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