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Partial Pressures and Surface Equivalents

Partial Pressures and Surface Equivalents. Why Do We Care?. Typically when diving we breathe a mixed gas. Our body cares more about the partial pressure of a gas than about the % of the gas in the overall mix.

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Partial Pressures and Surface Equivalents

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  1. Partial Pressures and Surface Equivalents

  2. Why Do We Care? • Typically when diving we breathe a mixed gas. • Our body cares more about the partial pressure of a gas than about the % of the gas in the overall mix. • During descent or ascent, divers are affected by changes in the partial pressure of a gas. • To understand and predict the effect of these changes, think in terms of surface equivalents.

  3. Objectives • Given the percentage of an individual gas within a mixed gas, correctly calculate the partial pressure of a gas, in atm. or psi, for a given depth. • Given the percentage of an individual gas within a gas mixture, and the depth at which it is being used, correctly calculate the surface equivalent.

  4. Main Points • Definition of Partial Pressure • Calculating the partial pressure of a gas • Calculating surface equivalents

  5. Partial Pressure • Pressure exerted by each individual gas within a mixed gas. • Expressed as PPg where g is the gas • Partial Pressure of Oxygen PP02 • Partial Pressure of Nitrogen PPN2

  6. Calculating PPg Calculating PPg in atm.. Multiply %g in atm.. x total # atm.. Calculating PPg in psi Multiply PPg atm. x 14.7 psi

  7. %g and PPg in Air

  8. Calculating PP02 at depth Calculate PP02 in air at a depth of 66fsw Pressure atm.. = (depth + 33)/33 Pressure atm. = (66+33)/33 Pressure atm. = 3atm. PP02 atm. = 3atm.. X .2 =.6 atm.. PP02 psi = .6 atm. x 14.7 psi = 8.82 psi

  9. Surface Equivalents • Breathing air at 297 feet of sea water (fsw) is the same as breathing what PP02 at sea level? % of 02 in air at any depth 20% Pressure in atms. @ 297 fsw (297' + 33‘)/33’ =10 atm PP02 in air at 297 fsw .2 x 10atm. =2atm

  10. Surface Equivalents Breathing air with 2% C02 at 99' sea water, what is the surface equivalent? % C02 in this mix at any depth 2% or .02atm. Pressure in atms. @ 99 fsw (99' + 33‘)/33’ = 4atm. Surface Equivalent of 2% CO2 @ 99fsw .02 x 4atm. = 8% or .08 atm.

  11. Have We Covered • Definition of Partial Pressure • Calculating the partial pressure of a gas • Calculating surface equivalents

  12. Are you able to • Given the percentage of an individual gas within a mixed gas, correctly calculate the partial pressure of a gas, in atm. or psi, for a given depth. • Given the percentage of an individual gas within a gas mixture, and the depth at which it is being used, correctly calculate the surface equivalent.

  13. Summary • Typically when diving we breathe a mixed gas. • Our body cares more about the pressure of the gas than about the % of the gas in the mix. • Even though the % of a gas does not change with depth, the partial pressure of the gas does change. • The effects of a gas are determined by the partial pressure of the gas and this explains phenomena such as ??????.

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