What is my Pressure Group?

This article addresses how to calculate your Pressure Group after a
the 2nd or 3rd dive.

I thought this subject would be a good one. As an Instructor, one of
the most common problems I find individuals having is calculating
the correct Pressure Group at the end of a second or third, etc. dive.

In my blogs on RDP, I showed you a little on how to use an RDP. We will  now work through an exercise that will demonstrate how to correctly
identify the pressure group you will be in at the end of your dive.

Pull out the AIR RDP. The process is the same for any RDP. We will start
this subject at the end of your first dive.

Let us begin

You just complete your first dive of the day and ended in a Pressure
Group M.  During your time on the surface someone mentioned going to a wreck for the next dive. You plan a second dive to 57 feet to check out a
new wreck and wish to spend 30 minutes on the dive. After spending time on the surface, you plan to  enter  the water for the second dive at a Pressure Group of D.

Questions:

(A): How much surface interval time do you need to spend to move
from Pressure Group M to Pressure Group D?

 You complete the second dive to 57 feet for 30 minutes.
(B): What Pressure Group are you in when you end your second dive?

Calculate the answers to the questions above before looking further in this post.

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Answers:
(A): You must spend between 56 minutes to 1 hour and 4 minutes to move from pressure group M to pressure group D.

(B): Your answer should be S. Did you forget to round up on table 3 and table 1? Your second  dive was to 57 feet. There is no 57 on table 3, you need to round up to the 60 foot depth. Moving to the box where 60 feet meets pressure group D,  you find  16/39. The 16 represents Nitrogen still remaining in the body in the form of time.  It is the 16 you must add to your 30 minutes of Actual Bottom Time for your second dive. 

Using Table 1, you find the 60 foot column, and move down until you find the sum of 30+16, 46. There is no 46 minute box in the 60 foot column and you round up to the next number which in this case is 47. The Pressure Group at 47 minutes at 60 feet is S.

Larry Davis

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