What Test Are Done on SCUBA Tanks?

Visual Testing

Scuba tanks require annual visual inspections to ensure the interior of the tank isn't corroded and the integrity of the walls can withstand high pressure. Tanks that are depleted to an air pressure less than 300 PSI must undergo a visual inspection before refilling to ensure no water entered the tank. A visual inspection consists of depleting the air tank and removing the tank valve. Dive operators and technicians insert a bright light to inspect the interior of the tank for any discoloration that may indicate corrosion or weak spots. Additionally, they inspect the outside of the tank and the threads for the tank valve for any cracks or excessive wear. These signs may indicate possible problems with the integrity of the tank to withstand high pressures from the internal air and depth underwater. Tanks that passed visual inspections contain a sticker imprinted with the dive operator or technician and the inspection month and year. Tanks that maintain annual visual inspections may be safely refilled and reused.

Hydrostatic Testing

Air cylinders require hydrostatic testing at least every five years. Dive operators and technicians usually don't perform hydrostatic testing on-premises because the battery of tests require a specialized pressure chamber in case of possible explosion. Hydrostatic testing involves depleting the air from the tank and testing the material. The tank is filled with water and subjected to high pressure, up to 1.6 times the tank's rated pressure capacity. Once pressurized, the level of expansion of the walls is measured and compared to established tensile strengths. Hydrostatic testing requalifies the working pressure or load capacity of the tank for compressed air. Tanks that passed hydrostatic testing bear an imprint near the tank valve with the inspection code, data and pressure rating (part of the tanks crown markings).

Enriched Air

In addition to visual inspection and hydrostatic testing, specialized cylinders require additional cleaning and tests, such as enriched air scuba tanks. Enriched air (or nitrox) consists of higher levels of oxygen for breathing, usually above 21 percent and below 40 percent oxygen (air normally contains 21 percent oxygen). Preparing and testing cylinders that will be filled with oxygen involves cleaning (or tumbling) the interior of the tank with solvents to remove materials that may react with oxygen. After flushing and rinsing the tank, further tests check for the presence of reactions or residue. Technicians also apply special, nonreactive lubricants on the tank valve mechanism. Nitrox-approved scuba tanks bear a special sticker indicating "Enriched air." If you fill a nitrox tank with air (containing 21 percent oxygen), it must be recleaned and re-certified before it can be filled with enriched air.