Scuba Tanks
Purpose
A scuba tank (also called a cylinder or bottle) stores the compressed breathing gas you use underwater. Most recreational divers use a single tank on their back; technical divers may use doubles or sidemount configurations. Tanks are filled at a dive shop or compressor facility and are the component you're most likely to rent rather than own — particularly if you dive abroad, where transporting tanks is impractical. That said, many local divers who dive frequently own their own cylinders for convenience.
Features
Type of Material
Aluminium tanks are the most common rental tank worldwide — particularly the aluminium 80 (80 cubic foot / ~11 litre), which is the standard in North America. Aluminium is less expensive and highly corrosion-resistant; however, it is less rigid than steel, so the walls must be thicker, making aluminium tanks heavier and more buoyant. An aluminium 80 is typically neutrally buoyant at the beginning of a dive and becomes positively buoyant as air is consumed — many divers add extra weight to compensate, which can be inconvenient.
Steel tanks are stronger, allowing thinner walls and a more compact, lighter cylinder for the same volume. Steel tanks are negatively buoyant and become slightly less negative as air is consumed but typically remain negative throughout the dive, which can reduce or eliminate the need for weight belt weights. They are more prone to internal corrosion if moisture enters the cylinder, which makes proper care critical. Steel tanks are popular with drysuit divers and frequent divers who want to minimise lead weight.
Capacity
Tank volume is expressed in cubic feet (imperial) or litres (metric). The most common recreational tank is the aluminium 80 cubic foot (approximately 11 litres at 200 bar). Steel tanks of similar volume are often more compact. Smaller tanks (63–72 cubic foot) are popular for children's programs and shallow reef diving. Larger steel tanks (100+ cubic foot / 12–15 litre) extend bottom time and are preferred by cold-water divers with higher gas consumption. Pony bottles (small backup tanks of 6–30 cubic feet) provide an independent emergency air supply for deeper or longer dives.
Pressure
The pressure rating of a tank does NOT indicate how much air it holds, in otherwords, a higher pressure tank does not necessarily hold more air. The pressure rating indicates at what pressure the tank is considered full to its capacity. So an Aluminum 80 tank with a pressure rating of 3000 psi will still hold more air than an Aluminum 63 at 3000 psi. And a steel 95 with a working pressure of 2400 psi holds more air than both.
High vs Low
High pressure steel tanks are usually a bit smaller and more compact compared to their low pressure counterparts. Because they require a higher pressure (3300-3500 psi) to be "full" it can be difficult to get a good fill. The higher the pressure, the closer the air molecules are compressed and the hotter the air will get. A tank can get very hot to the touch while being filled to the maximum pressure. Once the tank cools off, the air molecules cool and the pressure in the tank will drop. For example, with a high pressure steel 80 tank, if it has less than its service pressure of 3442 psi once it has cooled, you will have less than 80 cubic feet of air in the tank. Getting a good, full fill on a high pressure tank can be difficult and may require you to leave your tank at the dive shop overnight to cool and get topped off the following day. This is one drawback of a high pressure scuba tank.
Buoyancy
Aluminum tanks tend be become positively buoyant as they lose pressure, so you may need to add more weight at the start of the dive to offset the slight positive buoyancy effect that will occur at the end. Steel tanks tend to stay negatively buoyant throughout the dive.
Tank Valves
Most tanks use either a yoke (A-clamp) valve or a DIN valve. Yoke valves are standard in North America; DIN valves (rated to higher pressures) are common in Europe and with technical gear. Many modern valves are switchable between the two. An International or Pro-Valve looks like a yoke style, but has also has an insert that can be removed with an allen key which then allows use with DIN regulators. The tank valve also includes an integrated burst disc that ruptures at a specific over-pressure to act as a safety relief mechanism. Keep the valve handwheel completely open when diving and check the o-ring before attaching a regulator.
Care & Maintenance
In Canada, scuba tanks are regulated under Transport Canada requirements. All cylinders must have a visual inspection (VIP) performed annually by a certified inspector to check the interior and exterior for corrosion, damage, or contamination. Additionally, tanks require a hydrostatic pressure test every five years, which verifies the structural integrity of the cylinder under pressure. A tank that is out of date on either inspection cannot legally be filled. When purchasing a used tank, verify the inspection date stamped on the neck and confirm a current VIP sticker is present.
Always store tanks with a small amount of air pressure inside — never completely empty — to prevent moisture from entering the cylinder through the valve. Store upright or in a secure horizontal rack, away from extreme heat and direct sunlight. Rinse the valve and exterior with fresh water after diving. Have the valve o-ring inspected whenever you have the tank filled and replace it if there are any signs of wear. Keep up to date with annual VIP inspections to avoid disruptions to your diving schedule.