Diving Cylinders (Diving Tanks)
The diving tank is an essential part of SCUBA (Self-Contained Underwater Breathing Apparatus) diving. The tank is what gives you your high pressure breathing gas so that you can breathe underwater. Your tank is also where your breathing gas is stored, so obviously your diving tank is one of the most important pieces of gear you will take with you on your dive.
Some new divers refer to diving tanks as “oxygen tanks,” but in fact diving tanks usually contain an oxygen-rich air mix rather oxygen itself. Most tanks hold  0.11 to 0.64 cu ft and carry a maximum pressure rating of  2,900 to 4,400 psi (or 200 to 300 bars). In addition to being used in the water, diving tanks are sometimes used to treat diving disorders or are used for first aid above water.
Another pertinent fact: American divers are more likely to refer to their tanks as “diving tanks” or “scuba tanks” but British divers and some divers from overseas call them “diving cylinders.” Despite the different terms, everyone is talking about basically the same equipment.
Many divers choose to rent their diving cylinders – at least at first. While buying your own means you will have to maintain it as well, owning your tank is useful if you want to dive far from the nearest rental shop. It’s also handy to own your own diving tank if you want to dive at a popular destination – rental tanks can sell out quickly during tourist season.
When you make a decision to buy your own diving tank, consider:
1) Tank materials. Most tanks are made either from steel or aluminum. Each has its drawbacks and advantages. Steel tanks are more expensive and much heavier. They can also be prone to rust, so they do require a little extra care. On the other hand, they are much more durable and can withstand just about anything you might run into on a dive. Aluminum tanks are less expensive, a little larger, lighter, and tend to show up dents pretty quickly. Steel tanks have been around much longer than aluminum tanks, but most rental shops rent aluminum tanks (because of their price) so you may be more familiar with aluminum tanks. Tank materials will affect your weight needs. An empty steel diving tank is about three pounds negative while an empty aluminum diving tank weighs about two to four pounds positive. If you have been diving with an aluminum rented tank and switch to a steel tank, make sure to do a proper weighting test with your new empty tank as your weight needs will change.
2) Pressure. If you have decided on an aluminum tank, you will likely have a low-pressure tank. However, if you opt for a steel diving tank, you will have the option of choosing between a high-pressure and low-pressure tank. Again, there are disadvantages and advantages to each choice. Low-pressure tanks are larger than their high-pressure counterparts and contain less air. However, high-pressure diving tanks need an adapter for your regulator to switch your Yoke setup to a DIN first stage. In fact, most high-pressure diving tanks will need a DIN regulator set up. High-pressure tanks also generally require more maintenance and service than low-pressure tanks because the larger amount of pressure inside the tank results in more wear and tear, especially on the O-rings and valves, which will need to be replaced more often on high-pressure tanks. The higher internal pressure also means that high-pressure air tanks fail hydro inspections at a more frequent rate than their low-pressure counterparts.
3) DIN or Yoke. In order for your tank to work, it must be attached to your regulator. DIN and Yoke are the two varieties of coupling fittings which can attach your tank to the regulators 1st stage. Yoke coupling fittings tend to be the most popularly used. They are the coupling fittings used on aluminum 80 tanks, which are the tanks you get when you rent your diving tank. DIN coupling fittings are necessary for high-pressure tanks and are also considered to be safer than Yoke varieties. With a DIN coupling fitting, the O-ring cannot stick out, because the fitting screws right into the tank valve, keeping the O-ring in place. To make filling the tank easier, a yoke adapter is often recommended with a DIN coupling fitting. However, tanks with yoke coupling fittings can have a DIN adaptor for the same purpose. If you often use DIN valve tanks, get a DIN adaptor – it will need to be installed at a scuba shop by a scuba repair technician.
4) Size. Tanks are measured not in terms of external size, but rather in terms of holding capacity, which is described in pressurized cubic feet. Many divers who dive for recreation find that tanks in the 80 to 100-cubic-foot range meet their needs. Most rented tanks, for example, have 80 cubic feet of air. In addition to the size of your main tank, you might want to consider a supplemental tank. Sometimes called a spare tank, pony bottle, or pony tank, supplemental tanks are usually 40 cubic feet or smaller and are designed for use in an emergency. Since diving tanks are your lungs under water, it is a good idea to carry a spare. Keep in mind that your pony tank will require a separate regulator (1st and 2nd stage).