Design And Testing Of Scuba Tanks
Description Of The Process For Scuba Dive Tank Inspection And Test
Sep 12, 2008
Scuba tanks have undergone many years of research and design modifications. Initially tanks were made of steel but these were heavy and rusted badly in the salt water environment. Aluminium alloy tanks were developed which are lighter and have an in-built corrosion resistance.
Modern tanks are designed to be negatively buoyant when full and slightly positive when empty. An 88 cubic foot aluminium tank is about one kilogram negative when full and nearly two kilograms buoyant when empty. This difference should be considered when planning a dive.
Aluminium Alloy 6351
Before 1990, some scuba tanks were made of aluminium alloy 6351. It since become apparent that these tanks can develop hairline cracks around the neck; especially if the tanks are left in storage for a period of time at high pressure. Every two years these aluminium alloy 6351 scuba tanks should undergo an ultrasonic test to check for cracks.
Testing Tanks
Australian Standard 2030 require scuba tanks be tested every year. The following is the basic process for an annual tank inspection.
External Visual Inspection
First an outside inspection to check for damage. Any major decrease in the wall thickness can decrease the strength of the tank. If roughly treated and there are a few dents and deep scratches a tank could be rejected.
Some divers use car body filler to cover up indentations. Most testing technicians are aware of these tricks and easily pick them up.
Internal Visual Inspection
Next the tank is drained of air, the valve removed and the tank inspected internally using special lighting apparatus. This is a check for corrosion, cracks or foreign matter. Special attention is paid to the neck and the thread where the valve screws in, a common place for cracks.
At this stage the internals of the tank might be wire-brushed to remove any minor corrosion. Or it could undergo a full internal polish.
Hydrostatic Test
The tank is then weighed to check any significant metal loss compared to its original weight, as stamped on the tank by the manufacturer.
The next test is the hydrostatic test. Before it undergoes the test the tank is measured to check its shape. It is then filled with water and pressurised to the test pressure as marked on the tank. As water doesn’t compress, the tank has to absorb this pressure. The shape of the tank is compared with its empty shape and any major deformation will result in rejection of the tank.
Reassembly
The tank is dried out and re-inspected internally to check any problems that could have occurred during the hydrostatic test. The valve is fitted and the tank filled.
Following the successful completion of the test, the month and year are stamped on the tank along with the testing station stamp. Most test stations have a system for recording tank tests. These detail the specifics of the tank, last test date, and any items to look for on the next test.
Valve Test
Every second tank test is a good time to have the valve tested. Most test technicians will get an idea at the tank testing stage whether the valve needs further inspection. It can pay to have the burst disk replaced at every valve service. The disks eventually wear because of age and it won’t add much to the cost of the service.
Regular testing of scuba tanks is plain common sense. A tank failing explosively can be catastrophic. Even if a diver has their own compressor, a tank should be regularly tested.
For further information on scuba tanks, refer to:
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