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cylinder was manufactured according to its standards. • The service pressure of the cylinder, which is the pressure to which the cylinder can be filled (for example, 200 bar or 3000 psi). • A “+” mark on steel tanks, authorizing a 10 percent pressure overfill beyond the stamped service pressure. • The material of which the cylinder is composed (for example, 3AL stands for a particular aluminum alloy). • The hydrostatic testing date of the cylinder (for example, 8/07 signifies that the cylinder was tested in August of 2007). In the United States, hydrostatic testing must be performed every five years. In Japan, hydrostatic testing must be performed every three years. In Australia, hydrostatic testing must be done every year. Accessories Cylinders sometimes have boots on the bottom to protect the bottom of the cylinder and its environment. Some steel cylinders have a rounded bottom and need a boot to be able to temporarily stand upright. You must remove the boot on a cylinder periodically to properly rinse the bottom of the cylinder. Cylinders can also be covered with a rubber or plastic net. The net protects the exterior of the cylinder and its paint. You might also see divers with two cylinders hooked together with a manifold sharing a common valve. Valves Every cylinder must have a valve to hold the air in the cylinder when it is not in use, to control the flow of air out of the cylinder, and to provide an attachment point for the scuba regulator. A valve acts like a water faucet. You open the valve by turning its knob counterclockwise and close the valve by turning the knob clockwise, as seen from the top of the valve. “K” Valves The United States “K” valve is one valve you will see often when diving (figure 2-12). It is designed like a post with an on/off knob. The first stage regulator yoke fits over the post and the regulator is tightened against the post with a screw. These valves are not usually used at pressures higher than 200 bar (3000 psi). An O-ring or gasket found on the cylinder valve makes a seal between the regulator and valve. If the O-ring is damaged or missing, the regulator will not seal to the cylinder and air will escape. You should replace O-rings frequently because of wear. “J” Valves A “J” valve looks like the “K” valve with a lever opposite the on/off knob. This lever is known as a reserve mechanism. The mechanism is designed to begin to restrict airflow at about 20 bar (300 psi) to 33 bar (500 psi) of pressure in the cylinder. When the mechanism is manually opened, the airflow is no longer restricted. “J” valves were popular before submersible pressure gauges (SPGs) were commonly used. The reserve mechanism had to be placed in the up position at the start of the dive. The lever was moved to a down position by means of a cord or rod when the diver noticed NAUI Scuba Diver 22 Diving Equipment FIGURE 2-12. “K” VALVES ARE THE MOST COMMON TYPE OF VALVE FOUND IN THE UNITED STATES.


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