Scuba cylinders or tanks hold the air you breathe while diving. They are usually the last piece of gear a diver purchases because cylinders are easily rented. There are many different sizes of cylinders, but they are either made of aluminum or steel. Capacity sizes range from 40 cubic feet to 130 cubic feet.
There are even smaller capacity cylinders that are used for dry suit inflation or as pony cylinders. A pony bottle is a separate and independent air supply to be used in the case of an emergency. It attaches to the side of your primary tank with cam straps or pony tamer type of device. Pony bottles require a separate regulator, which should be routed so that it is easy to access in case of an emergency. The regulator hose must be long enough to accomplish this. The ideal placement is to hang the second stage around your neck using a bungee necklace. A loop in the bungee necklace goes around the mouthpiece of the regulator and allows it to hang around your neck. This allows hands-free access and you always know where the regulator is; it’s not dangling behind you out of sight.
Cylinders must be inspected every year with a visual inspection (VIP), which is used to detect rust, oxidation, and cracks in the cylinder. In addition, a hydrostatic test is required every five years, which tests the physical integrity of the cylinder. With a hydrostatic test, water is placed inside the cylinder. The water is pressurized to five-thirds of the cylinder’s working pressure, and the expansion of the cylinder is measured by how much additional water is displaced in the water filled container. When pressure is reduced, the cylinder must return within 10 percent of its original volume. It the cylinder is permanently expanded by more than 10 percent, the cylinder is discarded. Each time the cylinder is tested, a stamp is placed on the shoulder of the cylinder with the date of the test.
To use a cylinder, it must have a valve that screws into the top of the cylinder. The first stage of your regulator connects to this valve. The standard type of valve is a K valve. K valves have a built-in snorkel device that lowers the air intake of the valve. This prevents water or debris from entering the cylinder. The valve also has a burst disk, which is extremely important. The burst disk is designed to fail if pressure inside the cylinder gets too high to due overfilling or heat. This prevents the cylinder from blowing up and causing serious injury or death.
The other type of valve that many divers are using today is the DIN valve. DIN valves provide a more secure connection for your regulator because the first stage of your regulator is screwed into the valve. You must have a DIN fitting on your regulator to use a DIN valve. Many manufacturers make DIN adapters for their regulators.
Aluminum cylinders are made of aluminum, are generally filled to 3,000 psi, are lighter, more buoyant, and are generally easy to take care of. Aluminum cylinders are more common and cost less than steel cylinders. The aluminum 80 is the most standard cylinder in the diving industry. Aluminum cylinders oxidize if moisture gets inside the cylinder. If taken care of, aluminum cylinders can last for up to 10 years.
Cost: $125 to $150
Steel cylinders are made of steel, generally rated to either 2,400 psi or 3,444 psi, are heavier, less buoyant or negative, and are a little harder to take care of. Steel cylinders are less common than aluminum cylinders and are more expensive. Using a steel cylinder usually allows you to remove some weight from your weight belt depending on the type of exposure suit you’re diving. Steel cylinders can rust if moisture gets inside the cylinder. If caught quickly, they can usually be tumbled and returned to normal use. If taken care of, steel cylinders can last for up to 20 years or more.
Cost: $200 to $350