Background Essay

All objects, when placed in a liquid, will either float or sink, and still some others will remain at the same depth. All of these functions are known under the same property: buoyancy. There are three different "types" of buoyancy: positive, negative and neutral. An object that floats towards the surface of the liquid is positively buoyant while an object that sinks is negatively buoyant. Objects that are at a weight and volume such that they do not sink or float are called neutrally buoyant, neither positive or negative.
People have known about buoyancy for a long time and have been using its properties even longer. Prehistoric people who built simple log rafts used the buoyancy of water to keep themselves and their cargo above the water and dry. Eventually they realized that there was something about the water (that was absent in air) that made rafts possible. Of course air also has density therefore it also has a certain amount of buoyancy. An example of an object using neutral buoyancy of air is a hot air balloon, which floats in air, the way that other objects float in water.


Eventually, like so many great ideas, it was a Greek who summed up buoyancy into a useable, workable and understandable fact. His name was Archimedes and his idea is known as the Archimedes Principle. This principle states that, "Any object wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object."


Displacement is used frequently when talking about of buoyancy. It is a fairly simple concept that is extremely important in scuba diving. When an object displaces a fluid, it is basically pushing the fluid out of the way to make room for itself. Objects with a larger volume will displace more water than objects with a small volume. Also, the more fluid an object displaces the more buoyancy it will have. This is even more important to why things float than weight, although the two are tied closely together.


According to Archimedes Principle, an extremely heavy object like an ocean liner can keep its original weight but still float because of the pressure exerted on it by the water. It will only float, however, if it displaces water, with the same or more weight as itself. To displace enough water the object must be a certain shape so that its weight is spread over a large area. Basically, if a ship weighs 200 tons, and it displaces at least 200 tons of water, it will float. But if it only displaces 190 tons of water, it will sink.


When diving, you must take into account the fact that saltwater and freshwater are different weights and, therefore, something that floats in one may not float in the other. Saltwater has salt dissolved into it, so it weighs a little more than freshwater. Freshwater has a weight of about 62.4 pounds per cubic foot. Saltwater is only slightly heavier at 64 pounds per cubic foot. This may not seem like much of a difference, but if you had an object one cubic foot in size, and it weighed 63 pounds, this object would float in saltwater, but sink in fresh.


Also, an invaluable skill for diving is the ability to stay at neutral buoyancy, or stay at the same depth. For if a diver were to be negatively buoyant, as soon as a diver jumped off a boat they wouldn't stop descending until they hit the ocean floor. But it is equally dangerous to be positively buoyant. This would cause you to rise at an uncontrolled rate and as you ascended, the air in your lungs would expand. Once the loung had expanded beyond their capacity and the diver would be severely injured. This is why all divers are required to wear a Buoyancy Control Device or BCD and a weight belt. A diver must regulate the BCD to keep at neutral buoyancy to make any and all dives safe. This is done by inflating and deflating the BCD through the inflation and dump valves. Another essential item to scuba diving is a weight belt, which keeps you from floating and allows you to gain neutral buoyancy more easily. For a more detailed description of scuba diving gear click this link.


A diver must take all of the things that we have discussed in this essay into consideration in order to have a safe and productive dive. Even with all of these precautions, however, scuba dives cannot be taken lightly and you must be "awake" at all times. Sometimes there are coincidences and situations that no one could have planned for. If you do follow the guidelines and understand why these things are important, then it will minimize your chances of being in danger.

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