Aluminum Tank Controversy Continues, August, 2000 Undercurrent feature story

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Aluminum Tank Controversy Continues
safely separating truth from fiction

from the August, 2000 issue of Undercurrent

Since our March article on the dangers of Sustained Load Cracking (SLC) in a small number of older aluminum tanks, we’ve been hearing lots of misinformation within the industry regarding this problem and what’s being done about it. Let us set the record straight.
There’s no question that SLC has led to catastrophic ruptures of aluminum scuba tanks manufactured before 1990 from alloy 6351-T6. The U.S. Department of Transportation (DOT) has files on three such aluminum tank explosions, including the 1998 maiming of a Riviera Beach, Florida dive shop worker that we reported.
While a San Diego newspaper reported that on June 4 an instructor was injured “when a scuba tank exploded at La Jolla Shores Beach,” that wasn’t correct. Bill High, whose firm Professional Scuba Inspectors, Inc. (PSI) monitors such incidents, tells us that the instructor actually was burned when he turned on the valve of a cylinder containing a high percentage of oxygen and the mixture spontaneously combusted (a risk of gas with high oxygen mixes). There was no explosion or internal damage to the cylinder. A few days later a fatal explosion did occur at a popular dive site in Ontario, Canada. But the tank that ruptured was a steel military surplus ballast cylinder being used for diver air storage. The mass media don’t always make such fine distinctions, so be wary of accepting news reports at face value.
Additional confusion has arisen from DOT’s listing of scuba tank brands and models most likely to be made from 6351-T6, which we published in March. Some shop owners and divers have mistakenly assumed that the manufacturers are recalling the listed tanks. In fact, no recalls have been announced. The tanks are still in service, providing they pass a visual inspection (VIP) annually and a hydrostatic test every five years. That's for U.S. tanks - other nations have their own testing requirements. The leading manufacturer, Luxfer, has just issued a new policy statement requiring that every Luxfer 6351-T6 aluminum scuba cylinder be visually inspected at least every 2.5 years and that the cylinder neck be tested with an eddy-current device such as Visual Plus or Visual Eddy (using electromagnetic waves to detect cracks in tank threads that might not be visible to the naked eye) or equivalent non-destructive testing equipment. These requirements are in addition to DOT-mandated VIP and hydro tests. For cylinders in heavy use (for example, those filled five or more times a week), Luxfer recommends visual inspection every four months.
Luxfer also recommends that eddy-current devices be used only on 6351-T6 cylinders, because the procedure has produced "false positive" readings in its newer 6061-alloy tanks, which it began producing in mid-1988 in the United States. There have been no reports of SLC in tanks made from the harder 6061 alloy. PSI further recommends that VIPs be performed just before and after each hydro-test, since the hydrostatic procedure (in which the tank is pressurized with water to 5/3 of its working pressure) can make cracks more visible.
Not all testing facilities follow these procedures. High points out that some dive shops require and charge for eddy-current testing not only on all aluminum cylinders, regardless of their alloy, but also on steel tanks, though there is no history of SLC in 3AA steel cylinders. So don’t pay for unnecessary tests. Ask a few questions about testing procedures, and be sure you’re comfortable with the answers.
If you’re unsure whether a tank you may own or rent was made from 6351-T6, check the earliest hydro test date (date of manufacture) stamped on the tank’s shoulder. If it’s earlier than 1990, assume it’s made from 6351-T6 and follow the above procedures just to be on the safe side.
If a U.S.-made original-owner Luxfer tank is more than 10 years old and fails either a VIP, hydro or eddy-current test, you can return to Luxfer with $50 for a replacement. All returned tanks are subject to Luxfer’s own examination of the tank. (Call Luxfer at 1-909-684-5110 for specific details.) These policies apply only to condemned Luxfer tanks. No other manufacturers offer a return policy.
If your tank is made of 6351-T6 and passes all the tests, it’s probably safe – especially since Sustained Load Cracking takes years to develop to the detectable (or dangerous) point. Only you can determine your own comfort level in continuing to use it.
-D.L.


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Aluminum Tank Controversy Continues safely separating truth from fiction

Aluminum Tank Controversy Continues
safely separating truth from fiction