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. 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.

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.

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

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

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

%g and PPg in Air GasPP02 % atm.. 1 atm.. Total gas psi PPg psi PPg atm. 2 atm.. Total gas psi PPg psi PPg atm psi2.94psi.2 atm psi5.9 psi.4 atm. N psi11.76 psi.8 atm psi23.52 psi 1.6 atm.

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

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 297 fsw (297' + 33‘)/33’ = 10 atm PP02 in air at 297 fsw.2 x 10atm. = 2atm

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 99 fsw (99' + 33‘)/33’ = 4atm. Surface Equivalent of 2% 99fsw.02 x 4atm. =8% or.08 atm.

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

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.

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 ??????.