Education

Partial Pressures and Dalton's Law

Dalton's Law is the foundation of all gas physics in diving. Learn how partial pressures work and why every diver should understand them.

Every other gas calculation in diving — MOD, EAD, END, best mix — is built on one foundational principle: Dalton’s Law of Partial Pressures. If you understand this, everything else follows.

What it is

Dalton’s Law states that the total pressure of a gas mixture equals the sum of the partial pressures of each individual gas. In other words, each gas in a mix contributes its own pressure independently, proportional to its fraction of the total.

For divers, this means the pressure of each gas you breathe increases with depth. At the surface, 21% oxygen exerts a partial pressure of 0.21 bar. At 30 meters (4 bar absolute), that same 21% oxygen now exerts 0.84 bar.

The formula

ppX = fX × P
VariableMeaning
ppXPartial pressure of gas X in bar
fXFraction of gas X in the mix (e.g., 0.21 for oxygen in air)
PAbsolute pressure at depth in bar: (depth in meters / 10) + 1

The absolute pressure formula itself is straightforward: every 10 meters of seawater adds 1 bar of pressure, and you start at 1 bar (atmospheric pressure at the surface).

Worked example

What are the partial pressures of each gas on air at 30 meters?

Step by step

P = (30 / 10) + 1 = 4.0 bar
ppO2 = 0.21 × 4.0 = 0.84 bar
ppN2 = 0.79 × 4.0 = 3.16 bar

At 30 meters on air, your oxygen partial pressure is 0.84 bar and your nitrogen partial pressure is 3.16 bar. Both are within safe limits — oxygen toxicity starts around 1.4-1.6 bar, and nitrogen narcosis becomes noticeable around 3.2 bar.

Now consider the same depth on EAN36:

EAN36 at 30 meters

P = 4.0 bar
ppO2 = 0.36 × 4.0 = 1.44 bar
ppN2 = 0.64 × 4.0 = 2.56 bar

The oxygen partial pressure of 1.44 bar is right at the recreational limit. This is why EAN36 has a MOD of only about 28 meters — go any deeper and you exceed safe oxygen levels.

Why it matters

Partial pressure is the mechanism behind almost every physiological effect of breathing gases under pressure:

  • Oxygen toxicity occurs when ppO2 exceeds ~1.4-1.6 bar (CNS toxicity) or during prolonged exposure above ~0.5 bar (pulmonary toxicity)
  • Nitrogen narcosis increases with ppN2 — noticeable above ~3.2 bar, significant above ~4.0 bar
  • Hypoxia occurs when ppO2 falls below ~0.16 bar — relevant for hypoxic trimix at shallow depths
  • Decompression obligation is driven by the partial pressure of inert gases (primarily nitrogen and helium) absorbed into tissues

Understanding partial pressures is not just academic. It is the reason you check your MOD before every nitrox dive and the reason technical divers use helium to manage narcosis at depth.

Safety considerations

  • Oxygen limits: Recreational diving uses a ppO2 max of 1.4 bar during the working portion of a dive. Decompression stops may use up to 1.6 bar
  • The 1.6 bar ceiling: A ppO2 of 1.6 bar is widely considered the absolute maximum for any diving. Many agencies and dive planners treat this as a hard limit
  • Hypoxic mixes: Some trimix blends have less than 21% oxygen and can be hypoxic at the surface. These mixes must not be breathed until the diver has descended to a depth where the ppO2 is at least 0.16 bar. See Minimum Operating Depth
  • Total pressure matters: A gas that is harmless at the surface can be dangerous at depth purely because of increased partial pressure

Sources