2-24 U.S. Navy Diving Manual — Volume 1 2-12 GAS MIXTURES If a diver used only one gas for all underwater work, at all depths, then the general gas law would suffice for most of his necessary calculations. However, to accom - modate use of a single gas, oxygen would have to be chosen because it is the only one that provides life support. But 100 percent oxygen can be dangerous to a diver as depth and breathing time increase. Divers usually breathe gases in a mixture, either air (21 percent oxygen, 78 percent nitrogen, 1 percent other gases) or oxygen with one of the inert gases serving as a diluent for the oxygen. The human body has a wide range of physiological reactions to various gases under different conditions of pressure and for this reason another gas law is required to predict the effects of breathing those gases while under pressure. 2-12.1 Dalton’s Law. Dalton’s law states: “The total pressure exerted by a mixture of gases is equal to the sum of the pressures of each of the different gases making up the mixture, with each gas acting as if it alone was present and occupied the total volume.” In a gas mixture, the portion of the total pressure contributed by a single gas is called the partial pressure (pp) of that gas. An easily understood example is that of a container at atmospheric pressure (14.7 psi). If the container were filled with oxygen alone, the partial pressure of the oxygen would be one atmosphere. If the same container at 1 atm were filled with dry air, the partial pressures of all the constituent gases would contribute to the total partial pressure, as shown in Table 2 - 3 . If the same container was filled with air to 2,000 psi (137 ata), the partial pressures of the various components would reflect the increased pressure in the same proportion as their percentage of the gas, as illustrated in Table 2 - 4 . Table 2‑3. Partial Pressure at 1 ata. Gas Percent of Component Atmospheres Partial Pressure N 2 78.08 0.7808 O 2 20.95 0.2095 CO 2 .03 0.0003 Other .94 0.0094 Total 100.00 1.0000 Table 2‑4. Partial Pressure at 137 ata. Gas Percent of Component Atmospheres Partial Pressure N 2 78.08 106.97 O 2 20.95 28.70 CHAPTER 2 — Underwater Physics 2-25 Gas Percent of Component Atmospheres Partial Pressure CO 2 .03 0.04 Other .94 1.29 Total 100.00 137.00 The formula for expressing Dalton’s law is: Pp pp pp p To ta lA BC = +++ ... Where: A, B, and C are gases and pp PV ol 1.00 A To ta l A = × % A simple method to solve problems of Dalton’s law is to arrange the variables in a “T” formula. To use the T formula there can only be one unknown value; Multiply the known values if the unknown value is partial pressure or divide if the unknown is ata or volume of gas. The T formula is illustrated as: partial pressure ata % of Gas (in decimal form) Sample Problem 1. Use the T formula to calculate the partial pressure of oxygen given in air at 190 fsw. 1. Convert feet of salt water to ata: 190 fsw + 33 = 6.75 ata 33 2. Convert the percentage of oxygen in air to decimal: 21% = .21 pp02 100 3. Substitute known values: pp 6.75 .21 2. Multiply the pressure by the volume to solve for pp: 6.75 x .21 = 1.41 ppO2 Sample Problem 2. In diving we have the option of using gas mixtures other than air. However, we must control the level of oxygen in those mixtures to avoid

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