Repetitive Dive Planning: Surface Intervals on the DECO 2000 Table

Why residual nitrogen matters on the second dive

You surface from any dive with more nitrogen in your tissues than you started with, and it leaves slowly: Taucherpedia describes it washing out over the following 24 hours or so. A table turns that leftover load into time. The residual nitrogen time is the bottom time you have, in effect, already spent at the new depth before you descend. You add it to the planned bottom time and read the table with the total.

The extra load is measurable. In a DAN study that monitored recreational divers with Doppler ultrasound during multi-day trips, high bubble grades were about 20% more common after repetitive dives than after first dives. The CDC Yellow Book lists repetitive dives and multiple consecutive days of diving among the factors that raise the risk of decompression illness.

The DECO 2000 planner has a Repetitive Dive tab that runs the lookup for you. The steps below are the same lookups done by hand, so you can check its answer.

Worked example: two dives in one day

Dive 1 goes to a maximum of 20 m (66 ft) for 22 minutes. After 1 hour 10 minutes on the boat, dive 2 is planned to a maximum of 14 m (46 ft) for 30 minutes. Result: 37 minutes of residual nitrogen, a 67 minute total, no stops and group G. Here is each lookup.

• Dive 1, front of the table: 20 m rounds up to the 21 m block and 22 minutes rounds up to the 26 minute row. No stops. Group E.
• Surface interval: in row E, 1:10 is past the 1:00 threshold but not past 1:30. Follow the arrow under 1:00 down into the residual nitrogen part of the table.
• Residual nitrogen: dive 2's depth, 14 m, rounds up to 15 m. The 15 m line in that column reads 37 minutes.
• Dive 2: 30 minutes of real bottom time plus 37 minutes of residual nitrogen gives 67 minutes. At 15 m that rounds up to the 72 minute row: no stops, group G.
• The ceiling: the 15 m no-stop limit is 72 minutes, so dive 2 can last at most 72 minus 37, which is 35 minutes. At 36 minutes the total is 73, which falls on the 84 minute row with a 4 minute stop at 3 m.
• End of the day: group G, which the table pairs with a 24 hour wait before flying.

How to read the surface interval table

Go to the row for your group letter, find the largest printed time you have passed and follow the arrow under it down to the line for your next dive's depth. Depth rounds up here too.

Both ends of each row matter. Before the first printed time there is no residual value at all, so the table cannot plan the dive and our planner asks you to wait. For group G that means more than 2 hours out of the water. Past the last printed time, residual nitrogen is dropped and the next dive is planned as a first dive. A time that lands exactly on a printed threshold counts as not yet passed in our planner, which gives the more conservative column.

How long should a surface interval be?

As long as the day allows, and in practice at least 2 hours, which is Taucherpedia's recommendation. The table shows where the time pays off. After a group E dive, a second dive to 15 m gets 30 minutes of no-stop time after a 31 minute break and 45 minutes after a 2 hour 1 minute break. The next 90 minutes add another 11. Then nothing changes until the 8 hour mark, when the table stops counting dive 1 at all.

Short breaks are also where the models disagree most. Writing for DAN, Michael Lang noted that there is little agreement among dive computers on repetitive dives with surface intervals of one hour or less.

Why the second dive should be shallower

Because the same leftover nitrogen costs far more bottom time at depth. The DECO 2000 rules on our table page say a repetitive dive is always planned shallower than the dive before it, and the residual nitrogen table shows why. Take the example day again: group E, 1 hour 10 minutes on the surface.

Going back to dive 1's table depth of 21 m leaves 5 minutes. At 27 m the residual time alone is longer than the no-stop limit, so any dive there needs a stop.

Current guidance on reverse profiles is less absolute. New Zealand's workplace regulator WorkSafe, citing the 1999 Reverse Dive Profiles Workshop, allows a later dive to be deeper only if every dive in the series is a no-stop dive, none goes below 40 m and none is more than 12 m deeper than an earlier dive. For occupational diving it still calls deeper-dive-first the commonly accepted safest method. With a table, the arithmetic above usually settles the question anyway.

Multi-day diving and flying home

The table resets faster than your body does. Row G drops a dive after 12 hours, half the 24 or so hours of off-gassing described above, and an overnight break of more than 12 hours sends every group back to the first-dive limits on the front of the card. Nothing on the table tracks the third or fourth day of a dive week.

DAN's case files show what that can look like. A diver on a liveaboard made three dives a day for two days, all to 27-30 m (90-100 ft) for 55-66 minutes, all within the limits of her computer, and developed decompression sickness about an hour after the sixth dive. DAN's comment: "The No. 1 limiting factor for your dives should not be the pressure gauge."

The DAN Doppler study cited above found high bubble grades fell by about 25% over 6-8 days of multi-day diving, which the authors compared to adaptation. That is a bubble measurement in one group of divers, not a reason to push the later days.

For flying, DAN's consensus guideline is a minimum of 18 hours after multiple dives per day or multiple days of diving, and the CDC Yellow Book gives the same figure. The DECO 2000 column can be shorter: a day that ends in group E reads 16 hours and group D reads 12. Use whichever is longer. Taucherpedia's own advice is 24 hours.

Table or computer on repetitive dives?

Use one method for the whole day, and know what each one gets wrong. A table charges dive 1 as if you spent all of it at maximum depth, rounded deeper. A computer, as DAN's Michael Lang puts it, works from the depth you actually dived and runs repetitive dives through its full decompression model. After a multilevel reef dive, that difference feeds straight into the second dive's allowance.

Tables disagree with each other too. divetable.info ran the same day through seven tables: a first dive to 18 m for that table's no-stop limit, a 60 minute break, then a second dive to 18 m. The no-stop time left for the second dive came out between 2 minutes (US Navy, 2008 revision) and 30 minutes (PADI RDP and DCIEM). DECO 2000 is listed at 6. On our planner the same DECO 2000 day gives 2 minutes if the break is exactly 60 minutes and 6 minutes from 61 minutes, because a threshold only counts once you are past it.

Computers vary too. DAN's dive computer guidance warns that a model can set conservative limits for a first dive and liberal ones for repetitive dives, and that you cannot move in and out of relying on a computer unless it has recorded all of your dives. A computer switched on or borrowed after dive 1 knows nothing about dive 1.

Other repetitive-dive mistakes

Each of these changes the column or the row you read.

• Planning dive 2 with the no-stop limit printed on the front. After group E and 1:10 on the surface, 15 m allows 35 minutes, not 72.
• Timing the interval to the planned entry and then jumping in early. Read the column for the break you actually took.
• Carrying dive 1's letter into the evening. After dive 2, the next interval and the no-fly time start from dive 2's letter.
• Adding residual nitrogen to the gas plan. It is a bookkeeping figure, not time you breathe from the cylinder, so plan gas for the real bottom time with the gas planner.
• Rounding dive 2's depth down on the residual table. 14 m is read on the 15 m line, and 16 m on the 18 m line.