Why Does a SCUBA Diver Need High Pressure in the Air Tank?
SCUBA (self-contained contained underwater breathing apparatus) diving appeals to a variety of individuals. Tourists give it a try at beach resorts all over the world, while others make a career out of it. One of the most important components of the SCUBA setup is the air tank, or cylinder. Without it, divers might as well snorkel. To provide fresh air to the diver, these tanks are filled to very high pressures.
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High-Pressure Rational
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Above water, humans breathe approximately one-cubic foot of air each minute. For example, if the typical person exhaled each breath into a balloon for one minute, the resultant balloon would take up one-cubic foot of space. Air tanks are much larger than balloons, but if they were not filled with extra air, the tank would only last a few minutes. For this reason, extra air is forced into the tank to allow for longer dives. The amount of air expelled from the tank during each breath is controlled by the regulator.
Effect of Depth
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The amount of air consumed equals breathing rate multiplied by bar pressure multiplied by breathing time (in minutes). The deeper a diver swims, the higher the water pressure will be. Therefore, the air in the tank will last for less time. While a tank filled with 3,000 psi (pounds per square inch) of air may last 80 minutes near the surface of the water, the tank will last only 40 minutes when the water pressure doubles at 33 feet below the surface.
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Tank Composition
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Most SCUBA tanks are made from either aluminum or steel. Aluminum tanks are cheaper, softer and thus more prone to damage, and require thicker walls. This is because aluminum is a weaker metal than steel, and without thick walls, the high pressures of contained air would rupture the tank. Steel tanks are more expensive and heavier, despite having thinner walls. They are less prone to scratches and dents, and do not change weight as much as aluminum tanks as the high pressure air is released.
High-Pressure Decisions
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SCUBA divers can determine how much air they would like in their tanks. Some opt for the lower limit of 1,500 psi, while others opt for the higher limit of 4,500 psi. The pressure selected depends on how long the diver will be in the water and how much added weight they are looking for from their tank. Highly-pressured tanks allow for longer dives but also weigh more, requiring a careful adjustment of weight belts. For divers taking a quick dip, a tank with less pressure is preferable.
Air Tank Safety
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All SCUBA tanks are required by law to have three codes stamped into the cylinder. The first lists the tank construction regulations along with the tank material. These codes are followed by the service pressure, which is the maximum recommended pressure within the tank. The second code lists the tank manufacturer and serial number, and the third code lists the dates of any hydrostatic tests performed on the tank. Never fill your tank above the service pressure listing because this can rupture the tank or disrupt the regulation system that provides your air.
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References
Resources
- Photo Credit two scuba divers image by Janet Wall from Fotolia.com
