DALTON'S LAW: The total pressure exerted by a mixture of gases is the sum of the pressures that would be exerted by each of the gases if it alone were present and occupied the total volume.

Simply stated, Dalton's Law declares that in a mixture of gases, the whole is equal to the sum of its parts. Air is a mixture of 78% nitrogen and 21% oxygen at one atmosphere, or at sea level. By Dalton's Law 78% of atmospheric pressure is due to the nitrogen, with oxygen comprising the other 21% of the same 14.7 psi. and the remainder being trace elements.

Mathematically The Partial Pressure Of Air Comes Out To:

The Partial Pressure of Nitrogen equaling 78% of 14.7 or 11.6 psi

The Partial Pressure of Oxygen is 21% of 14.7 or 3.09 psi.

The partial pressure of a gas may be insignificant at atmospheric pressure, or 1 ATM, it can become significant as the pressure and density increases.

For example: 2% carbon dioxide in a mixture at 5 ATM, or 132 feet, will have the same physiological effect as 10% carbon dioxide at the surface. This is known as its SURFACE EQUIVALENT VALUE or SEV. The physiological effect of a gas at a given partial pressure at depth is the same as if it breathed at the surface.

The most dangerous gas to the diver is carbon monoxide. It can be seen that if 0.02% carbon monoxide is introduced into a scuba tank by a bad compressor then at 5 ATM its SEV would be 0.10%.

To determine the SEV of the carbon monoxide take 0.02 X 5 ATM = 0.1%

Gas diffusion is the process of the mixing of gas molecules. When two gases are placed together in the same container, they will mix completely, though the weight of one molecule may be heaver than the other. This takes place because of the constant motion of the molecules. Gases tend to diffuse or move through the surface of a liquid (from air to liquid), throughout a liquid (mix in a liquid), as well as through membranes (pass into a cell) of the body, and many other materials.

We are interested in how much of a gas is taken in by the body. To determine the amount of a gas that will diffuse through a membrane depends on its partial pressure on both sides of the membrane. If the partial pressure is higher on one side, the gas will tend to move to the other, until both sides are equal. When a liquid is exposed to gas, molecules of that gas will diffuse into the liquid. This process is similar to that of gases passing through a membrane. However, the point of equilibrium is influenced by the solubility of the gas to that liquid. A physicist name Henry studied these effects and came up with Henry's Law..

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