Air Pressure
Effects of Unequal Air Pressures While Scuba Diving: Ear Squeeze,
Sinus Squeeze, Air Embolism and Other Forms of Barotrauma WHAT
DOES BOYLE'S LAW PREDICT ABOUT UNEQUAL PRESSURES?
Once again we come to all-important Boyle's law: for a given mass of
gas at a constant temperature, the product of pressure (P) and
volume
(V) is constant (K):
P x V = K.
We have already seen how, for the breath-hold diver, Boyle's law
predicts that compressible air spaces will shrink on descent and
re-expand on ascent, and that the situation is different for scuba
divers because compressed air is continuously inhaled. Even if the
scuba and breath-hold diver go to the same depth and spend the same
amount of time under water (e.g., one minute), the effects of water
pressure are radically different on the two divers. Because tank air is
inhaled at the ambient pressure, the scuba diver's lungs and other
compressible air spaces do not shrink.
Consider that all the body's air-containing spaces are in contact with
inhaled air. At the same time, there is a natural tendency for air
anywhere in the body - in the lungs, middle ears, sinuses and other
spaces - to diffuse into the blood. As air is absorbed into the blood
it
is replenished by fresh air inhaled from the scuba tank.
If the absorbed air was not replenished the air spaces would, over
time,
shrink and close. (This in fact happens in some lung diseases. Any
part of a lung that becomes plugged, such as from mucus or a tumor,
will shrink completely until it becomes airless.) Shrinkage and
closure
of an air space will not happen as long as it is in contact with a fresh
supply of air. Does Boyle's law still apply to such spaces?
Unequivocally, yes. When fresh air enters the diver's lungs it is at
the
same pressure as the surrounding water pressure. This pressure, of
course, is higher than at sea level and so the air is correspondingly
denser.
As long as the diver continuously breathes from the scuba tank, the
density of inhaled air will change with ascent and descent. For this
reason the "given mass of air" stipulated in Boyle's law, which is
really
the number of air molecules, changes as the depth changes. On
descent the "given mass" increases as the inhaled air becomes denser;
on ascent the "given mass" decreases as the inhaled air becomes less
dense. Stated another way, when breathing compressed air under
water, the actual number of air molecules, in the lungs and all other
air spaces, increases on descent and decreases on ascent.
Assume two people dive from a boat to a depth of 99 feet; one diver
holds his breath and the other diver uses compressed air (scuba).
Also
assume that the lungs of each diver contain 10 billion air molecules
and the lungs behave like balloons. (Actually, the breath-hold
diver's
lungs, being tethered by a rib cage, don't behave exactly like
balloons.
Also, some gas exchange takes place even with breath holding, since
oxygen is taken up from air in the lungs while a smaller amount of
CO2 is added to that air; as a result, the number of molecules does
not remain exactly the same.)
For the breath-hold diver, the mass of gas (the number of
molecules)
in the air spaces remains about the same during the dive, so as water
pressure increases the lungs must shrink (Boyle's law). For the scuba
diver, however, the mass of gas (number of molecules) increases
along
with the increase in water pressure, so the lungs do not shrink.
Thus the lungs (and other air spaces) of a scuba diver at 33 fsw
contain twice as many air molecules as at sea level, and also twice as
many air molecules as a breath-hold diver at the same depth
(allowing
for slight variation due to absorption of oxygen into the blood).
Since the volume of air in the scuba diver's lungs doesn't change, the
density of air (how close the molecules are to one another) must be
greater.
At 99 fsw the breath-hold diver's lungs contain about the same
number of molecules as on the surface, but in only 1/4 the volume;
hence the air, being compressed by the increased pressure, is four
times denser than on the surface. At 99 fsw the scuba diver's lungs
are
filled with air just as dense, but since there are four times the
number
of molecules as on the surface the lung volume is preserved. As long
as there is good communication with inhaled air, the scuba diver's
compressible air spaces will fill up with the air extra molecules and
will not shrink.
The most critical result of Boyle's law occurs when the mass of gas is
fixed, as would occur in the lungs of a scuba diver if breath is held. If
the scuba diver was to breath-hold and ascend, the fixed mass of gas
at
the time of breath-hold would inexorably expand in volume (as
predicted by Boyle's law) until the diver exhaled or the lungs
ruptured. This, of course, is why a breath-hold ascent from any
scuba
dive is so dangerous.
The glottis is the voice box, located just under your chin inside the
neck. Also called the larynx, it leads directly into the trachea (the
"windpipe") which then leads to the lungs. We "close" the glottis
when we hold our breath, which is why you can't speak and hold
your
breath at the same time. (The universal sign of someone choking on
food is fingers to the throat and inability to speak.)
What happens when the glottis is open or closed relates directly to
the difference between compressible and non-compressible structures.
Non-compressible structures include the blood, bones and all solid
organs; compressible structures include the lungs, sinuses, middle
ear, and the hollow organs such as the stomach and intestines.
A scuba dive with breath held (glottis closed) is tantamount to a
breath-hold dive, the lungs will be squeezed by the increased ambient
pressure. An initial volume of 6 liters (e.g., after taking in a regular
breath) could theoretically shrink to only one 1.2 liters at a depth of
132 feet. However, the scuba diver has the option (which should
always be exercised) to continuously breathe compressed air, in
which
case lung volume will stay the same at all depths (allowing for slight
variation with regular breathing). In both situations (glottis closed
and breathing compressed air) the density of air in the lungs will
increase with increasing depth.
Air in the tank is highly pressurized, approximately 3000 psi
(equivalent to 204 atmospheres) for an 80 cu. ft. tank filled to
capacity. The two-stage regulator allows compressed air to be inhaled
at the ambient pressure, so the scuba diver can maintain normal lung
volume at all depths. (The increased density of the inspired air at
depth is seldom great enough to impair work of breathing during
recreational diving.)
Tank pressure should be at least 500 psi before beginning any ascent
(ideally, one should arrive at the safety stop with no less than 500
psi). Five hundred psi are equivalent to 34 atmospheres. Since the
maximum RSD depth of 130 feet equals only about five atmospheres,
there should always be a large gradient for air to flow from the tank
to
the diver's lungs.
Adventure Dominica