The only thing that happens when you go deeper, is that your NDL becomes shorter......but my question is that what if i accidentally exceed 30m in a dive (say 31m) and I also miss safety stop as PADI RDP required (but I'm not in deco), what should I do after the dive?
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If you miss a safety stop, you off-gassed less nitrogen at the ideal depth. The RDP is based on surfacing without a safety stop (above 30m). With a safety stop, the SI for your next dive is probably shorter, but there's no reliable data to support this, so stick to the table. Diving the near limit of the RDP (greyed and black boxes) strongly recommends a safety stop.
PADI does a fair job in explaining the theory about NDL in recreational diving, but I always spend an extra hour of theory during the Open Water course on this subject.
Exceeding the NDL does not harm you, nor does exceeding the maximum depth for recreational diving.
Not knowing what to do if this happens, is what might get you injured.
Getting entangled at +30m/+100ft could result in exceeding your NDL, getting caught in a downcurrent might take you deeper than you planned.
In the first case, the guideline of staying 8 minutes at 5m (less than 5 minutes exceeding the NDL) and 15 minutes at 5m (more than 5 minutes exceeding the NDL) should in general be save enough to get you healthy back to the surface. Use the advised time on the surface of 6 or 24 hours to check for any signs and symptoms of dcs.
In the second case, the RDP won't help you unless you put it in your bcd and you can check during the dive how much shorter your NDL has become.
The RDP does not go beyond 42m. At that depth, you only have 8 minutes bottom time and you used 2 or 3 minutes to descend to that depth. Going deeper means less time and will inevitably result in a deco obligation. To dive safely beyond the recreational depth, you need additional knowledge. And a good instructor!
Computers have become fairly cheap over the last decade. The manual will tell you what your computer will do in case you exceed the NDL. Read it! And understand it.
An ascent time of 9 minutes with a ceiling of 3m does not mean go immediately to 3m and stay there for 9 minutes. There's a difference between ascent time and stop time.
Just ask yourself - if you are often diving near or on the recreational limits, why not extend your knowledge and skills?