I am sure you have seen firemen holding some tanks on their back. That’s because they need air when they are trying to extinguish the fire, staying in anoxic environments due to the smoke. Just like firemen, we need air underwater.
What is inside the dive tank is air, not pure oxygen. Air consists of oxygen, nitrogen and a very small percentage of other gas. Differences of air in the tank and the one you breath now are:
- There is no moisture, dirt and dust in the tank. They are filtered while getting filled by compressor.
- Air in dive tank is compressed. Because the largest dive tank is 18 lt. But we need much more than that underwater. So it becomes 3600 lt when the pressure is 200 atm/bar (3000 psi).
Tanks have variety of sizes. But it is not like XS, S, M or L, but like 10 lt, 12 lt, 15 lt or 18 lt. They may also vary in colors but the cylindrical shape is constant. You may see net on the dive tanks. They are used for protecting the paint of the tanks and the corrosion is minimized.
The most important piece of a dive tank is the o-ring in the valve. It surrounds the space between the regulator first stage and the tank valve, preventing the air leak.
Dive tanks must pass hydrostatic test on a regular base (frequency change regional). In hydrostatic test, tanks are filled with water in a pool (for safety) above the working pressure. Volume before and after is checked and if the difference is above the tolerance rate, the tank fails the test. Failed tanks cannot be used anymore. When a tank passes the test, test date is stamped with a seal on the tank.
There are some markings on each dive tank including the test date as mentioned above. Other markings include volume of the tank, working pressure, brand, material, serial number and such information.
A hard plastic base is generally used that hold the tank straight while mounting scuba. But when unused, dive tanks should be laying on the floor. Because they may fall and cause injury and/or equipment failure. Plastic base is generally (90%) used for steel tanks, because aluminum tanks already have flat base. You can easily differentiate steel and aluminum tanks by this.
There are two kinds of tank valves: K-Valve and DIN (Deutsche Industrie Norm) Valve
This is generally a standard valve for dive tanks. It is used on dive tanks that have a working pressure of 200 atm/bar (3000 psi). Consists of a simple open/close mechanism.
Used on tanks that have a working pressure above 200 atm/bar (3000 psi). DIN tank valves can only be used with DIN regulators. Regulator is screws this valve on the tank. By using a simple adapter, DIN valves can transform for usage with Yoke regulators.
Dive tanks are made of two material: Aluminum and steel.
Aluminum Dive Tanks
- These dive tanks are much more lighter than the steel tanks. An aluminum dive tank becomes positively buoyant when the air is decreased in it. That’s why it requires the diver to take more weights compared to the steel dive tanks.
- Because aluminum tanks are softer, they scratch more.
Steel Dive Tanks
- Steel tanks are negatively buoyant. Even when the air is decreased in it, they don’t pretend to float. They only become “less negative“. This makes them heavier than the aluminum tanks.
- Main disadvantage of the steel tanks is the corrosion possibility. When filled improperly, moisture can damage steel. That’s why steel dive tanks require to get examined for oxidation and should be removed periodical.
- With proper care, steel tanks generally last longer than the aluminum ones.
Dive tanks also vary in pressure capacity. More pressure means more air, but also means thicker walls for safety. Higher the pressure capacity a tank has, higher the price should be paid. Here is the chart:
- High pressure: 220 – 235 atm/bar (3300 – 3525 psi)
- Standard pressure: 200 atm/bar (3000 psi)
- Low pressure: 160 – 175 atm/bar (2400 – 2625 psi)
I don’t really advise you to buy a tank unless you will dive with your buddy by using your own dive boat. They are heavy and need to be re-filled. Don’t ravel with a dive tank but rent one from the dive center you’re planning to dive.