The air supply on your back
Throughout your experience as a diver, you will likely have a conversation a number of times on what the proper name is for a scuba diving tank. The most appropriate name is a tank or cylinder, although you will also sometimes hear gas bottle. But gas bottle would be wrong because a gas bottle is only designed to store flammable gas, and it is manufactured differently, with visible welding. So it can withstand less pressure than a scuba diving tank.
Luxfer and Catalina are the two largest makers of scuba tanks, and tanks are usually made of either stainless steel or aluminum. Each material has its own advantages and drawbacks. Steel is heavier and doesn’t require carrying as much ballast in our belts, which is great, but it will oxidize quicker in contact with the water due to its carbon composition. Aluminum is lighter and withstands oxidation better, but is less shock resistant. Depending on the country where you are diving, you will come across one or the other. Nowadays, aluminum tanks are somewhat cheaper to make, and they have become common in a large number of countries.
Sizes range from 6 liters, up to 18 liters, although the 12-liter ones are most common. There are also personal tanks called Ponies, starting from a half-liter in capacity, priced from 170 euros, which are used for emergency situations when you are alone. Remember that you should always dive with a partner and that you should be responsible and check your air on a regular basis. Tanks are filled with the same air we breathe normally, with special compressors, filters to remove impurities and to eliminate humidity, at a pressure of 200 to 300 bars, depending on the features. All tanks have a plug or protector of its valves, indicating that it is full. A diver is responsible for checking and smelling the air in the tank before an immersion, to make sure that the air is clean and uncontaminated. If the air smells strange, do not dive with the tank, because it can be hazardous to your health. If you multiply the tank’s capacity by the filling pressure, you can determine exactly how many liters of air you are actually carrying. A 12-liter tank with 200 bars of pressure would have a capacity of 2400 liters of air, which is enough to dive for nearly an hour in normal breathing conditions.
All tanks need two safety tests to ensure they are in good working order. One is a yearly visual inspection, where a qualified technician will check the exterior and interior of the tank and its valves for any corrosion or defects. If all is in order, the inspector places a sticker on the tank showing the inspection date. Otherwise, the tank must be discarded if any irreparable damage is found.
The second safety test is the hydrostatic test, where a tank is subjected to higher-than-normal pressure while being filled with water, in order to verify the metal’s flexibility under changes of pressure over a period of several hours. This test is mandatory in all countries, although the frequency will depend on the country’s regulation. In Spain, this test must be performed every three years, although five years is common in many countries. You can see the test in the silk-screen printed nomenclature towards the top of the tank, showing the month and year, with a logo in the middle of the two dates naming the company that conducted the test. Any tank that fails to pass the hydrostatic test must be discarded. The useful life of a tank is approximately 20 years.
Let’s talk about tank valves. There used to be a model called “J”, but it did not yield good safety results because it involved a mechanical system of levers for activating the air reserve compartment in tanks. Because this system was prone to mechanical and human failure when filling a tank, and caused accidents, it is no longer used. At present, the K valve system is used for both recreational and technical diving, and it includes a safety system called a burst disk. This system is activated when the tank is filled beyond capacity, and all the air is released through a copper spiral on the side.
Like regulators, international/Yoke valves are often used for recreational diving, or DIN valves for technical diving. The main feature of an international/Yoke vale is the metallic core with rubber called a thoracic joint around the entry point, which has the job of preventing air leaks and providing perfect water tightness.
Then there are DIN valves. Their design allows for winding the regulator directly on the valve, making it a more compact and resistant block. The thoracic joint is mounted on the regulator and not on the valve.
All valve systems must show their maximum pressure and you must never exceed it. Can you mix different valve types with different types of recreational or technical diving? Yes and no. A safety-conscious diver must adapt his or her gear to their precise immersion profile to minimize any risks. The rest is up to you.
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