Scuba diving tanks or cylinders come in two types: aluminum and steel. The tanks also come in a variety of sizes.
The common sizes of a tank / cylinders are: 8, 10, 12 and 15 liters in the metric system. Using the imperial system, the most common sizes are 50, 71.2 and 80 cubic feet, although many other sizes (both larger and smaller) are available as well.
The scuba tanks markings are on the neck of the cylinder identify and provide important information for the filling station:
- Serial number
- Working pressure
- The manufacturer
- Date of all pressure tests
- Tank / Cylinder material
Typically there is a sticker on each tank that shows the last inspection. In the United States, the tanks must be inspected annually, checking the tank internal integrity, valve, O-rings replaced and only after approved refilled. The Department of Transportation (DOT) and Transport Canada (TC) require in the United States and Canada that the tank be hydrostatically tested every five years. In Europe the aluminum tanks must be tested every five years and steel tanks must be tested every two years
Scuba tank valves come in two types, DIN, which is common in Europe. Using a DIN valve you will screw regulator into the valve. The other and most common type is the yoke, which the diver assembles the regulator by the Yoke assembly. Both valves are made from chrome-plated brass.
Though not very common anymore, the J-valve has a built in mechanism that signals the diver when the air in your tank is running low. A unique feature with the J-valve is the internal spring operated shut off valve that is held open by the pressure until 40 bar or 300 – 500 psi. Once the tank pressure stops holding the shut off open there is a resistance the diver will experience in their breathing to let them be aware that their air supply is running low. The K-valve is simply an on/off valve. The tank valve cap indicates to the fill station operator that the tank is filled, so when you bring your tank into the shop or when you’re on the dive boat between dives, you want to be sure and leave the tank valve cap off.
The valves also have a burst disk with a copper wafer that is designed to rupture if the tank reaches 140% of its rated pressure. Over pressurization can happen if the tanks are overfilled or excessive heat in which case the pressure would be relieved by the burst disk rather than the tank/cylinder rupturing.
When handling the tanks it is important to pick up the tank at the valve so that the opening is out or carry it from behind. The clockwise rotation of tightening the valve is not compromised and also the opening of the valve is away from your hand. If the valve were to rotate in the counterclockwise direction it could open and high-pressure air would be directed towards your hand. So it is important to remember to carry dive tanks in the correct manner. When transporting and loading tanks when they are laying on the side it is important to put the tank valve in first. Never lay a tank down with the valve facing the rear of the vehicle.
Most dive shops and training centers are going to rent aluminum tanks to you. Typically they are less expensive, $150.00 – $170.00 and comparable steel is $250.00 – $280.00 per tank. The steel tanks are more durable and much longer life span, while aluminum tanks are typically rated to last 15 to 20 years. The steel lasts much longer if you take care of them. The 80 cf steel tanks weigh approximately 4 lbs. less than the aluminum, depending on the manufacturer. They are smaller in size, which allows you to carry a bit less weight
Master Scuba Dive Trainer Marcus Gee leaves you with a few points to remember:
- You should have a minimum of 500 lbs. remaining in the tanks when you complete your dive.
- When diving in kelp or wreck diving, I recommend that you increase this margin.
- Your dive buddy is counting on you, as you are they, so the safety reserve is not just for you, it is for your dive buddies also.