eRDPml — PADI Divemaster and Instructor Exam Study Notes

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eRDPml — Topics

Dive Planning Mode Surface Interval Mode Multilevel Dives Practice Questions
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What Is the eRDPml?

The eRDPml (Electronic Recreational Dive Planner, metric/imperial) is a physical dive planning device used in PADI Divemaster and Instructor exams. It performs the same calculations as the printed RDP Table — working out pressure groups, no-decompression limits, and adjusted no-decompression limits for repetitive dives — but it does so faster, with fewer manual steps, and with built-in rule warnings that the table cannot provide.

The biggest advantage of the eRDPml over the RDP Table The eRDPml warns you when a special rule applies. When a dive puts you in pressure group W, X, Y or Z, when a safety stop is required, or when extended surface intervals are needed — the device flags it automatically. This makes it very difficult to miss a rule in the exam. The printed table gives no such warnings.

The second advantage is working speed. When using the eRDPml, you only need the depths and times from the question. You enter those values directly into the device and it steps you through the answer. You do not need to work manually through every column of the repetitive dive worksheet — the device handles the calculation. For this reason, always use the eRDPml when you have access to it in the exam.

RDP Rules Overview — eRDPml and RDP Table general rules explained by Will Welbourn, Go Pro Caribbean

The Four Operating Modes

The eRDPml has four operating modes. Selecting the correct mode is the first step in answering any exam question — entering the right information into the wrong mode produces a wrong answer.

Dive Planning Mode

Use Dive Planning Mode for questions where you know the depth and time of each dive and need to find the ending pressure group. If a question gives you a starting pressure group rather than a full first-dive description, Dive Planning Mode handles that too — it asks whether you are beginning the first dive of the day or starting from a known pressure group. This is the most commonly tested mode across both Divemaster and Instructor exams. Full teaching notes and worked examples →

Maximum Depth Mode

Maximum Depth Mode is used when you are partway through a multilevel dive and need to know the shallowest allowable depth for your next level, given the time spent at the current level. It has a narrower application than the other modes and is less frequently tested. It is covered as a named subsection of the Dive Planning page. See the Dive Planning page →

Surface Interval Mode

Use Surface Interval Mode for questions where you know the depths and times of both dives and need to find the minimum surface interval between them. You enter either the ending pressure group from the first dive (if the question gives it to you directly) or the depth and time of the first dive, then the details of the second dive. The device calculates the minimum required surface interval. Full teaching notes and worked examples →

Multilevel Dives

The multilevel function is accessed within Dive Planning Mode. Use it for questions describing a three-level staircase dive — where the diver descends to the deepest level first, then ascends to progressively shallower levels. The device tracks pressure groups through each level in turn and gives the ending pressure group after all three levels are complete. Full teaching notes and worked examples →

Ready to test your skills? Go to the eRDPml practice questions →

RDP Rules — What the Exam Tests

The rules below apply whether you are using the eRDPml or the printed RDP Table. Several are tested directly in Divemaster and Instructor theory exams. The eRDPml will warn you when most of these rules apply — but you still need to know the rules themselves so you can choose the correct answer.

Emergency Decompression

If a diver accidentally exceeds the no-decompression limit (NDL), what happens next depends on how much the NDL was exceeded. This is one of the most commonly tested rules because it splits into two distinct outcomes.

The 5-Minute Threshold — Two Outcomes NDL exceeded by no more than 5 minutes:
Make an 8-minute stop at 5 m / 15 ft. Upon surfacing, remain out of the water for 6 hours.

NDL exceeded by more than 5 minutes:
Make a stop of no less than 15 minutes at 5 m / 15 ft, air supply permitting. Upon surfacing, remain out of the water for 24 hours.
Exam trap — "air supply permitting" When an exam question covers the more-than-5-minutes scenario, the answer option sometimes includes the phrase "as long as your air supply lasts" or "air supply permitting." Students often assume this phrasing is wrong. It is not — it is the correct wording. Air supply permitting means until you are getting low on air. Once low on air, you end the stop and ascend safely. It does not mean staying until the tank is empty. If you see this phrase as an answer option, do not dismiss it.

To answer an emergency decompression question, look up the NDL for the given dive depth. Compare it to the actual dive time. If the dive time exceeds the NDL by more than 5 minutes, the 24-hour surface rule applies. If by 5 minutes or less, the 6-hour rule applies.

Flying After Diving

Pre-Flight Surface Intervals After a single dive: minimum 12 hours before flying.

After multiple dives or multi-day diving: minimum 18 hours before flying.

After a dive requiring emergency decompression: at least 18 hours before flying — and better to be more than 18 hours.
Altitude diving and flying — same rules apply The flying after diving section of the RDP makes no separate mention of altitude dives. That means the pre-flight surface intervals for altitude dives are exactly the same as for sea-level dives. There is no special flying rule for altitude diving.

Altitude Diving

If a question tells you that a dive is taking place at altitude, the first thing to establish is whether altitude rules apply at all.

The 300 m / 1,000 ft Threshold If the dive site is above 300 m (1,000 ft) altitude, special altitude diving procedures are required.

If the altitude given is below 300 m (1,000 ft), treat the dive as a sea-level dive — no special procedures needed.
What Information Do You Need to Plan an Altitude Dive? Two pieces of information are needed: the altitude of the dive site and the actual planned depth. Using those two values together, you look up the adjusted planning depth — which will always be deeper than the actual depth you are diving to. You then use that adjusted depth when looking up the NDL.
Worked Example — Altitude Dive Planning You plan to dive to 20 m at a site 2,400 m above sea level. What depth do you use for planning?
  1. Identify the two required values: altitude = 2,400 m, actual depth = 20 m.
  2. Look up this combination in the altitude conversion table. At 2,400 m altitude and 20 m actual depth, the planning depth is 27 m.
  3. Take the NDL from the 27 m column — not the 20 m column.
You plan the dive using 27 m, even though you are only going to 20 m.

The WXYZ Rule

After a long, shallow dive, a diver can end in a high pressure group — W, X, Y or Z — despite having stayed well within the NDL. The RDP imposes extended minimum surface intervals at these pressure groups.

WXYZ Minimum Surface Intervals Ending pressure group W or X: minimum surface interval before the next dive = 1 hour.

Ending pressure group Y or Z: minimum surface interval before the next dive = 3 hours.
Why Does the WXYZ Rule Exist? The WXYZ rule exists to account for long, shallow dives. A diver can spend a very long time at a shallow depth — completely within the NDL — and still absorb a large amount of nitrogen. The RDP imposes this extended surface interval rule to manage that nitrogen load. If an exam question asks why the WXYZ rule exists, the answer is: to account for long, shallow dives.
The eRDPml warns you automatically If your dive puts you in pressure group W, X, Y or Z, the eRDPml flags the extended surface interval requirement automatically. This is one of the key advantages of the device over the printed table — you cannot miss this rule if you are using the eRDPml.

Cold Water and Strenuous Diving

If a question states that conditions are cold, or that the dive will involve strenuous swimming (such as against a current), the NDL must be taken from a deeper column than the actual depth.

Cold Water / Strenuous Diving Rule Plan the dive as if you are diving 10 ft (4 m) deeper than the actual depth. Take the NDL from that deeper column.
Worked Example — Cold Water Dive A question states a diver is going to 70 ft in a cold water lake. What is the NDL?
  1. Actual depth = 70 ft. Add 10 ft → planning depth = 80 ft.
  2. Look up the NDL for 80 ft on the eRDPml or RDP Table.
Take the NDL for 80 ft, not 70 ft.

Rule 9 — Plan Each Successive Dive Shallower

Rule 9 Plan repetitive dives so that each successive dive is to a shallower depth than the one before it. You must not do a dive deeper than any previous dive on the same day.
Exam trap — dives listed out of order An exam question may list a series of dives that are not in the correct order. For example: 40 ft for 30 min, 100 ft for 15 min, 70 ft for 25 min. Before you start any calculation, rearrange the dives from deepest to shallowest — 100 ft first, 70 ft second, 40 ft third. Applying Rule 9 first is the correct starting point for any multi-dive planning question.

Rule 10 — Maximum Depth

Rule 10 The maximum depth for recreational diving is 40 m / 130 ft. If a diver accidentally descends below this depth, they must ascend immediately at a safe ascent rate and perform an 8-minute emergency decompression stop at 5 m / 15 ft.

Rule 11 — No Pressure Group

After a long enough surface interval, a diver can lose all residual nitrogen and have no pressure group at all. Once that happens, their next dive can be planned as if it is the first dive of the day.

Pressure group A is not the lowest group A common mistake is to assume that pressure group A is the lowest possible pressure group. It is not. Below pressure group A there is a state of having no pressure group — sometimes described as "new diver" status. If a question or answer option states that a diver has no pressure group, that diver's next dive is planned as a first dive. "No pressure group" is a valid — and sometimes correct — answer in the exam. Do not dismiss it.
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