2-26 U.S. Navy Diving Manual — Volume 1 exposing divers to harmful side effects of increased ppO2. Use Dalton’s law to determine the maximum O2 % allowed when diving to 300 fsw given a limit of 1.3ppO2. 1. Convert fsw to ata: 300 + 33 = 10.09 33 ata = = 10.09 ata 2. Substitute known values: 1.3 ppO2 10.09 ata % of Gas 3. Divide pp by ata to solve for percent of gas: 1.3 ppO2 = 0.1288 % of Gas 10.09 4. Convert from decimal to percentage: 0.1288 x 100 = 12.88 max % O2 allowed Sample Problem 3. Determine the maximum safe depth of an 11% mix of HeO2 given a 1.3ppO2 limit: 1 . Convert 11% HEO2 to a decimal: 11% = 0.11 100 2. Substitute known values: 1.3 ppO2 ata 0.11 3. Divide pp by percentage of gas to solve ata: 1.3 ppO2 = 11.81 ata 0.11 4. Convert from ata to fsw: (11.81 x 33) - 33 = 356.73 fsw Round down to a max safe depth of 356 fsw CHAPTER 2 — Underwater Physics 2-27 2‑12.1.1 Calculating Surface Equivalent Value (SEV). Dalton’s law explains the potential consequences of exposure to increased partial pressures of various gasses. For example, if the surface air were contaminated with 2 percent (0.02 ata) CO2, a level that could be readily accommodated by a person on the surface, the partial pressure at an increased depth could be dangerously high. The correlation of a gas inspired at depth to its equivalent physiological effect if the same concentration were breathed on the surface is referred to as surface equivalent value (SEV). The formula for calculating SEV is: pp SEV = 1 ata Example: When breathing air on the surface 21% (0.21 ppO2) oxygen is being inspired. At 33 fsw (2ata) the pressure doubles to 0.42ppO2, the percentage by volume stays the same but the number of molecules inspired increased. Move the decimal point 2 places to the right to get a surface equivalent of 42% oxygen. It makes sense that we are breathing twice the molecules of O2 at 33 fsw since we are at twice the pressure. Sample problem 1. Your recompression chamber is on ascent from treatment depth at 1fpm and is at a depth of 127 fsw. The chamber CO2 monitor reads .23% CO2. The limit for chamber CO2 levels is 1.5 SEV. Is the chamber within safe limits for CO2? 1. Calculate ppCO2 at 127 fsw. SEV = ATA x % of gas (decimal form) SEV = 4.84 x .023 CO2 = 1.11 SEV SEV = 1.11 which is lower than 1.5. The chamber is within acceptable limits. Sample problem 2. What is the maximum permissible CO2 reading on the monitor for the same scenario in problem 1?

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