Cited Passage
Physics of Diving 2-13
Gas can dissolve in water and fat in the human body as
they make up a large percentage of the body’s total mass.
The deeper one dives, the greater the pressure exerted upon
the body, and the higher the total pressure of the breathing
gas. It follows that more gas will dissolve in the body tis-
sues. During ascent, the dissolved gases will begin to be
released.
If a diver’s rate of ascent (including decompression
stops) is controlled properly, the dissolved gas will be car-
ried to the lungs by the tissue’s blood supply and will be
exhaled before it accumulates and forms bubbles in the tis-
sues. If, on the other hand, ascent is too rapid and/or
decompression stops are missed or reduced so that the
pressure is reduced at a rate higher than the body can
accommodate, gas bubbles may form, disrupting body tis-
sues and systems, and producing a condition known as
decompression sickness (the bends).
The various gases are dissolved in the body in proportion
to the partial pressure of each gas in the breathing medium.
The amount of gas dissolved is also governed by the length of
time and the pressure at which you breathe it. However, as
gases vary in their solubility, the exact amount dissolved
depends on the specific gas in question. If a diver breathes a
gas long enough, his body will become saturated; but this
occurs slowly. Depending on the gas, it will take anywhere
from 8 to 24 hours.
Some gases are more soluble than others and some liq-
uids are better solvents than other liquids. For example,
nitrogen is five times more soluble (on a weight-for-weight
basis) in fat than in water. These facts and the differences
in blood supply have led to the postulate of tissues with dif-
ferent saturation halftimes (5-minute tissues, 10-minute tis-
sues, 20-, 40-, 75-, etc.). This serves as the basis for
calculating decompression tables.
2.8.5 General Gas Law
Pressure, volume, and temperature are interrelated. A
change in one factor must be balanced by a change in one
TABLE 2.6
Air at 14.7 psi (1 atm)
Percent of Partial Pressure Partial Pressure
Air component atm psi
N
2
78.08% .7808 atm 11.478 psi
O
2
20.95% .2095 atm 3.080 psi
CO
2
.03% .0003 atm .004 psi
Other .94% .0094 atm .138 psi
Total 100.00% 1.000 atm 14.700 psi
TABLE 2.7
Air at 2,000 psi (136.05 atm)
Percent of Partial Pressure Partial Pressure
Air component atm psi
N
2
78.08% 106.23 atm 1561.6 psi
O
2
20.95% 28.50 atm 419.0 psi
CO
2
.03% .04 atm .6 psi
Other .94% 1.28 atm 18.8 psi
Total 100.00% 136.05 atm 2000.0 psi
Absolute Pressure in Atmospheres
1 ata
sea level
2 ata
33 ft
3 ata
66 ft
4 ata
99 ft
FIGURE 2.7
Partial Pressure
PP = Partial Pressure
P
t
= Total Pressure
= O
2
= N
2
Partial Pressure % Density
PPO
2
= 0.2 ata O
2
= 20%
PPN
2
= 0.8 ata N
2
= 80%
P
t
= 1.0 ata Total = 100%
PPO
2
= 0.4 ata O
2
= 20%
PPN
2
= 1.6 ata N
2
= 80%
P
t
= 2.0 ata Total = 100%
PPO
2
= 0.6 ata O
2
= 20%
PPN
2
= 2.4 ata N
2
= 80%
P
t
= 3.0 ata Total = 100%
PPO
2
= 0.8 ata O
2
= 20%
PPN
2
= 3.2 ata N
2
= 80%
P
t
= 4.0 ata Total = 100%
Twice as dense
as the surface at
2.0 ata - 33 fsw
Three times as
dense as the surface
at 3.0 ata - 66 fsw
Four times as dense
as the surface at
4.0 ata - 99 fsw
Partial pressure of a 20/80
mixture of oxygen/nitrogen at 1
ata (sea level), 2 ata (33 feet), 3
ata (66 feet), and 4 ata (99 feet)
Air Composition
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