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

Gas PP02

% atm..

1 atm..

Total gas psi

PPg psi

PPg atm.

2 atm..

Total gas psi

PPg psi

PPg atm.

02 .2 14.7 psi 2.94psi

.2 atm..

29.4 psi 5.9 psi

.4 atm.

N2 .8 14.7 psi 11.76 psi

.8 atm.

29.4 psi 23.52 psi

1.6 atm.

Calculating PP02 at depth

Calculate PP02 in air at a depth of 66fswPressure atm.. =

(depth + 33)/33Pressure atm. =

(66+33)/33Pressure 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 depth20%

Pressure in atms. @ 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 atms. @ 99 fsw (99' + 33‘)/33’ = 4atm.

Surface Equivalent of 2% CO2 @ 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 ??????.