minimum surface interval help!

Question
how do you find the minimum surface interval required between a  dive to
18m/40 mins followed by a dive to 14m/60mins?

Can you please explain and answer using step by detailed step using the padi
rdp?

thank you so much!

Answer
Hi Kelsi,

I'm sorry for the delay in getting back to you but your question came in on Saturday and I didn't get it until today (Monday).

PADI has three RDP's, the Table, the Wheel and the Electronic RDP (E-RDP).  I'm not sure which one you've got so I'll handle the problem using the table.  The Wheel is no longer produced by PADI so you probably don't have one of those.  If you have a Wheel, write back and I'll go through the same problem using the Wheel RDP.  The E-RDP is simply a matter of reading the instruction book and plugging in the numbers.

My table is in Imperial so I've converted your meters to feet in order to use it.

18 meters = 59 feet...round off to 60 feet
14 meters = 46 feet...round off to 50 feet

Using the table:  Find your pressure group from Table 1 for a dive to 60 feet for 40 minutes.  Follow 60 feet down the column while looking for 40 minutes.  Since 40 is not on the table, round up to 42 minutes.  Following the column down from 60 feet until you get to 42 minutes and then going to the right gives you a pressure group of Q.

Turn the table over to get to table 3.  Your second dive is 50 feet for 60 minutes.  Start at the left side of the table and find 50 feet.  Follow the horizontal row until you get to 60 minutes in the blue box (Actual Bottom Time).  Since 60 is not there, use the closest time which is LESS than the desired time.  In this case, it's 59 minutes.  Now follow the column containing the 59 minutes vertically to find that you'll need to be in pressure group E to do that dive.

Now go back to table 1 and find pressure group Q.  Then find Pressure Group E at the bottom of Table 2.  Follow Pressure Group Q across the table until you intersect the column with Pressure Group E.  There you'll find two numbers which represent a range of times.  In this case 1 hour and 4 minutes to 1 hour and 11 minutes.  You need to select the greater of these times for safety so the minimum surface interval between these two dives is 1 hour and 11 minutes.

If you have to work in meters on your RDP just follow the instructions I have given you and you should be able to solve the problem.  Your answer should be similar to the one I got.

Remember when rounding off, use the next greater depth and the next shortest bottom time when planning your dives.  That adds an additional safety factor.  For surface intervals, use the longest time for an additional safety factor since you're trying to lose nitrogen and the longer the surface interval, the more nitrogen you lose.

Kelsi, I hope this helps you to figure out how to do Minimum Surface Interval problems using PADI's table version of the RDP.

The thought process behind all this is as follows:  You handle your first dive just as you always have and get a pressure group at the end of it.  You treat your second dive as a dive to your maximum bottom time (NDL) and you see what pressure group you would have to be in at the start of your dive in order to make this second dive.  Then you see how many minutes (minimum surface interval) it would take to get from your first pressure group to the second one.

Keep in mind that, in order to plan the minimum surface interval, we're planning the second dive right up to our NDL so a safety stop will be mandatory for this second dive.

Please write back if this doesn't answer your question.

Sincerely,

Mike Giles
Mike's Dive Center
mikescuba.com