How to Read a Dive Table

Why learn tables?

Even though dive computers are the norm today, knowing how to read a decompression table remains essential. It's mandatory for FFESSM exams (N2, N3, N4), it's your backup if your computer fails, and most importantly, it gives you a deep understanding of what happens in your body during and after a dive.

Step 1: Identify your dive parameters

Before checking the table, you need two pieces of information:

• Maximum depth reached — rounded up to the next table value (e.g., 23m → 25m)
• Dive duration — from start of descent to start of ascent, rounded up to the next minute

Golden rule: always round up. If the exact depth or time isn't in the table, take the next higher value. This provides a safety margin.

Step 2: Read the stops

In the MN90 table, cross-reference the depth column with the time row. You get:

• Stops — mandatory stop duration at 3m, 6m, or 9m during ascent
• TDT (Total Decompression Time) — total time from start of ascent to surface
• SDG (Successive Dive Group) — letter from A to P indicating residual nitrogen loading

Concrete example

Dive to 25m for 30 minutes:

→ MN90 table: 5 min stop at 3m, TDT = 7 min, SDG = F

This means: ascend at 15-17 m/min to 3m, stay 5 minutes at 3m, then complete the ascent at the required rate.

Step 3: Managing a successive dive

If you dive again within 12h (excluding consecutive dives <15min):

• 1. Note your SDG from the first dive (e.g., SDG = I)
• 2. Wait the minimum surface interval of 15 min (ideally 1h+)
• 3. Table I — Cross SDG + surface interval → residual nitrogen coefficient
• 4. Table II — Cross coefficient + planned depth → time penalty in minutes
• 5. Add the penalty to the actual time of the 2nd dive and read stops normally

Common mistakes to avoid

• Rounding down — always round depth AND time UP
• Forgetting the penalty — for successive dives, dive time = actual time + penalty
• Ignoring ascent rate — 15-17 m/min from depth to first stop, 6 m/min between stops
• Neglecting surface interval — minimum 15 min, recommended 2h+ for high SDG (>H)
• Diving beyond the table — if your depth or time exceeds the table, the dive is NOT ALLOWED with those parameters

What do TDT and SDG mean?

TDT means Total Decompression Time. It includes ascent time and mandatory stops before surfacing. SDG means Successive Dive Group: it is a letter used to estimate residual nitrogen before a second dive.

If you plan another dive after a surface interval, the SDG helps calculate a time penalty. This penalty is added to the actual bottom time of the second dive before reading the decompression stops.

Dive table vs dive computer

A dive table assumes a square profile: the diver is treated as if they spent the entire bottom time at the maximum depth. A dive computer tracks depth changes continuously and may therefore show a different profile. For training and exams, use the required table. In real diving, follow your instructor, your dive plan and the most conservative instruction.