Scuba Tank Testing and Maintenance
Let's take a look at scuba tanks and their testing requirements.
1. Controlled by the U.S. Department of Transportation
Pressure vessels are regulated by the DOT for the safety of transport vehicles, their passengers and the environments through which they pass. Every scuba tank model must pass an initial DOT certification, then EACH manufactured scuba tank must pass a test before it can be sold. This is 100 percent inspection that even racing tires are not subjected to! It is somewhat equivalent to space specifications, so this is an extremely well-controlled industry!
2. Hydrostatic testing
After the initial test, the results are stamped into the body of the scuba tank, along with the date (year and month) it passed. After that, the tank must be retested hydrostatically every five years. The test for each tank is on file at the DOT, and the testing station must contact them to obtain the correct test specification. A common hydrostatic test is to pressurize the scuba tank to 5/3 of its working pressure and to measure the flexing of the tank walls. Five-thirds of a 3,000 psi working pressure means that the tank will be pressurized to 5,000 psi. This is done by replacing the valve with a special hydraulic testing connection and filling the scuba tank with water under pressure - NOT air! Air would be too explosive if the tank were to let go during the test, as it sometimes will. Because water cannot be compressed, the tank cannot explode if it fails.
Testing is done inside an armored vessel. The one my hydro station uses is sunk into the shop floor for added protection. The tank is also under water, and they measure the amount that a water column rises as the tank is pressurized. This measures the amount of tank expansion. The water inside does not expand, but the aluminum or steel tank casing certainly does! They are looking for tanks that DO NOT expand as much under pressure, indicating that their walls have been work-hardened over the years. Like a piece of steel that is repeatedly flexed, the tank walls get hard to the point that they may suddenly fail with a snap. This is EXTREMELY rare outside the testing station. When it DOES happen, it's almost always fatal to people standing close by.
3. You know it when they fail!
When a tank fails during hydro, it's usually the valve threads that let go. It sounds like a loud "whump" if you are in the building when one fails. The other thing that sometimes happens, but not as often, is that the bottom of the tank bursts open. This is due to corrosion. If the tank fails to flex enough during the test, it can never be filled with air again, and no dive shop will do so. It will not be stamped with a current test stamp (one within the past five years) and is "out of hydro" as far as any dive shop is concerned. One possible use for a failed tank is to cut off the bottom with a carbide saw (a lengthy process!) and hang it far downrange on a rifle range to be used as a gong.
4. Will a dive shop ever cheat?
Yes, it does happen. The few fatalities that occur usually happen at dive shops when shop operators fill out-of-hydro scuba tanks. So, in a cruel way, this is a Darwinian control over cheating! If you follow the rules, you are as safe as an airline passenger, and several times safer than any car passenger. I've been in three auto accidents in my life, but no airplane or scuba tank accidents - thank God!
5. The annual visual inspection
The annual visual inspection is performed by a dive shop. They let the air out of the tank and removes the valve to have a look inside. They're looking for signs of corrosion and will refuse to put a visual inspection sticker on the tank if they find any. These stickers are paper or plastic and they stick to the tank. Every dive shop I have used has refused to fill a tank that has an out-of date inspection sticker. But they can do the inspection right there - the tank does not have to be sent to an outside party. They open the valve a little to slowly let the air out. This prevents condensation from rapid cooling. It takes 12 hours or more for a tank to bleed down - especially since airgunners seldom let their tanks drop below 2,000 psi. Let the dive shop bleed your tank for you; they know what they're doing!
6. Questions and Answers
Question: Why is it necessary to clean scuba tanks for oxygen service?
Answer: To remove contaminants that can become the fuel for an oxygen-fed fire.
Oxygen, an oxidizing gas, does not burn, but it supports combustion and causes other materials to burn. In the presence of pressurized oxygen concentrations exceeding that in standard air, most materials become progressively more flammable and ignitable as the oxygen concentration and pressure increase. Even materials that would not easily catch fire in standard air under normal atmospheric pressure will ignite and burn furiously in a pressurized oxygen-enriched environment. It cannot be overemphasized that rising pressure increases the risk of ignition.
For an oxygen-fed fire to occur, three things must be present: oxygen, fuel (something to burn), and heat (an ignition source)—this is the classic “fire triangle.” When dealing with pressurized oxygen or EAN in a scuba cylinder, remember this important variation on the classic three factors needed for a fire:
Any of these contaminants—many of which are very difficult to detect—can be the initial fuel for an ignition event, the technical term for starting a fire. Once an oxygen-fed fire gets going, even metal components—including a scuba cylinder itself—can burn vigorously. Aluminum melts at 1,220 degrees F (660 degrees C); after an oxygen-related cylinder fire, it is not unusual to find large areas of melted aluminum—which shows just how hot and energetic such a fire can be.
That’s why it’s so essential to remove contaminants and to keep the oxygen system clean. Proper oxygen cleaning need not be excessively costly or time-consuming, but it does need to be effective. Removing contaminants and keeping equipment oxygen-clean is the best way to make sure that an oxygen-fed fire never starts.
A number of agencies and organizations in the United States provide certification courses in oxygen cleaning. For example, American Nitrox Divers International (ANDI) offers a particularly comprehensive training course that defines “oxygen cleaning” as “the process of contaminant removal from component parts or assemblies intended for contact with gases containing Oxygen concentrations of 23.5% or more by volume.”
Question: How clean does my cylinder actually need to be for oxygen service?
Answer: It needs to be “oxygen clean”—which means free from contaminants that can be the fuel for an oxygen-fed fire (see the list of contaminants in Question 3, above). Cleanliness on surfaces is measured by milligrams (mg) of hydrocarbon per square foot of surface area (mg/ft2). In the U.S., DOT (in RRC 901c) requires that the cleanliness of cylinders used in oxygen service be proven to be less than 2.5 mg/ft2, which is a level considerably below what can be seen by the naked eye. For this reason, a visual inspection after cleaning is not enough. A test must be performed to verify that the required cleanliness level has been achieved. In CFR49 173.302(b), DOT specifies an approved cleaning method and a standard test to determine the cleanliness of cylinders.
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