History of MN90 Dive Tables
How the French Navy divers tested your deco stops
ℹ️ Read time: 6 minutes. Discover how these tables, the FFESSM references, were designed.
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From GERS 65 to MN90
Before 1990, French divers used the famous "GERS 65" tables, created by the Underwater Research and Study Group of the French Navy. These tables were pioneering but began to show their limits, particularly concerning residual nitrogen management during repetitive dives and safety margins, which were no longer considered optimal with the evolution of hyperbaric medicine knowledge.
In the 1980s, the French Navy decided it was time to overhaul the system to ensure better safety for its clearance divers and combat swimmers.
Choosing the mathematical model
For the MN90s, the French Navy didn't start from scratch. It relied on existing decompression models, notably the "Bühlmann" model, but chose an algorithm based on 12 tissue compartments (compared to 16 for the Bühlmann ZH-L16 model). This technical choice allowed for modeling gas exchanges (nitrogen saturation and desaturation) accurately enough without unnecessarily complicating the calculations, which had to be transcribed onto easily readable tables.
The Testing Phase: The Military Method
This is where the history of the MN90s becomes fascinating. Mathematical models are not enough; they must be confronted with physiological reality. The French Navy therefore organized a vast testing campaign in "wet chambers" (hyperbaric chambers filled with water) and then in open water.
The test subjects? The military divers themselves. For months, they performed series of dives with extreme profiles (deep dives, close repetitive dives, intense bottom efforts) to validate the robustness of the tables. After these dives, doctors actively looked for "silent bubbles" (asymptomatic micro-bubbles) using Doppler detectors to ensure the planned stops were sufficient.
Adoption by Recreational Diving
Initially designed for military personnel in excellent physical condition, the MN90s were quickly adopted by the French Underwater Sports Federation (FFESSM). A choice that might be surprising, but is explained by the rigor of their design and their readability.
To adapt them to a recreational audience (and therefore less trained), safety margins were implicitly added via the usage instructions: strict compliance with ascent speed (15 to 17 m/min max, today often reduced to 15 m/min or less), no holding your breath, etc.
A living legacy
Today, even though the dive computer reigns supreme, the MN90s remain the teaching standard in France for levels N2 to N4. Understanding these tables is a bit like paying tribute to those French Navy divers who lent their lungs to define the safety standards we still apply today.
Continue with MN90 resources
To move from history to practice, open the interactive MN90 tables, read the guide to reading dive tables, or compare MN90 vs MT2019.