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Scuba cylinders serve one purpose—to hold gas. Cylinders are also called tanks or bottles. They can come in various materials, pressure ratings, shapes, and sizes. Most recreational open circuit divers carry a single cylinder. Divers who require more gas can choose from other cylinder layouts.

Cylinders are commonly made from either steel or aluminum. They are fitted with a cylinder valve. The cylinder valve allows the first stage of the regulator to be connected to the cylinder. This provides easy access to the gas for breathing.

Cylinders must be inspected and tested periodically. At regular intervals, all cylinders have to be visually inspected. They need to be checked for corrosion and condensation. They also have to be hydrostatically tested. This test ensures cylinders can safely contain the required high levels of pressure.

Only a qualified technician can inspect and test cylinders. The technician will update the cylinder markings or decals if the cylinder passes all the tests. These markings are stamped onto the cylinder neck. Out-of-date markings on a cylinder will prevent it from being refilled.

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