Partial pressures and surface equivalents
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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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Presentation Transcript

Why do we care
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
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
Main Points

  • Definition of Partial Pressure

  • Calculating the partial pressure of a gas

  • Calculating surface equivalents


Partial pressure
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

Calculating PPg in atm..

Multiply %g in atm.. x total # atm..

Calculating PPg in psi

Multiply PPg atm. x 14.7 psi



Calculating pp02 at depth
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
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


Surface equivalents1
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
Have We Covered

  • Definition of Partial Pressure

  • Calculating the partial pressure of a gas

  • Calculating surface equivalents


Are you able to
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
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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