Nitrogen saturation
Play a dive and compare the nitrogen in the ambient air with the nitrogen stored in your body: it loads at depth, then off-gasses as you ascend. A teaching tool: it replaces neither a dive computer nor tables.
Your dive
Three settings are enough. Change them and watch what moves on the chart.
More settings
Watch nitrogen go in and out
Why ×5.3 at 40 m, and not ×5?
At 40 m, ambient pressure is 5 bar, 5 times the surface pressure. You might think the body can hold 5 times more nitrogen.
But the Bühlmann model accounts for water vapour in the lungs. It always takes up about 0.063 bar (0.0627 exactly), whatever the depth. Inspired nitrogen is calculated on what is left:
PiN₂ = (Pamb − 0.0627) × 0.79
At 40 m: (5 − 0.0627) ÷ (1 − 0.0627) = 4.937 ÷ 0.937 ≈ ×5.3
This holds at every depth: ×2.1 at 10 m, ×3.1 at 20 m, ×6.3 at 50 m.
Careful, it is not instant: this is the most the tissues could hold at that depth. Each one takes up nitrogen at its own pace: how much is actually stored depends on the depth and length of the dive.
Detailed view: the body does not absorb nitrogen at the same speed everywhere. Fast tissues (blood, brain) fill and empty in a few minutes, slow tissues (fat, bone) take hours: they even keep loading a little above 12 m.
The same tissues as curves
Dotted line: the nitrogen breathed. Fast tissues follow it closely, slow ones barely.
Going further: the 16 compartments of the model
The Bühlmann model splits the body into 16 compartments. Each bar shows how close one of them is to its limit (its M-value). ▲ loading, ▼ off-gassing. Tap a bar to follow it.
How the Bühlmann model works
16 compartments, 16 speeds
The model does not know the real tissues of the body. It represents it with 16 theoretical compartments, each with its own half-time, from 5 min to 635 min: the time it takes to cover half the way to balance with the nitrogen breathed. After 6 half-times, a compartment is 98 % saturated. Fast ones are reminiscent of blood or the brain, slow ones of fat or bone: it is an image, not anatomy.
Loading and off-gassing
A compartment loads when the partial pressure of nitrogen in the ambient air (PiN₂) is higher than its own nitrogen pressure. It off-gasses when it is lower. The bigger the gap, the faster it goes. The calculation uses the Haldane equation at constant depth and the Schreiner equation during descent and ascent.
The limit: the M-value
As you ascend, a compartment can hold more nitrogen than the ambient air: it is supersaturated. Each compartment tolerates a maximum supersaturation, its M-value (M = a + Pamb / b). The slower it is, the lower its tolerance: at the surface, 2.96 bar for compartment 1, 1.27 bar for compartment 16. A dive computer plans the ascent and stops so that no compartment exceeds its M-value. It often adds a margin, gradient factors, which are not used here. The compartment closest to its limit drives the ascent: it is the leading compartment.
Why the off-gassing zone starts at 12 m by convention
In the model, there is no single depth where everything empties. Each compartment off-gasses as soon as PiN₂ drops below its own nitrogen pressure, and that moment depends on its load. After 40 m for 20 min on air, the heavily loaded fast compartments off-gas from the very start of the ascent. The lightly loaded slow ones keep loading a little, even at a 3 m stop: compartment 16 is at 0.82 bar there while the air breathed brings 0.98.
The 12 m limit is therefore a teaching convention. After a saturating dive, we consider that you still load below 12 m: do not linger between the bottom and 12 m. Above it, the compartments that matter for decompression off-gas.
Why slow tissues do not lengthen the stops
Above 12 m, slow tissues may keep loading, but they stay far from their limit. On air, PiN₂ is 0.98 bar at 3 m and 1.21 bar at 6 m. Even if the diver stayed at the stop forever, a slow compartment could not exceed that value. And it stays below its surface M-value: 1.27 bar for compartment 16, 1.40 bar for compartment 11. It will therefore never reach critical supersaturation, and the computer does not lengthen the stop for it. Stops are driven by the fast and medium compartments, which do off-gas. After 40 m for 20 min and 5 min at 3 m, compartment 16 is only at 64 % of its M-value on surfacing, and the leading compartment is number 4 (half-time 18.5 min).
Understanding nitrogen saturation
1. Nitrogen follows pressure, with a delay
You breathe nitrogen at a pressure that depends on depth and gas (PiN₂). The body loads as long as it holds less, and off-gasses as soon as it holds more. That is saturation, then desaturation.
2. Each tissue has its own pace: the half-time
In one half-time, a compartment covers half the way. After 6 half-times, it is almost full. The fastest has a half-time of 5 min, the slowest 635 min (over 10 h).
3. Each tissue has its limit: the M-value
It is the maximum nitrogen the model tolerates at a given pressure: M = a + Pamb / b. The half-time says how fast a tissue changes, the M-value says how far it can go.
4. Switching gas speeds up off-gassing
A gas rich in oxygen drops the nitrogen breathed at once. The body does not empty at once: it is the bigger gap that pushes nitrogen out faster. To choose your mix and its maximum depth, use the nitrox calculator; to prepare it, the gas blender.
Model assumptions and limits
- Bühlmann ZH-L16C model, nitrogen only, 16 compartments (Bühlmann 2002 a and b coefficients, 5-minute compartment 1).
- Pamb = Psurface + depth / 10; PiN₂ = (Pamb − 0.0627) × FN₂.
- Exact calculations (Haldane and Schreiner equations), independent of the animation.
- Diver with no excess nitrogen at the start. Descent at 20 m/min. After surfacing, the diver breathes air.
- No gradient factors or helium in this version.
- “Water”: partial pressure of nitrogen breathed at this depth with the current gas. “Body”: in the simple view, a reference tissue of the model (compartment 3, half-time 12.5 min), chosen so the loading shows clearly over a dive; in the detailed view, 3 tissue families and the average of the 16 compartments. ×1 = nitrogen breathed on air at the surface.
- Simple view: the 12 m teaching rule. Below 12 m you load, above it (at the end of the dive) you off-gas. The loading speed (line colour and thickness) follows the gap between the nitrogen in the water and in the body: it grows with depth and shrinks as the body fills.
- Discovery view: no calculation, an illustration. At the bottom, the body approaches balance with the ambient air, faster the deeper it is, and almost reaches it at the end of the bottom time. Above 12 m, it gives its nitrogen back.
- Detailed view: calculation by tissue family. Fast tissues empty deeper, slow ones shallower.
- A model is not physiology: staying below M-values does not guarantee you will avoid an accident. No ascent instructions are given here.
Frequently asked questions
What is nitrogen saturation in diving?
At depth, you breathe nitrogen at a higher pressure than at the surface. The body absorbs it until it reaches balance with the gas breathed: it is then saturated. During the ascent, the pressure drops and nitrogen leaves through the lungs: that is desaturation, or off-gassing.
Why do we talk about 12 m for off-gassing?
It is a teaching convention. After a saturating dive, we consider that you still load below 12 m and off-gas above it. In the model, each tissue switches at its own moment; slow tissues can even load a little above 12 m, without ever approaching their limit.
Why 5.3 times more nitrogen at 40 m, and not 5 times?
At 40 m, pressure is 5 times the surface pressure. But the Bühlmann model subtracts water vapour in the lungs, about 0.063 bar at any depth: (5 − 0.063) ÷ (1 − 0.063) ≈ 5.3. Tissues can therefore hold up to 5.3 times more nitrogen than at the surface, but only given enough time: each loads at its own pace.
Does nitrox reduce nitrogen loading?
Yes. Nitrox contains less nitrogen than air: at the same depth, the body loads more slowly and surfaces with less excess nitrogen. In exchange, its maximum operating depth (MOD) is shallower. The tool compares each nitrox dive with the same dive on air.
What is the half-time of a compartment?
It is the time a model compartment needs to cover half the way to balance with the nitrogen breathed. Bühlmann ZH-L16C uses 16 compartments, with half-times from 5 min to 635 min. After 6 half-times, a compartment is 98 % saturated.
What is the M-value?
It is the maximum amount of nitrogen a compartment can tolerate at a given pressure according to the model (M = a + Pamb / b). A dive computer plans the ascent and stops so that no compartment exceeds it.
Can this tool plan my dive?
No. It is a teaching tool: it never calculates mandatory stops and gives no ascent instructions. To dive, follow your dive computer or tables, and your training.