Buoyancy is a major part of scuba diving. The sign of
a good diver is one who has mastered their buoyancy. Being able to go up
and down safely, and hover in one place takes a lot of practice and experience.
The buoyancy of a diver is caused by the diver's density. What is really
happening when a diver adjusts their buoyancy?
Styrofoam floats and granite sinks in water. Why is that?
It is all due to density. Water has a density of one. Man made it come out
that way because everyone has access to water. Anything that floats on water
has a density of less than one. Anything that sinks in water has a density
of more than one. If you were told a piece of granite, no matter how large
it was, had a density of 2.7 you would know it would sink in water. Divers
have a density of less than one sometimes. At other times their density is
more than one. And, it would follow that a diver sometimes has a density
of exactly one and can remain suspended in water. Incidentally, we are talking
about pure water. If seawater is to be considered the density of that is
How does one find the density of an object. There are
two things that have to be known: How much does the object weigh, and how
big is it? If the weight is divided by the size the density is determined.
The math would look like:
Density = Mass/Volume.
That's it in a nutshell.
Divers have different densities. The following are a
list of factors that affect the density of a scuba diver:
The amount of weight worn.
The size of the lungs.
How much air is in the lungs.
The size of the wet or dry suit.
The thickness of the wet suit.
How much undergarmet is worn under the dry suit.
How much air is in the scuba cylinder.
How much air is in the buoyancy compensator.
Hard back packs on BC's contain about 3 lbs. of air that may or may not be
How much fat the diver has. (Fat has a density of approximately 0.8 gm/ml
and floats on water.)
How much muscle and bone the diver has. (Muscle has a density of ~1.1
and bone ~ 1.9. They sink.)
The depth of the water.
How do you change your density (buoyancy)? Let's say
you are scuba diving at the bottom of a lake and want to go to the surface
without kicking. If you made your density less than one you would move upward.
There are only two possibilities. Either you could change your mass (weight)
or your volume (size). Looking at D=M/V, if you made your mass less, or made
your volume more the density in the equation would have to go down. You could
make your mass less by dropping your weight belt. Up you would go and down
would go $45! Without a weight belt your density would suddenly get less
than one and you would be in an uncontrolled free ascent. There are only
a few ways to change your mass underwater other than the removal of the weights.
But, it is easy to change the volume! You can easily make yourself bigger
by inflating your lungs, dry suit, and/or BCD. The latter is the most convenient,
and that is what is done to control buoyancy.
The average 170 pound male with inflated lungs has a
volume of approximately 2.8 cubic feet. The weight of seawater is 64 pounds
per cubic foot. (Freshwater is 62.4 pounds per cubic foot.) The density of
the male is (170 lbs./2.8 cu ft) is 61 pounds per cubic foot. Since seawater
is 64 pounds per cu ft, it denser than the male. If that male jumped into
the sea he would float with a force of 3 pounds per cu ft, or 8.4 pounds.
That would lift his 9 pound head out of the water!
Before a diver goes down with a scuba they should do
a "buoyancy check." This check should be done every time the diver changes
into a new diving pattern. For example, if the diver gains or loses weight,
changes the size or thickness of the exposure suit, goes from fresh to salt
water, etc., then a new check should be done prior to diving. It is easily
done. The diver should put on all the gear they are to wear diving except
the BC and the tank and regulator. They should enter the water (fins are
a safety must!) and fill their lungs completely. Without moving the arms,
hands, or legs, and holding the breath, the diver should float to a level
where the water covers the top of the mask while they are looking forward.
They now have perfect buoyancy for skin diving. If scuba is to be used an
extra 3 pounds should be added to the weight system to compensate for the
decrease in density due to the loss of air from the 71.2 or 80 cu ft. tank.
(Note: If another type of scuba tank is to be used the additional weight
requirement may change due to the density of the tank.