Altitude diving
Your table is made for the sea. In a mountain lake the air weighs less: the depth to look up in your table, your ascent rate and the real depth of your stops.
The lake
Altitude of the water surface. Correct from 300 m.
Pressure from the standard atmosphere: the most cautious of the three methods.
Your dive
Max depth read on your depth gauge, and your mix.
Your table
Ascent rate planned at sea level.
Depth to look up in your table
23m
instead of 20 m real. In the MN90, read the 25 m row.
Stops: real depth in the lake
Step by step
- Lake surface pressure0.899 bar
1.01325 × (1 − 0.0000225577 × 1.000)^5.25588
- Altitude coefficient1.127
1.01325 ÷ 0.899
- Table depth23 m
20.0 × 1.127 = 22.55 → rounded up to the metre
- Real stops2.7 / 5.4 / 8.0 m
3, 6, 9 ÷ 1.127 → rounded up to the decimetre
- Ascent rate13.3 m/min
15 ÷ 1.127 → rounded down to the tenth
- Between stops (MN90)5.3 m/min
6 ÷ 1.127
- ppO2 on the bottom0.61 bar
0.21 × (0.899 + 2.0)
Why it works, and the limits
Decompression depends on the ratio between the pressure on the bottom and the pressure at the surface. At altitude, the surface is below 1 bar: at the same depth, this ratio is larger than at sea. So we look for the sea depth that gives the same ratio (Commander Chauvin’s method).
coefficient = sea pressure ÷ lake pressure
table depth = depth × coefficient
real stop = table stop ÷ coefficient
real rate = table rate ÷ coefficient
- For a nitrox, first work out the equivalent air depth (EAD) at sea, then apply the coefficient.
- Lakes are fresh water: 1 m of fresh water weighs slightly less than 1 m of sea water. It is not counted, which leaves a small margin in your favour.
- Depth gauge: a computer or electronic gauge resets to zero at the surface and reads the right depth in the lake. An old mechanical gauge set for the sea reads too shallow: ask your instructor before relying on it.
- Three ways to get the lake pressure: the standard atmosphere (default, the most cautious), the French course rule “1 − altitude ÷ 10,000”, or the real pressure read on your computer, the most accurate on the day.
- Recent arrival at altitude: your body still carries sea-level nitrogen, as after a first dive. Allow time before diving or follow your agency’s procedure. If your way home crosses a pass higher than the lake, that is another pressure drop: plan it with your instructor.
A calculation aid: it does not replace your training or your computer. Always check with your table.
Understanding altitude diving
Why correct an altitude dive?
Tables such as the MN90 are calculated for a surface at 1 bar. In a mountain lake, the surface pressure is lower: 0.90 bar at 1,000 m, 0.79 bar at 2,000 m. At the same depth, the ratio between bottom and surface pressure is larger than at sea, and decompression more demanding. So the dive is converted into a deeper “sea” dive, the theoretical depth, which you look up in the table.
The Chauvin method, step by step
Coefficient = sea pressure ÷ lake pressure. Theoretical depth = real depth × coefficient, rounded up to the metre. Stops are shallower (stop ÷ coefficient) and the ascent slower (rate ÷ coefficient). French FFESSM course example: 24 m at 2,000 m gives 30 m in the table. The tool also offers the standard atmosphere, more cautious, or the pressure read on your computer.
Nitrox in a mountain lake
With a nitrox, first work out the equivalent air depth at sea, then apply the altitude coefficient. For the EAD alone, see the EAD tool. The bottom ppO2 is calculated with the real lake pressure: check your max depth with the MOD tool and your ppO2 with the PpO2 tool.
Frequently asked questions
How do you work out a theoretical depth at altitude?
Theoretical depth = real depth × (sea pressure ÷ lake pressure). With the course rule (pressure = 1 − altitude ÷ 10,000), 24 m at 2,000 m gives 24 ÷ 0.8 = 30 m.
At what depth should you do stops at altitude?
Real stop = table stop ÷ coefficient. At 3,000 m with the course rule, the 3 m stop is done at 2.1 m and the 6 m stop at 4.2 m.
From what altitude should you correct?
From 300 m. Below that, the pressure difference with the sea is negligible.
Should you correct for fresh water?
No: 1 m of fresh water weighs slightly less than 1 m of sea water. Not counting it leaves a small margin in the diver’s favour, as in the FFESSM course material.