The SCUBA air tank, or cylinder, is used to store compressed air for diving. The amount of air stored depends on the size of the cylinder and the pressure of the air in the tank. The more air that is put into the tank, the higher the pressure. The size of the cylinder determines how much air can be put into a cylinder to reach a given pressure. It is an essential element of the diver's life support system and should be cared for as if your life depends on it. It does. Although gases other than air are sometimes used for diving, including oxygen and nitrox, the mainstay of most recreational diving is air. SCUBA cylinders used for gases other than air have additional requirements. This article will detail only those SCUBA tanks used in air diving.
The SCUBA Cylinder
SCUBA air cylinders are made from steel or aluminum. Aluminum cylinders have less weight than steel cylinders and are less prone to corrosion. Steel cylinders tend to resist physical damage and can hold slightly more area for a given external volume due to thinner walls. The most common size for cylinders is 80 cubic feet. The 80 cubic feet approximates how much air at normal atmospheric pressure the cylinder can hold at 3,000 pounds per square inch. Sizes can vary from small pony bottles that hold 6 cubic feet to oversized tanks that hold 120 cubic feet.
The valve attaches to the cylinder at the neck and is threaded with an o-ring seal between the valve and cylinder. The valve serves as the attachment point for the first stage of the regulator. The valve allows the air to be sealed inside the cylinder and released when ready. Valves may be j-valves, k-valves or DIN valves. The k-valve is commonly used and simply allows full use of the air at all times. The j-valve has a spring-loaded mechanism that restricts a portion of the air supply, which can be accessed with the turning of a second valve. The DIN valve is used for cylinders rated over 3,500 psi. Valves are required by the Department of Transportation to have a burst disc that allows the air to escape in case of over-pressurization.
SCUBA Cylinder Markings
Along the neck of the SCUBA tank are a series of markings etched into the cylinder. These markings provide information on the manufacturer, country manufactured in, manufacture date, working pressure, test pressure, cylinder specifications and other useful information. Perhaps of greatest importance is the date and location the cylinder was last hydrostatically tested. In the United States, cylinders are required to be hydrostatically tested every five years. A hydrostatic test is performed by measuring changes in the cylinder walls when pressurized. After each hydrostatic test, a new marking is added to the cylinder indicating the date and test center.
Safety and Maintenance
SCUBA air tanks can outlast their owners if properly cared for. The greatest threat to the tanks is moisture and physical abuse. To get the most life out of the tank:
1. Rinse your tank after use. Ensure that the valve is closed when rinsing.
2. Do not over-pressurize the tank.
3. Ensure that the tank is visually inspected every year and hydrostatically tested every five years by a reputable test facility.
4. Do not use a tank that has cracks or dents.
5. Do not overtighten the on/off valve.
6. Transport the tank in the trunk or rear of the vehicle with the valve pointed out away from passengers.
7. Never store your tank fully pressurized for long periods of time, nor store it empty. Store with enough pressure to prevent moisture from entering the cylinder.
8. Store in an upright position where it will not get knocked over or banged around.
9. If you notice any abnormalities or suspect any problems, have your tank serviced immediately.
SCUBA Cylinders and Nitrox
Air is not the only gas mixture used in recreational SCUBA diving. Increasingly, nitrox is being used. Air is a mixture of approximately 21% oxygen, 78% nitrogen and less than 1% of other gases. Nitrox is any mixture of nitrogen and oxygen of which air is one formulation. Formulations with oxygen levels above 40% require equipment to be oxygen-cleaned. This involves removing hydrocarbons from the cylinder and valve and replacing rubber parts with other materials. Cylinders that have been oxygen-cleaned should be labeled appropriately.
Article Written By David Chandler
David Chandler has been a freelance writer since 2006 whose work has appeared in various print and online publications. A former reconnaissance Marine, he is an active hiker, diver, kayaker, sailor and angler. He has traveled extensively and holds a bachelor's degree from the University of South Florida where he was educated in international studies and microbiology.