Richard: When I took my initial training (2005) PADI's standard was 60 fpm below 60 fsw and 30 fpm above 60 fsw from what I recall.
It is possible tht you were told that in your training, but that is not the PADI standard. It is still 60 feet per minute.
The reason is that when the table was created, that was the standard ascent rate pretty much everywhere. Since your dive time ends when you start your direct ascent to the surface, all the numbers for NDL's are based on the assumption that you will ascend at that rate. Switching to a different ascent rate will force a change in the actual numbers on the table, and that has not happened.
The reason is because as you ascend, your body tissues are getting rid of their nitrogen. Different tissues give it off at different rates. The bigger the difference in pressure between the nitrogen in the tissues and the nitrogen in the air you are breathing, the faster the nitrogen is given off. In theory, if the difference is too great, bubbles form. Different tissues approach that critical level at different rates as you ascend. If you ascend at the proper rate, they will get rid of enough nitrogen as you ascend taht they will never reach that level.
The people who created the RDP determined that at the appropriate ascent rate, no tissues will reach that critical level. Going slower would seem to be safer, but it may not be because while some tissues (fast tissues) are giving off nitrogen as you ascend, but some (slow tissues) are still taking it on, and they can reach that critical level later in the ascent.
You should know, though, that the tables were padded for safety when created, so you should be able to go at the 30 fpm rate that is now considered to be safer without any worries at all. (I can explain this further if you like--I don't want to take this too far here.)
All that is basic theory, but in the quarter century since the RDP was created, much has been learned. DAN studies have found that 30 fpm is better than 60 fpm, and both are better than 10 fpm. That same study found that safety stops were more important than the ascent rate. That is one reason why people are emphasizing them more today than they used to.
The dive planning software options mentioned earlier are all based on theories that are different from the ones upon which the RDP (or just about any other standard table) is based. These theories are based on the fact that there are already bubbles in the system, so we have to worry about the ones that are there already, not just the ones we might create.
There are actually quite a few different theories on what causes DCS and how it should be avoided. The truth is, though, that even though there is so much disagreement, history shows that if you follow any of these theories, you are extremely unlikely to suffer from DCS. Even though standard tables do not take into account existing bubbles, they have a proven track record of safety. Nobody can say for absolute certainty what is right or wrong, but we can point to a number of models that work anyway.
