If you will recall, the pressure of the atmosphere may
be measured in pounds per square inch (psi) or in atmospheres (atm). There
are 14.7 pounds of air on every square inch at sea level, or 1 atmosphere.
Again, the pressure is caused by billions of moving molecules bombarding
the square inch. Most of those molecules are nitrogen (78%), and the rest
are oxygen (21%).

A diver is ready to enter the water and is standing at
the beach with the ambient (surrounding) pressure at1 atm due to the 15 miles
of air overhead. He or she enters the water and descends to 33' (34' in fresh
water) where the pressure is 2 atmospheres (29.4 psi). One of those atmospheres
is caused by the air and the other from the water. Divers refer to the total
pressure (2 atm.) as, "absolute pressure." When one refers to the pressure
caused only by the water it is called, "gauge pressure." Therefore, the absolute
pressure at 33' in seawater is 2 atm, and the gauge pressure is 1 atm. because
the air is ignored. Likewise, the gauge pressure at 66' would be 2 atmospheres
and the absolute pressure would be 3.

The average male human being has about 2800 square inches
on the outside. If one considers there are 14.7 pounds of molecules pounding
each square inch, the total pressure works out to be 21 tons! We should
be crushed to death. Fortunately there are molecules inside our body that
push out with an almost equal force. We are stabilized and don't even know
it. In reality, the internal molecules found in the blood and tissues push
with a greater force outward because of the power of the pump (heart). That
is why we flow out when a knife wound occurs. (Things flow from high pressure
to low!)

Now, if you filled a plastic bag with water and took
it down in the sea there would be little change to the shape of the bag.
As you found out with the hypodermic syringe, fluids are barely compressible.
If you filled that same bag with air it would get smaller as you descended
into greater pressure. In fact, the bag would be 1/2 the size at 33' because
the pressure there is double (2 atm absolute). If you went further down to
66', where the pressure is 3 atm., the bag would be 1/3 of the size, and
so on.

Robert Boyle stated the above in a mathematical way:
P1V1 = P2V2. That's Boyle's law and is very important for divers. The affect
of Boyle's law can kill a diver! How does the law work? The P1 is the pressure
at the first location such as sea level (1 atm.). The P2 is the pressure
at the 2nd location such as 33'. The V1 is the volume of the gas space, such
as our plastic bag, at the first location, and the P2 is the volume of the
gas space at the 2nd location. To make it clearer, let's say the plastic
bag at the beach is 1 quart in size. When you take it to 33' it should be
1/2 quart. Check it out doing Boyle's math:

1 Atm (beach) x 1 quart should = 2 Atm
(at 33') x 1/2 quart

1x1 = 2 x 1/2

If you didn't know one of the numbers in the above equation
you should be able to figure it out. If you didn't know how big the bag would
be at 33' you would have math that looked like this:

1 Atm (beach) x 1 quart = 2 Atm (33')
x WHAT?

1 x 1=2 x ?
The question mark is a number you should now be able to figure out.

Your body is composed mainly of fluids and solids. If
we were made totally of fluids and solids scuba diving would present far
fewer problems. We have gas spaces in our bodies and they act like a plastic
bag filled with air when we go down to greater pressures. That is why, for
example, your ears start to hurt when you go to the bottom of a swimming
pool. In the ears there are gas spaces that are being squeezed smaller as
one goes deeper. This crushing effect causes discomfort and pain. There are
other air spaces that also shrink and expand as the diver goes down and up.
The list includes the middle ears, sinuses, stomach, intestines, and the
lungs. The mouth, nose, and throat are open to the outside and are not usually
affected by Boyle's law.

It is important in skin and scuba
diving to keep the pressure inside body air spaces the same as the pressure
on the outside!

One further note: Pressure underwater
increases and decreases most rapidly when you are near the surface. Going
down 10 feet in a swimming pool is a much greater pressure change when compared
to going from 30 to 40 feet underwater. Descending from the surface to 33'
changes the pressure from 1 atm. to 2 and that represents a doubling of the
pressure. If you went down another 33' to 66', the pressure would increase
from 2 to 3. That is not another doubling. You would have to go from 33'
(2 atm) to 99' (4 atm) to equal what happens from the surface (1 atm) to
33' (2 atm).

You should be able to figure out what the water pressure
is for any depth. From memory, you probably know the answer to the question,
"What is the pressure of the water at 33' (fresh: 34')?" You would say 2
atmospheres, correct? If the question was for 66" (68'), you would answer
3 atmospheres. Now, what if you were asked for the pressure at 57' would
you be able to figure it out? Try to work out a simply equation for doing
this math. Use 33' and 66' in the search because you already know the answers
for those two. The answer is below:

Depth and pressure problem answer: Depth/33' = Gauge pressure. Add
1 for the atmosphere to get the absolute pressure. So, Depth/33' + 1 = Pressure
of the water in atmospheres.

Using 57": 57'/33' = 1.73; 1.73 +1=2.73. The pressure
at 57' is 2.73 atm.

Note: Use 34' instead of 33' if it is fresh water!