
shows others ways to use your RDP. For example, to calculate the amount of
surface time needed to support another dive to a specific depth for a specific
amount of time.

Let’s take a look at the RDP. What I am about to describe holds true for all
of the DSAT RDP’s and most other organizations rendition of the RDP.
The RDP contains three tables. On the front you may find two tables. Table 1,
the No Decompression Limits and Group Designation Table and table 2, the
Surface Interval Credit Table. On the reverse may be found table 3, the
Repetative Dive Timetable. Three tables.

It takes knowing only two of the three table values to figure out the third
table’s value. For example, if you know the depth and time at depth (table 1),
and the depth for the subsequent dive (table 3), you are able to calculate the
surface interval (table 2)  needed to support the time at depth for the
subsequent dive.

Let me word it a different way. Let’s say you want to dive to 80 feet for 25 minutes.
Then your second dive, you would like to dive to 60 feet for 40 minutes, how much
time must you spend on the surface that will allow you the 40 minutes at 60 feet?
The answer: between 1:09  and 1:30 hrs/mins.

So how do you use the RDP to calculate Surface Interval Time?
Let’s use the example just used above and we’ll walk through the flow on the RDP.
Following a dive to 80 feet for 25 minutes your pressure group is  N. You desire
to dive to 60 feet for 40 minutes for your next dive. Using table 3, locate 60 feet.
Follow the row until the 60 feeet and your current pressure group, N intersect.
You should find the box with the numbers 35 over 20, 35/20.

The legend on the bottom right states the bottom number is our No Decompression
Limit. In this case, 20 minutes. This is telling you that if you were to enter the water
with a Pressure Group of N and decend to 60 feet, you would have 20 minutes of
no decompression diving available. Yet, you desire to spend 40 minutes at 60 feet.
How do you get enough credit to perform a dive to 60 feet, simple, spend more time
on the surface. How much time? You know you want 40 minutes, so still on table 3,
look at the line for 60 feet and follow it to right right until you find a number in the
blue box that is 40 or greater. Scanning we find the column at pressure group C,
where Pressure Group C and the 60 foot row intersect the pair of nunbers 14/41.

What you just just learned is to move from pressure group N to pressure group C
you must spend some amount of time on the surface. Flip the card over to view
table 2. Finding the Pressure Group C located at the bottom of the card, work
your way up until the Pressure Group C column meet the Pressure Group N.
The pair of numbers in the box informed you of the amount of surface time
required to move from the N pressure group to the C pressure group. In this case
1:09/1:30. That’s 1 hour 9 minutes to 1 hour 30 minutes.

Exercise: (post your reponse, showing your work)What pressure Pressure Group did you enter the second dive with?
What was your ending Pressure Group at the end of your second dive?

How much surface interval must you spend between your second dive
and your third dive to support a dive to 40 feet for 45 minutes?
What pressure Pressure Group did you enter the third dive with?
What was your ending Pressure Group at the end of your third dive?

You are planning a three tank dive.
Your first dive you plan to decend to 90 feet for 20 minutes.
Your second dive you plan to decend to 60 feet for 45 minutes.
Your third dive you plan to decend to 40 feet for 45 minutes.

What was your ending Pressure Group for the first dive?
How much surface interval must you spend between your first and second
dive to support a dive to 60 feet for 45 minutes?


Larry Davis

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