How to Read Dive Tables & the RDP
Dive planning trips up more exam candidates than any other topic — not because it's hard, but because it's unfamiliar. Once the logic clicks, tables and the RDP become mechanical.
What dive tables actually do
Every dive table and planner exists to answer one question: how long can you stay at a given depth and still ascend directly to the surface without a required decompression stop? That time is your no-decompression limit (NDL). The deeper you go, the faster you absorb nitrogen, so the shorter your NDL. Tables, the Recreational Dive Planner (RDP) and the electronic eRDPml are just tools for staying inside those limits.
Pressure groups: your nitrogen 'score'
After a dive, a table gives you a letter — your pressure group — that represents how much nitrogen is still dissolved in your body. A deeper or longer dive leaves you with more residual nitrogen and a later letter. Think of it as a score you carry with you until enough time on the surface brings it back down.
Surface intervals and repetitive dives
Time on the surface lets your body off-gas nitrogen, so your pressure group moves back toward the start. That's what a surface interval buys you. But a repetitive dive begins with residual nitrogen already on board, so it counts against your next NDL — meaning less bottom time than if it were your first dive of the day. Planning that reduction correctly is the whole game.
The three steps of the RDP
The paper RDP works in three ordered steps, and doing them in order is what keeps you out of trouble:
- Step one: your first dive's depth and time give you an ending pressure group.
- Step two: your surface interval credits you back some nitrogen and gives you a new, lower group.
- Step three: that group tells you how much 'residual nitrogen time' to add to your next dive, which shortens its NDL accordingly.
How to not lose points on the exam
Table questions are mechanical, so the mistakes are almost always procedural rather than conceptual:
- Round depth and bottom time up to the next value on the table — it errs on the safe side.
- Follow the steps in order; don't jump straight to the repetitive-dive limit.
- Practise repetitive-dive problems specifically — one dive is easy, it's the second and third that catch people out.
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Frequently asked questions
What is a no-decompression limit (NDL)?
It's the maximum time you can spend at a given depth and still ascend directly to the surface (with a safety stop) without a required decompression stop. The deeper you dive, the shorter the NDL, because you absorb nitrogen faster.
What is a pressure group on a dive table?
A letter that represents how much residual nitrogen is still in your body after a dive. Deeper or longer dives give you a later letter; time on the surface moves it back toward the start as you off-gas.
Why does a repetitive dive give you less bottom time?
Because you start it with leftover nitrogen from your previous dive. That residual nitrogen counts against your no-decompression limit, so a repetitive dive has a shorter allowable time than the same dive would as your first of the day.
What's the difference between the RDP table and the eRDPml?
They plan the same way; the eRDPml is the electronic multilevel version. You enter depths and times and it does the lookups for you, and it can plan multilevel dives more precisely than the paper table. Both keep you within no-decompression limits.
Do you round up or down when using dive tables?
Round depth and bottom time up to the next value on the table. Rounding up assumes a slightly deeper or longer dive than you actually did, which is the conservative, safer choice.