where Pg is the partial pressure of the component gas;
Fg is the fraction of the component gas in the mixture;
and Ptotal is the total pressure of the gas mixture.
A useful way to state this for recall purposes is simply:
“The part is a fraction of the whole.” (In formulas,
“is” translates into “equals” and “of” into “times.”)
You will meet this statement and its formulation several
times and in many guises in this book.
The partial pressure of a gas is abbreviated using the
chemical symbol for the gas. PO2 means partial pressure
of oxygen, and PN2 means partial pressure of nitrogen.
Some scientific conventions use ppO2 and ppN2.
When using enriched air nitrox, we need to be aware
of the partial pressures of nitrogen and oxygen to which
our body is being exposed. By knowing our exposure to
nitrogen, we are able to estimate how much inert gas
(nitrogen) we will take up in our tissues compared to
breathing air, and we can plan our dive times accordingly.
By knowing our exposure to oxygen, we are able to
control and limit the risk of oxygen toxicity from too
high a partial pressure of oxygen. We take the nitrogen
partial pressure into consideration to determine our nodecompression
limits and our tissue nitrogen levels at the
end of the dive. We take the oxygen partial pressure into
consideration to determine our safe oxygen exposure.
It is easy to calculate the partial pressure of a gas if
we know the fraction of the component gas in the total
mixture and the absolute pressure.
Determining absolute pressure during a dive is a
simple conversion from the depth into an appropriate
pressure unit (usually atmospheres). For any depth, we
can find the absolute pressure, and then, knowing that
“the part is a fraction of the whole,” we find the partial
pressure of the gas at that depth.
CONVERTING BETWEEN DEPTH AND
PRESSURE
Recall from your entry-level scuba course the difference
between absolute pressure and gauge pressure.
Depth is a gauge pressure. Your depth gauge reads zero
at the surface even though you are actually under one
atmosphere of air pressure. Each 10 meters (33 feet)
that you descend in the ocean adds another atmosphere
of pressure to the one atmosphere of surface pressure.
The absolute pressure is “absolutely everything,” and it
is one atmosphere more than what your depth gauge is
telling you. When you are at a depth 20 meters of seawater
(msw) / 66 feet of seawater (fsw), the absolute
pressure is three atmospheres–the two atmospheres of
water pressure plus the one atmosphere of air pressure.
In diving calculations, it is always necessary to consider
absolute pressures.
In diving using the metric system, the bar is the
common pressure unit instead of atmospheres. Although
a bar is just slightly less than an atmosphere, the difference
is negligible for diving calculations and you will
usually find bars and atmospheres used interchangeably.
Ten msw equals one bar.
NAUI Nitrox Diver
22 Gases & Gas Mixtures
FIGURE 2-6: ABSOLUTE PRESSURE EQUALS WATER PRESSURE PLUS
ATMOSPHERIC PRESSURE
DEPTH
FEET
0
33
66
99
132
DEPTH
METERS
0
10
20
30
40
WATER
PRESSURE
0
1 atm
2 atm
3 atm
4 atm
ABSOLUTE
PRESSURE
1 ata
2 ata
3 ata
4 ata
5 ata