Surface Interval Mode — eRDPml Study Notes

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Surface Interval Mode

Use Surface Interval Mode when a question gives you two dives — depths and times — and asks for the minimum surface interval between them. This is the mode to use any time the surface interval is the unknown. Draw the standard two-dive diagram and fill in the depths and times from the question. The surface interval slot stays blank — that is the answer you are calculating.

Do not confuse this with Dive Planning Mode Both modes use the same two-dive diagram. The distinction is simple: if the surface interval is the question, use Surface Interval Mode. If the ending pressure group or ANDL is the question, use Dive Planning Mode. Selecting the wrong mode produces a wrong answer — always identify what the question is asking before pressing Mode.
Surface Interval Mode eRDPml tutorial — Will Welbourn, Go Pro Caribbean

Step by Step — Full Dive Details Given

When the question gives you the depth and time for both dives, the sequence is straightforward. The device label ABT1 stands for actual bottom time, dive one — the planned time you intend to spend at depth, not the NDL.

Surface Interval Mode — Full Dive Details Sequence 1. Select Surface Interval Mode → Enter
2. First dive? → Yes → Enter
3. Enter depth of dive 1 → device shows NDL for that depth (reference only; press Enter to continue)
4. Device displays ABT1 (actual bottom time, dive 1) → enter planned time of dive 1 → safety stop warning if required → Enter
5. Enter depth of dive 2 → device shows ANDL for that depth
6. Enter planned time of dive 2 (ABT2) → safety stop warning if required → Enter
7. Device shows minimum surface interval
Worked Example — Minimum Surface Interval, Full Details Two dives: 60 ft for 53 min, then 50 ft for 71 min. What is the minimum surface interval?
  1. Surface Interval Mode → First dive? Yes.
  2. Enter depth of dive 1: 60 ft. Device shows NDL = 55 min → press Enter.
  3. ABT1: enter 53 min. Safety stop warning → Enter.
  4. Enter depth of dive 2: 50 ft. Device shows ANDL = 73 min → press Enter.
  5. ABT2: enter 71 min. Safety stop warning → Enter.
Minimum surface interval = 2 hours 48 minutes.

Starting From a Known Ending Pressure Group

Some questions give you the pressure group at the end of dive 1 rather than the full dive 1 details. Draw the two-dive diagram and place the given pressure group in the end-of-dive-1 box. The device sequence changes slightly — it asks for this PG directly instead of working it out from depth and time.

Surface Interval Mode repetitive dive with known pressure group — eRDPml tutorial, Will Welbourn
Surface Interval Mode — Starting From a Known Pressure Group 1. Surface Interval Mode → Enter
2. First dive? → No
3. Device asks for pressure group at end of dive 1 → enter it
4. Enter depth of dive 2 → device shows ANDL
5. Enter planned time of dive 2 → safety stop warning if required → Enter
6. Device shows minimum surface interval
Worked Example — Minimum Surface Interval, Known Pressure Group You surface from a dive in pressure group V. You want to do a second dive to 50 ft for 60 min. What is the minimum surface interval?
  1. Surface Interval Mode → First dive? No.
  2. Device asks for PG at end of dive 1 → enter V.
  3. Enter depth of dive 2: 50 ft. Device shows ANDL = 73 min → press Enter.
  4. Enter planned time: 60 min. Safety stop warning → Enter.
Minimum surface interval = 1 hour 29 minutes.
The diagram tells you which sequence to follow Before answering the "first dive yes or no?" question, look at your diagram. If the end-of-dive-1 pressure group box is empty and you have depth and time for dive 1, answer Yes — the device will work out the PG from the dive details. If the end-of-dive-1 box already has a PG in it (because the question gave it to you), answer No — then enter that PG directly. The diagram makes this decision automatic.