Dive Tables Explained: PADI RDP, US Navy, DECO 2000 and BSAC 88

What is a dive table?

A dive table is a decompression model that was run in advance and printed as a grid. You give it two numbers, maximum depth and bottom time. It gives back the no-stop limit for that depth, any decompression stops and a group letter for the nitrogen you carry out of the water. A second part of the table turns that letter and your surface interval into a time penalty for the next dive.

Behind the grid sits a set of theoretical tissue compartments. PADI's Recreational Dive Planner (RDP) was computed from 14 of them, but its pressure groups and surface interval credit follow only one, the 60-minute compartment, because a printed table cannot track 14 numbers between dives. Ray Rogers, describing the DSAT dive computer model in the SPUMS Journal in 1994, gave this and the rounding of depth and time as reasons a table is more conservative than its own model.

Every table also assumes a square profile. The VDST's course notes for DECO 2000 say it plainly: the table charges the whole bottom time at maximum depth, even if you touched it only briefly, while a computer follows the profile you actually swam. Our DECO 2000 table and US Navy dive tables show the full grids, and each page has a planner that does the lookups.

Why learn dive tables if you dive with a computer?

Because a table shows the lookups your computer does out of sight. The CDC Yellow Book notes that dive computers have largely replaced tables for manual dive planning, and the VDST's 2020 course slides agree that computers have taken over. The same slides still teach table handling in the federation's one-star course.

Expect a table to give less time than a computer on a reef dive. Rogers listed three reasons: depth and time are rounded up, surface off-gassing is worked out on one compartment that is usually slower than the one limiting the dive, and divers log the deepest point although most of the dive was shallower. Our NDL calculator puts a Bühlmann ZHL-16C limit beside the DECO 2000 and US Navy figures: at 18 m on air with gradient factors 40/85 it gives 47 minutes.

Tables still earn their place in three ways:

• Training. Working a dive by hand shows why the no-stop limit shrinks with depth, why a second dive gets less time and what a group letter stands for. A computer shows only the result.
• Planning. You can check a plan before you get wet. On DECO 2000 the limit at 30 m (98 ft) is 15 minutes, so a 20 minute dive there is read on the 21 minute row and owes 4 minutes at 3 m.
• Backup. DAN's guidance is that no computer can factor in a dive it was not there for, and that a diver who forgets the computer early in a repetitive series is restricted to tables for the rest of that series.

PADI RDP, US Navy, DECO 2000, BSAC 88 and NAUI compared

The five tables below are the ones recreational divers most often meet in training. They differ in origin and units, and in how they carry nitrogen into the next dive. The limits are for a first dive on air at 18 m, or on the 60 ft row for tables printed in feet.

Some cells in the table need context. BSAC 88 counts the initial ascent in its dive time, so its 50 minutes is not a like-for-like figure. Writing in the SPUMS Journal in 1991, Greg Adkisson also noted that, to his knowledge, the BSAC 88 tables had never been tested in a controlled trial. The NAUI figure comes from the classic NAUI table, and the two PADI figures are the imperial and metric editions of the same planner.

Other tables are in use too. divetable.info lists 50 minutes at 18 m for both the SSI table and the Canadian DCIEM table, which handles a repetitive dive with a multiplier on its planned time instead of added minutes. Two sets of recreational tables based on Bühlmann's model were published in 1986, with group letters based on an 80-minute compartment.

Why do dive tables give different limits?

Because each rests on its own model, test history and idea of acceptable risk. At 18 m (60 ft) the gap between DECO 2000 and the US Navy is 18 minutes, and DECO 2000's 45 is about 29% shorter. Even one publisher's number moves: divetable.info lists 60 minutes at 60 ft for the older Navy tables, and Revision 7 allows 63.

The models differ in kind. In 2008 the Navy adopted Ed Thalmann's exponential-linear algorithm, in which nitrogen leaves the tissues more slowly than it enters, as InDepth explains. PADI's planner rests on a test programme at the Institute of Applied Physiology and Medicine that looked for the point where bubbles start to form while avoiding exposures likely to cause decompression sickness. The risk figures behind each limit are covered in no-decompression limits explained.

The gap grows on a second dive. divetable.info ran one day through seven tables: a first dive to 18 m for that table's limit, 60 minutes on the surface, then a second dive to 18 m. The no-stop time left came out between 2 minutes on the 2008 US Navy table and 30 minutes on the PADI RDP and DCIEM. DECO 2000 left 6 and NAUI 11.

How to read a dive table, step by step

The cards look different, but on every table that uses group letters the lookups run in the same order. BSAC 88 is the exception: its code points you to a different table for the next dive instead of adding minutes. The full DECO 2000 walkthrough, with a dive that falls between two rows, is in how to read the DECO 2000 dive table. The general order is:

• Maximum depth. Take the deepest point and use the row that equals it or is the next deeper one. The US Navy manual says to enter the table at the depth equal to, or next greater than, the maximum depth of the dive. A 16 m dive on DECO 2000 is read on the 18 m row.
• Bottom time. On DECO 2000 it runs from leaving the surface to starting the ascent, so the descent counts. Round it up to the next printed time.
• No-stop limit and stops. At or under the limit, ascend at the table's rate and add a safety stop. Over it, make the stops the row lists.
• Group letter. Read it at the end of the same row and log it with the time you surfaced.
• Surface interval. Find your letter on the repetitive part of the table and the interval you actually spent out of the water. The US Navy table gives the letter you hold at the end of the interval, while DECO 2000 sends you down a column. On both, a break longer than the last printed interval means the next dive is planned as a first dive. On DECO 2000, a break shorter than the first printed time means the two dives count as one.
• Residual nitrogen time. Read it on the line for the next dive's depth, add it to that dive's real bottom time and go back to the first step with the total.

Worked example: one dive day on DECO 2000 and the US Navy table

Dive 1 goes to 18 m on DECO 2000, or 60 ft on the US Navy table, for 35 minutes. After 1 hour 45 minutes on the surface, dive 2 goes back to the same depth for 15 minutes. Each table is read in its own unit. 60 ft is 18.3 m, which DECO 2000 would read on its 21 m row, so the two dives are close but not identical.

In the table below, the Navy leaves 15 more minutes for the second dive, close to the 18-minute gap between the two first-dive limits. The planners on our table pages give the same numbers for this day. How long to wait between dives, third dives and why dive 2 should be shallower are covered in planning repetitive dives.

Rules every dive table shares

The numbers differ between tables, the ground rules hardly at all. Break one and the table no longer describes the dive you made.

• Round depth and time up, never to the nearer row. PADI's instructions read a 57 ft dive on the 60 ft column, and the VDST reads any depth or time missing from the card on the next greater one.
• Use maximum depth, not the average depth from a computer log.
• Dive deepest first. The PADI instructions say each later dive should be shallower than the one before, and our DECO 2000 page applies the same rule. BSAC 88 allows a deeper repetitive dive, which Adkisson called one of its design flaws.
• Ascend at the table's rate: 10 m (33 ft) per minute on DECO 2000, and 30 ft (9 m) per minute on the US Navy and NAUI tables. The VDST also recommends no more than 5 m per minute above 10 m. divetable.info lists 18 m (60 ft) per minute for the PADI RDP, while the copy of its instructions on Divetalking asks for 30 ft (9 m) per minute. BSAC asks for at least one minute from 6 m to the surface.
• Add margin for cold and hard work. The VDST reads the next longer time row after short heavy exertion or in very cold water, and adds 50% to the bottom time after long heavy exertion. divetable.info gives PADI's version as reading the next greater depth.
• Keep one table for the whole day. Group letters do not carry over from one table to another.