How to Read a Decompression Table — Step by Step Guide

ℹ️ Educational guide for informational purposes. Always dive under qualified supervision and use a dive computer.

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 9 minutes at 3m, then complete the ascent.

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

FAQ

What happens if I skip my decompression stops?

Skipping stops can cause decompression sickness (DCS): dissolved nitrogen forms bubbles in tissues. Symptoms range from fatigue and itching to severe joint pain, paralysis, and in extreme cases, death. If you missed a stop, report it immediately and monitor for symptoms over the next 24 hours.

My computer and the table give different results, which do I follow?

In real diving, always follow the more conservative of the two. The computer calculates your exact profile in real-time, while the table assumes a square profile (max depth for the entire duration). The computer is therefore generally more permissive. For exams, use the table as required.

The table shows "—" for my parameters, what does it mean?

A dash means the depth/time combination is either in the "no-stop" zone (no mandatory decompression, only a recommended 3-min safety stop at 3m) OR exceeds the table limits and is therefore prohibited. Check whether you're in the upper (no-stop) or lower (prohibited) part of the table.