Dalton's Law
“The total pressure of a gas in a container is the sum of
the partial pressures of the individual gases in the container.” – John Dalton
Ptotal = P1 + P2 + P3 + . . .
P1 P2 P3 are the partial pressures of the various gases in the mixture.
In a mixture of gases, each individual component gas contributes to the total pressure of the mixture. The pressure exerted by each gas is called the partial pressure of that gas. Dalton’s Law of Partial Pressures is important to diving because the gas mixture that a diver breathes at depth is delivered at the ambient/surrounding pressure by the SCUBA regulator. As a diver descends the partial pressure of each component gas in that mixture increases as the ambient pressure increases.
The atmospheric air we breathe is approximately 21% oxygen and 78% nitrogen. Another way to express these amounts are as fractions: 0.21 and 0.78. As a diver descends, individual component pressures increase because the total absolute pressure increases. Therefore at 3 ATM the component pressure of oxygen would be 0.63 and nitrogen 2.34. At elevated partial pressures oxygen, nitrogen, and carbon monoxide can all have adverse and even dangerous effects on a diver. Nitrogen is the gas that SCUBA is most concerned about. This is another example that illustrates the vital importance of understanding the laws of SCUBA physics.
Boyle’s Law explains the inverse relationship of breathing gas and depth: as a diver descends breathing gas becomes denser. This increases exponentially with depth.
Dalton’s Law explains another aspect of diving at depth. At 5 atm (132 ft) the gas is denser by a factor of 5, therefore the component pressures of nitrogen would increase to 3.9% and oxygen to 1. 05%.
Also, a quantity of breathing gas that would last for 1 hr at the surface, would only last 12 minutes at that depth: disregarding descent/ascent time. That is why monitoring depth, dive time and tank pressure is crucial in diving.