2
30 NAUI Nitrox Diver
Dalton’s Law can be expressed another way: “The
partial pressure of any component gas in a mixture is
the fraction of that gas in the mixture times the total gas
pressure.” Expressed as a formula:
Pg = Fg x Ptotal
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 no-decompression 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
Depth
(meters)
Depth
(feet)
Water
Pressure
Absolute
Pressure
0 0 0 1 ata
10 33 1 atm 2 ata
20 66 2 atm 3 ata
30 99 3 atm 4 ata
40 132 4 atm 5 ata
Figure 2-6 Absolute pressure equals water pressure
plus atmospheric pressure.
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 (Figure 2-6). 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.
Atmospheres absolute is usually abbreviated “ata” or
“ATA” to distinguish it clearly from “atm,” which could
also mean gauge pressure. Many short-handedly use
“ata” as a word in itself - usually in the plural form “atas.”
Don’t be taken aback if someone asks: “What’s the Pee
Oh Two of Eee A En thirty-two at four atas?” As you’ll
discover shortly, the answer is 1.28 atmospheres.
Converting Between Depth and Pressure
by Formula
Converting depth to pressure uses a simple equation in
which you first find the water (hydrostatic) pressure in
atmospheres and then add one atmosphere for the sealevel
air pressure to convert to the absolute pressure.
Written as an equation using the U.S./Imperial system:
Pata = D f s w + 1 atm
33 fsw / atm
where P = the pressure in atmospheres absolute
D = the depth in feet of seawater