One thing we can use the PADI RDP for is to calculate the minimum surface interval we are allowed between two dives. Personally I prefer to try and have the longest interval that I can have between dives, and as I dive quite often with EANx the recommended minimum is one hour. So personally I prefer to have a surface interval of at least an hour.
So why would you want to work out the minimum surface interval, instead of just taking the longest one you can? Well there are several reasons, the most common is half day boat trips where you only have the boat for a short period of time and you are wanting to get as much diving done as possible.
If you haven’t already done so, I suggest you read my previous post How to use the PADI RDP – Part 1, as I explain a lot of terminology there and for brevity I won’t be repeating it all here.
So let’s plan the dive.
We are looking to do two dives today. One of 20m for 45 minutes and one of 14m for 60 minutes.
So let’s fill in our dive profile for the first dive. Remember from the previous post how to calculate the pressure group? If not, go back and use it as a guide on how to work it out. See if you get the same result as I did.
Now the next step is to fill in the second dive (leaving the surface interval blank). Flip to Table 3 and find the pressure group that will allow you to make a 14m dive for 60 minutes. Remember we are looking at the ANDL (the number in blue) to find the first one that is 60 or above. Once you have found it go up and notice that it is pressure group J.
Now fill this information onto your dive profile.
All we have to do now is use Table 2 to work out the minimum surface interval. Move right from N on Table 1 and move up from J on Table 2, where these two meet is the box with the minimum surface interval. The box where they meet is 00:45 to 00:50. That means the minimum surface interval is 45 minutes.
So here is the completed dive profile for the two dives. I have also filled in the final pressure group.
Although we worked out the shortest interval between these dives we have basically taken the RDP to its limits. This is not a safe way to dive. It is better to be more conservative; you can do this by increasing the surface interval times, decreasing your maximum depth, reducing your bottom time and by not making repetitive dives. No dive table or method can safely guarantee that you will not get decompression illness. Everyone is different and reacts differently to the effects of diving. Remember you dive at your own risk.
Here is a video explanation of the above method
Here is a video using the eRDPml
Let me state that you must receive formal training from one of the many dive organisations before attempting a dive. Without formal training you risk injuring yourself. Please get training before attempting anything you have read or seen in this post. I will not be held responsible for you attempting to use the methods contained in this post. The following post is for illustration purposes only.
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