How NDL is calculated (compartments)

Your dive computer uses nitrogen compartments to track gas absorption. It is a mathematical model of the human body.

Most computers use algorithms like Bühlmann ZHL-16C to simulate different tissues. These models divide your body into 16 distinct theoretical compartments because each tissue absorbs nitrogen at a different rate. While some compartments fill quickly during a shallow dive, others require much longer durations to reach saturation levels. This affects your NDL directly. You might have plenty of time in your lungs, although your slow-tissue compartments are already nearing their limit.

Always watch your ascent rate. I saw a diver in Koh Tao in 2019 ignore his computer because he thought his NDL was high, but his fast tissues were actually saturated. Never rely on a single tissue model without understanding how depth changes these values.