Dalton's Law — What It Is and Why It Matters
Dalton's law states that the total pressure of a gas mixture equals the sum of the partial pressures of each individual gas. In diving, this means that as you descend and total pressure increases, the partial pressure of each gas in your breathing mix increases proportionally — even though the percentages remain the same.
Dalton's law is the foundation for understanding oxygen toxicity (ppO2 increases with depth), nitrogen narcosis (ppN2 increases with depth), and why different gas mixes are used at different depths. It is the reason you calculate maximum operating depth based on oxygen fraction and ppO2 limits.
Frequently Asked Questions
How does Dalton's law relate to nitrox diving?
Dalton's law explains why nitrox has a shallower maximum operating depth than air. With a higher oxygen fraction (32% vs 21%), the ppO2 reaches the 1.4 bar safety limit at a shallower depth. For EAN32: 1.4 / 0.32 = 4.375 ATA, or about 33 meters — compared to about 56 meters on air.
Does Dalton's law only apply to oxygen?
No. It applies to every gas in the mixture. Nitrogen partial pressure increases with depth (causing narcosis), oxygen partial pressure increases (risking toxicity), and helium partial pressure increases in trimix (relevant for decompression calculations). The law governs them all equally.
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