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The formula for expressing Dalton's law is: PTotal = ppa + ppB + ppC+ -

The formula for expressing Dalton's law is: PTotal = ppa + ppB + ppC+ -

Where: A, B, and C are gases and PTotal X "/oVoU

Another method of arriving at the same conclusion is to use the T formula. When using the T formula, there can be only one unknown value. Then it is merely a case of multiplying across, or dividing up to solve for the unknown value.The T formula is illustrated as:

partial pressure atmosphere(s) absolute | % volume (in decimal form)

Sample Problem 1. Use the T formula to calculate oxygen partial pressure given 10 ata and 16 percent oxygen.

1. Fill in the known values:

2. Multiply the pressure by the volume to solve for the oxygen partial pressure (pp):

The oxygen partial pressure is 1.6.

Sample Problem 2. What happens to the breathing mixture at the operating depth of 130 fsw (4.93 ata)? The air compressor on the ship is taking in air at the surface, at normal pressure and normal mixture, and sending it to the diver at pressure sufficient to provide the necessary balance. The composition of air is not changed, but the quantity being delivered to the diver is five times what he was breathing on the surface. More molecules of oxygen, nitrogen, and carbon dioxide are all compressed into the same volume at the higher pressure. Use Dalton's law to determine the partial pressures at depth.

1. Calculate the oxygen partial pressure at depth. pp02 _ .21 (surface) x 4.93 ata

2. Calculate the nitrogen partial pressure at depth. ppN2 _ .79 (surface) x 4.93 ata

3. Calculate the carbon dioxide partial pressure at depth. ppC02 _ .0003 (surface) x 4.93 ata

2-12.1.1 Expressing Small Quantities of Pressure. Expressing partial pressures of gases in atmospheres absolute (ata) is the most common method employed in large quantities of pressure. Partial pressures of less than 0.1 atmosphere are usually expressed in millimeters of mercury (mmHg). At the surface, atmospheric pressure is equal to 1 ata or 14.7 psia or 760 mmHg. The formula used to calculate the ppCO2 at 130 fsw in millimeters of mercury is: