Dalton's Law

The gas law stating that total pressure of a mix equals the sum of its partial pressures, the basis for nitrox limits and narcosis predictions.

Dalton's law states that the total pressure of a gas mixture is the sum of the pressures each gas would exert alone, its partial pressures. Each component behaves as if it filled the space by itself. For divers this is the accounting rule of breathing gas: air at any depth is still 21 percent oxygen and 79 percent nitrogen, but the pressure each gas delivers to your body climbs in lockstep with depth.

The math is a multiplication: partial pressure equals the gas fraction times the ambient pressure in ATA. Air at 30 m (4 ATA) delivers oxygen at 0.84 ATA and nitrogen at about 3.16 ATA. This is how divers compute the maximum operating depth of a nitrox mix, why EAN32 at a PO2 limit of 1.4 tops out at 33.7 m, and why narcosis on air deepens predictably with depth even though the mix never changed.

Every gas planning decision in nitrox and technical diving, from choosing a bottom mix to scheduling a deco gas switch, is Dalton's law applied with a slate or a dive computer.