RPD question

Question
Hello Gustavo,

Thank-you for your time in answering my question.
I am a bit confused when calculating the following types of questions: Example

If you planned to dive to 82 feet and want to make two 20 minute dives . What is the minimum surface interval that I must have between these two dives.  Remembering that minimum surface intervals are for training purposes ONLY.   When diving, I always want to be conservative.

When you look on the RDP chart, is the answer always group A? So the answer to this question is goto depth 90 feet and down to 21 minutes which is (PG) M then across to A and minimal surface interval is 2:15?

I hope I didnt make this sound to confusing.

Thank-you.


Answer
Greetings Dough,

Before I give you my final calculations, there are several disclaimers I -MUST- give.

First of all, all certified divers are responsible for conducting their own dive profiles regardless of that of other divers. Therefore, any advice here does not dismisses your responsibility to "double check" the calculations and make sure for YOURSELF.

Second, I assume that you are a certified diver. If not, consult a PADI Instructor to conduct a PADI Discover Scuba Diving experience or a formal Open Water Course.

Third, an 82' dive is considered a deep dive, and you should be certified as an Advanced Open Water Diver in under to participate, for many safety reasons.

Fourth, assuming you are an Advanced Open Water Diver, you might want to use the Wheel and plan a multi-level dive which will give you both more bottom time and a shorter surface interval, of course, if your diving location allows for a multi-level dive.

Fifth, you might want to consider using a diving computer along with the RDP and/or Wheel to maximize the data collection of actual bottom time and remaining bottom time of your first and second dives.

Now, finally, this is my interpretation and calculation.

Not all dives are alike. So, yes, you do not need to go back to PG "A" to go back into another dive... AS LONG AS the RDP allows you to do it per the calculations. When you go to the back side of the RDP, look up the row of the intended depth for the 2nd dive. In this case, you would look at the 90' row. Now, slide your finger across the row, looking for the blue box (actual bottom time) to match the desired length of bottom time. As you can see, the only way you can stay 21 minutes at 90' is if you come down as a PG "A". So, in this case, YES, you have to wait 2:15 minimum of surface interval in order to repeat that very same dive at that depth and for 20 minutes.

When doing so, please be aware that you must add 4 more minutes to the actual bottom time (back side, white box of 90' row) into the actual bottom time. So, when you go back into the front part to calculate your new pressure group after the 2nd dive, you dive at 82, but calculate it at 90. You stayed at the bottom for 20 minutes, but the closest number is 21. When you add 4, that gives you 25... which is the non-decompression limit of the RDP at that given depth and bottom time for your second dive.

In other words, you are pushing the limit of the dive table.

Although the dive tables are very conservative in their very own nature, dive tables are impossible to accomodate for individual body habitus and metabolic differences. Some people are more prone to be at risk than others for decompression illness (the bends)... the so-called "undeserved hits", etc. Therefore, it is always advised to plan your dives even more conservative, when possible. That also gives you a little slack in case a problem occurs and you need to extend your bottom time.

The profile you are proposing is, again, within the very limits of what the RDP considers to be safe. Although it is still considered "safe", it does not take into account any unforeseen events, which you should.

Remember that all dives ending within the last three PG of the table at the given depth must end with a 3 minute safety stop at 15'.

Again, this information is just an advice and does not substitute your own judgement. After all, each certified diver is responsible for his/her own profile. Divers must dive within their level of training. Feel free to contact your local PADI Dive Center or any other instructor for any formal explanation and detailed instruction on the use of the RDP or Wheel for calculation of the dives.

I hope this information answers your question. If you have any further questions feel free to ask again.

Sincerely,

Gustavo E. Flores