You are surrounded by air. You have heard it many times:
"Air has pressure." So, you are surrounded by pressure. How can this be?
Air is made up of molecules. They are so light they float
around you without sinking to the ground. There are basically 2 groups of
molecules in the air, nitrogen and oxygen. The nitrogen makes up a huge portion
of the air: 78%! About 3/4 of the molecules causing atmospheric pressure
are tiny nitrogen molecules. Oxygen makes up 21% of the air. Combined, nitrogen
and oxygen make up 99% of what is going into your lungs at sea level. All
the other gases make up only 1% of the atmosphere and most of that is argon.
Not only are these molecules floating around you, but
they are vibrating. They do that because they are warm. If you heat them
up they will vibrate more. If you cool them down they will vibrate less.
If you cool them down enough they will stop vibrating entirely. That temperature
is so low (-273 degrees Celsius, -459 degrees F.) scientists have never been
able to get to that point although they have come close.
If you took a vibrating jackhammer and put it against
your arm it would do great damage. Even though we cannot see molecules of
nitrogen and oxygen, billions are vibrating against our bodies and they can
also do great damage! Even though they are tiny, there are so many of them
they are capable of killing a human! If you could see an oxygen molecule
at room temperature next to your skin, the effect on the skin resembles a
bullet fired out of a gun. Although a bullet is bigger, would penetrate,
and would do more damage, the gas molecules number in the billions on each
square inch and in combination they have the capability to do far more damage.
That is where the air pressure comes from: Tiny molecules of nitrogen
and oxygen vibrating against your body.
The atmospheric pressure nitrogen and oxygen molecules
produce has been measured. Draw a square inch on a piece of paper. The molecules
vibrating against it exert a force of 14.7 pounds at sea level. On a common
sheet of paper (8" x 11") there are 88 square inches. The air pushing
against that paper has a force of 1,294 pounds! Yet, when you hold the paper
in your hand it does not seem to have a pressure of over a half a ton on
it. Because there are molecules on the other side of the paper pushing up
with the same force things are equaled out. If you were to take the oxygen
and nitrogen molecules away from one side of the paper there would be an
obvious and violent reaction. It's no wonder airplane windows have to be
strong so they don't get blown out as the molecules on the outside get less
when the plane goes up into the atmosphere.
The average atmospheric pressure at sea level is 14.7
pounds per square inch due to the 15 miles of air above.. That may be abbreviated
as, "14.7 psi." To make things easier that number can be referred to
as, "1 atmosphere or 1 atm." Incidentally, as one gets closer to space the
number of molecules gets less. At an altitude of 18,000 feet the pressure
is about 1/2 of what it is a sea level. That would make it 7.4 psi or 1/2
atm. At that altitude there are so few oxygen molecules you would probably
pass out. Commercial airplanes commonly fly above 30,000 feet and stepping
outside would not be conducive to one's well being!
In AOPA Pilot (2/2006)
meteorologist Brian Guyer described how he launches weather balloons twice
per day in Virginia. The balloons carry GPS radiosondes that measure and
transmit the temperature, humidity, pressure, wind speed, wind direction,
and GPS location as it ascends in the atmosphere. When Brian fills the balloon
with helium it has a diameter of 6 feet. About 1 1/2 hours after launch it
reaches and altitude of 100,000' and has a diameter of 100'! That's
when the balloon bursts.
Something else divers need to know about gases: They
move from one place to another. They usually more from where there is a lot
of a gas to where there is little. For example, if you had gasoline fumes
(molecules) in a bottle in your kitchen, and the kitchen air was not polluted,
when you opened the bottle the gasoline molecules would immediately move
from the bottle into the kitchen air and make it stink. Because the pressure
of the gasoline in the bottle was high, and the pressure of the gasoline
in the kitchen prior to opening the bottle was zero, the gasoline molecules
flowed from high pressure to the low pressure. Gases always do that. They
flow from high pressure to low pressure and they will continue to flow until
the pressure is the same both inside the bottle and outside. The same would
be true with an automobile tire. The pressure in the tire is high compared
to the pressure outside the tire. If you forced a nail through the rubber
of tire the gas will flow from the high pressure interior to the low pressure
outside until the tire is flat and the pressures are equal. Incidentally,
Graham's Law states that gases flow to areas of lesser pressure.
Oxygen molecules are slightly heavier than nitrogen
molecules. In fact, 32 nitrogen molecules weigh as much as 28 oxygen molecules.
Since oxygen is heavier one would think the oxygen would be found at the
bottom of your living room with the nitrogen floating on top of it. But remember,
gases flow from high pressure to low. If the bottom of your living room were
pure oxygen it would represent high pressure compared to the ceiling where
no oxygen would be found. Since gases flow from high pressure to low (or
zero), the oxygen molecules would vibrate themselves from the floor to the
ceiling until the pressure was equal. The nitrogen on the top would do the
same. So if you test the air in your living room, the mixture will be the
same from floor to ceiling: 78% nitrogen and 21% oxygen.
The same thing happens to a gas that sits on top of a
liquid. Let's say you have a glass of water in the bathroom. The oxygen in
the air above that water will actually flow into the water until the pressure
is the same. The oxygen molecules dissolve in the water just as if it were
sugar! That is Henry's Law. If a fish were in the water and took a little
of the oxygen out of the water to stay alive, more oxygen would flow into
the water to keep the pressure in the air the same as that in the water.
If the fish breathed out a little carbon dioxide that would increase the
pressure of that gas in the water. Since the pressure of the carbon dioxide
in the bathroom air is very low, the gas from the fish would race to the
surface and enter the bathroom air. In that way the fish will keep getting
oxygen from the air and the glass of water will not become poisoned by the
fish's exhaled carbon dioxide.
Gases also compress easily. In class, you took a syringe
and tried to compress the pool water. It was almost impossible. When you
did the same for air some of you were able to compress it by over 1/2! That
is an important point for divers. If we were made out of only liquids and
solids we would have little problem with pressure under water. Our legs,
being only composed of fluids and solids, do not compress during descent.
We do have several gas spaces that do compress as we go down in the water.
They will cause discomfort unless measures are taken prior to it becoming
a problem. In the next few lessons a large amount of time will be spent on
taking care of areas in your body that contain gas such as your ears, sinuses,
lungs, stomach, intestines, and external items added to the diver such as
the face mask.
How much of a pressure change would you expect underwater?
Remember it was said you would have to ascend in the atmosphere 18,000 feet
to reduce the pressure by 1/2? You only have to go up from 33 feet in salt
water to the surface in order to create the same affect! (Because fresh water
is lighter, it takes 34'.) Pressure underwater changes rapidly, very rapidly.
You can drive down a mountain and "fix" your ears without being in any great
hurry. Diving down in the ocean does not give you that same time luxury.
You must make adjustments rapidly and repeatedly to enjoy diving and avoid
injury! It is important to master this in the pool and in open-water!!!
Not that it has much to do with diving, meteorologists
do not use 14.7 psi or atmospheres to measure air pressure. Instead they
use millibars and/or inches of mercury (Hg). The average pressure of the
atmosphere is 1013.2 mb or 29.92" of mercury at 15 degrees C (59 degrees
F) at sea level. If there is an air mass coming that contains more pressure,
that is, more molecules banging on each square inch, the weather will tend
to be drier, sunnier, and cooler. The pressure may be 1028 mb. If a warmer,
wetter, and stormier air mass arrives it will have less molecules per square
inch and the pressure may drop to 988 millibars. The highest recorded pressure
was 1084 mb or 32.01" Hg. That was on a cold winter day in Siberia when the
molecules were so close together there were many more of them on each square
inch. The lowest recorded pressure was in Typhoon "Tip" in Japan. The pressure
dropped to 870 mb. or 25.69" Hg!