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Diving Physics |
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Diving Physics |
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In order to stay underwater we need a means of breathing in order to supply our body with the oxygen it needs. What makes this task difficult is pressure. Pressure is defined as the amount of force per unit area. At sea level we are exposed to 14.7 pounds of pressure per square inch (psi). In other words the air surrounding us is pushing on each square inch of our body with 14.7 pounds due to its weight. These 14.7 psi is what is called one atmosphere. The same thing happens in the water. Since water is much denser than air it takes a lot less sea water to exert enough pressure to total 14.7 psi. If one were to take a column of sea water one square inch in diameter, the column would only have to be 33 feet (10 meters) tall to exert one atmosphere of pressure. This means, that if we dive to a depth of 33 feet our body is exposed to 2 atmospheres of pressure. 1 Atmosphere due to the weight of the air and one atmosphere due to the weight of the water. At a depth of 66 feet we are exposed to a pressure of 3 atmospheres or 3 * 14.7 psi = 44.1 psi. The following is a little Java Script calculator giving you the amount of pressure per square inch at any given depth in fresh or sea water (The reason for the differentiation between fresh and salt water is due to the fact that fresh water is not as dense as salt water.):
| Salt Water | Fresh Water | |
| depth in feet | ||
| answer |
Due to this pressure on a diver's body, a diver needs to breath pressurized air as the lungs are not capable of expanding enough to take in air which is not under pressure. This is the reason why we can't just use a snorkel of the necessary length to explore underwater without the worries of running out of air. In SCUBA diving the diver breathes air which is at the exact pressure of his surrounding environment, which means his lungs do not have to fight against the surrounding pressure. This fact poses other problems however that need to be taken into consideration every time a dive is undertaken with compressed air.
The first rule learned when breathing compressed air underwater is to never hold your breath. The reasoning behind this is simple. Gases are composed of molecules and under pressure these molecules get packed more tightly together. This means that with a higher pressure more gas molecules will occupy a given volume. At a depth of 33 feet the same amount of gas will occupy only half the volume it would occupy at sea level. At 66 feet it would occupy one third of the volume. If a diver were to fill his lungs at a depth of 33 feet and then ascend to the surface while holding his breath. The air in his lungs would expand to twice the volume. This of course would lead to major medical problems (see pulmonary overinflation syndromes). The above phenomenon is explained in Boyle's Law:
This leads us to the General Gas Law:
P1 x V1 P2 x V2 _______ = ________ T1 T2
The second thing we need to consider when breathing compressed air under pressure is the effect that the individual gases making up this air have on our metabolism. The air we breath is made up of approximately 20% oxygen and 80% nitrogen. Since these gases have effects on our body when breathed under pressure, it useful to understand the concept of partial pressure. Dalton's Law states:
| Depth in Feet | Salt Water | Fresh Water |
Now that we've understood partial pressure there is one more concept that needs to be understood in order to appreciate the complexity and dangers involved in breathing compressed air. This concept is stated in Henry's Law:
The following links lead to articles with further in depth information on dive physics:
