Buoyancy refers to the tendency of objects immersed in water to float or sink. Objects that are positively buoyant float on top of water; objects that are negatively buoyant sink; and objects that are neutrally buoyant neither float nor sink, but stay where you place them.
This principle was first stated by Archimedes (Greek mathematician and engineer, lived about 287 B.C. to 212 B.C.).
He observed that an object immersed in water sinks or floats depending on the weight of water it displaces. If the weight of water displaced is less than the object's weight, it sinks; if the displaced water weighs more, the object floats; and if the displaced water is the same the object is neutrally buoyant.

For example, a cubic foot of solid wood weighing 63 lbs. would float in sea water (cu. ft. = 64 lbs) and sink in fresh water (cu. ft. = 62.4 lbs.). A cubic foot of solid steel weighs much more than 64 lbs., and so will sink in any body of water, whereas a cubic foot of Styrofoam will always float. The steel displaces much less than its weight of water, the Styrofoam much more.

What Is Buoyancy Control?

"Buoyancy" also refers to an all important scuba diving skill, perhaps the one most difficult to master. A diver who has learned to control buoyancy has learned: the proper amount of lead weight to carry for each dive; how to use natural breathing to inflate or deflate the lungs to control vertical position; how to position equipment in order to stay horizontal; when (and when not) to put air in the BC; and how to ascend at a slow and steady rate.

Good buoyancy control is perhaps the single skill that most distinguishes experienced from novice divers. For most divers it does not come easily or quickly. Consider the many factors that can affect a diver's buoyancy:

Innate buoyancy differs among individuals, and depends on the amount and distribution of body fat, bone and muscle; as a result, some people naturally float and some sink. Fat tissue has a lower density than bone or muscle, so people with a lot of fat are more likely to float (i.e., displace an amount of water weighing more than their body), whereas lean, muscular folks are more likely to sink (i.e., displace an amount of water weighing less than their body).

The amount of air in the lungs affects buoyancy. If you establish neutral buoyancy during quiet breathing, a full inhalation will make you rise (become positively buoyant), whereas a full exhalation will make you sink (negatively buoyant).

A wet suit will be positively buoyant on the surface but will change to neutrally or negatively buoyant at depth.

A typical 80 cu. ft. aluminum scuba tank filled with air is negatively buoyant by 2-3 lbs., but when near empty the same tank will be 2-3 lbs. positively buoyant. (80 cu. ft. or air weighs about 6 lbs.)

The typical buoyancy compensator is positively buoyant without any air, and becomes more so as it is inflated.

Most ancillary equipment carried on a dive, such as a light, knife, or camera, tends to sink and therefore adds to negative buoyancy.
Some fins are positively buoyant, some negatively buoyant.
Finally, salt-water weighs more than fresh water (64 lbs. vs. 62.4 lbs. per cu. ft.), so divers are more buoyant in salt than in fresh water.

Because most people have slight positive buoyancy naturally, and because the BC and wet suit add to positive buoyancy on the surface, it is usually necessary to carry lead weights while diving.
Weights tip the scale to negative buoyancy and allow the diver to sink easily below the surface. Depending on body makeup and type of equipment carried or worn, warm water divers need anywhere from 4 to about 12 pounds of lead weight. The single item most affecting the amount of weight required is the wetsuit (the thicker the suit, the more weight needed to establish negative buoyancy).
As the dive progresses buoyancy is constantly changing. Consider:
Pressure compresses air cells in the wet suit, so the positive buoyancy contributed by the suit on the surface decreases significantly at depth.
As air is used up, the tank's buoyancy changes from negative to neutral to positive (steel tanks remain negative buoyant throughout the dive).

Any air in the BC at depth will expand as the diver rises, increasing buoyancy steadily with ascent; the higher the diver ascends the greater the increase in positive buoyancy.

Any change in the breathing pattern during the dive (e.g., a tendency to hyperventilation) may alter the diver's average lung volume, and thus affect buoyancy.

For the novice diver the net result of all the factors affecting buoyancy can be confusion and a bad dive. Consider two typical scenarios.

Novice A goes to 60 feet with a group of other divers. He has trouble emptying his BC of air and kicks hard to stay at depth and with the group; no matter how hard he tries to stay down he keeps rising. He looks in vain for something to hold on to. His depth gauge shows 40 feet...then 30...then 20.
He simply cannot dump any air out of his BC and a few seconds later breaks through the surface of the water.

Novice B claims trouble sinking on her dives.
She enters the warm Caribbean water with 16 pounds of lead, even though the dive master warns, "it is too much."
At 50 fsw she has difficulty staying off the bottom and puts some air in her BC. She then starts to rise, so she dumps some air.
All the while she notices herself kicking hard to stay level and with her buddy.
About 20 minutes into the dive she alerts her buddy that her tank has only 800 psi; her buddy still has 1800 psi. Together they begin an ascent to the safety stop line.

Diver A's problem was a BC with residual air that he could not properly vent. The closer he came to the surface the quicker he rose, as air in his BC expanded further. Fortunately he did not hold his breath and suffered no barotrauma from the rapid ascent.

Diver B's problem was carrying too much weight; this caused her to work extra hard under water just to stay level.
As a result, she used up air much faster than her buddy. As with Diver A, she had a less than optimal dive because of poor buoyancy control.

What Causes Weightlessness Under Water?

To "float" as if defying gravity - a feeling of weightlessness - is one of the many thrills of diving.
The feeling comes from establishing neutral buoyancy under water.
At that point the gravity force pulling your body down is equal to the water pressure pushing it up, and you neither sink nor rise.

The sensation is reportedly similar to what is experienced in outer space, which is why astronauts train with scuba gear before some space missions.
However the mechanisms are different. In outer space the astronaut is far enough away from earth to escape the effects of gravity, and it is gravity, which makes us have (and feel) weight.

On the surface or under water, gravity still exerts an effect, but it can be nullified by establishing neutral or positive buoyancy.
In outer space lead weights will float just like the astronaut because they are also free of earth's gravity.
When a diver is neutrally buoyant, his weight belt, if detached, will quickly drop because it is negatively buoyant and subject to earth's gravity.

How Does One Learn To Acquire Proper Buoyancy?

First, understand that buoyancy is not a static phenomenon, but one that changes constantly in the water, even if you stay at a level depth (because the quantity of tank air is constantly decreasing, and also because breathing affects buoyancy).
Appreciating buoyancy as a constantly changing factor during every dive is the first step to mastering it. As a rule, one should not have to rely on the BC for buoyancy control.

For novice divers this piece of advice may seem paradoxical; a BC is, after all, a buoyancy compensator (sometimes also referred to as a buoyancy control device). No matter.
Experienced divers learn how to use their lungs to fine tune buoyancy, and reserve the BC for an occasional adjustment, mainly to compensate for the decrease in wet suit buoyancy with depth.

Ideally, inflating the BC should only be necessary on the surface (for flotation), for wet suit compensation, and for emergency ascents.
However, because wet suit compression will alter buoyancy at depth, it is often necessary to add a little air to compensate and re-establish neutral buoyancy. >From that point of the dive, buoyancy should be controlled mainly by breathing.
(It will also be necessary at the end of the dive to dump out any air added to the BC, in order to prevent too rapid an ascent.)

Third, one can take a formal buoyancy control course, offered in many resorts around the world. These courses teach the nuances of buoyancy control, including how to position equipment on your body so weight is evenly distributed.

Finally, gain experience.
There is no substitute for experience, which means making many dives.

Some divers find that learning good buoyancy control takes perhaps 100 or more dives.
It also helps to practice such basic open water skills as the fin pivot.
(In this skill the diver establishes neutral buoyancy with only his fins touching the bottom; using his fins as the fulcrum, he can then rise up with each deep inhalation, and fall back toward the bottom with each exhalation.) Only with many dives can one experiment with equipment, weights, wet suits and breathing techniques, and learn to master buoyancy control.