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Fill Express™   Serving Fort Lauderdale and Southeast Florida

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Frequently Answered Questions About Aluminum 6351-T6 SCUBA Cylinders We are sometimes asked about the rationale behind our recommendations and policies regarding the the issues surrounding Sustained Load Cracking (SLC) in aluminum 6351-T6 SCUBA cylinders.

Will Fill Express fill my aluminum 6351-T6 alloy SCUBA cylinder?
Fill Express will fill an aluminum 6351-T6 alloy SCUBA cylinder if Fill Express has performed the annual visual inspection on the cylinder including an eddy current exam or if it has been hydrostaticly tested within 12 months AND you can provide documentation that the DOT testing facility also performed a combined visual and eddy current examination to PSI standards. However, Fill Express does not recommend renewing the visual inspection or hydro-static test for aluminum 6351-T6 alloy SCUBA cylinders because of issues associated with sustained load cracking (SLC) in those cylinders. We recommend that aluminum 6351-T6 SCUBA cylinders be removed from service.

How do I know if my aluminum cylinder is made from the 6351-T6 alloy?
Different manufacturers ceased using the alloy for production of SCUBA cylinders at different times. Luxfer phased 6351-T6 out over two years and had completely ceased using it by mid-1988, Cliff Impact ceased using 6351-T6 by mid-1990, Walter-Kidde ceased SCUBA cylinder production using 6351-T6 in 1989, and Catalina never used 6351-T6. The US Department of Transportation (DOT) issued an Advisory which identified in detail most 6351-T6 alloy cylinders (including many non-SCUBA cylinders) manufactured in the US. You should consult the advisory to precisely determine which cylinders are made of from the 6351-T6 alloy.

What is an eddy current exam?
This examination uses a tiny current of electricity and is performed on some types of aluminum cylinders to assist in the detection of cracks in the threaded neck area. It alerts the inspector to specific locations on the neck threads that need further evaluation. The devices using eddy current technology are sold under the brand names Visual Plus, Visual Eddy, and Simple Eddy. The exam is notorious for false results as a result of poor operator technique and is intended only to supplement, not replace, the visual inspection. Eddy current exams are not meaningful on any type of steel.

Why do aluminum cylinders made from the 6351-T6 alloy require an eddy current test to help detect cracks?
This particular alloy, used during the early days of aluminum cylinder production, has a history of a phenomenon known as sustained load cracking (SLC) that can cause a cylinder to fail. to read an article by Bill High, President of Professional Scuba Inspectors, Inc. (PSI), for more information about sustained load cracking in aluminum cylinders.

Does Fill Express require an eddy current exam on all cylinders before filling?
No. We require an eddy current exam only on cylinders made from aluminum 6351-T6 alloy. By mid-1990, the US aluminum cylinder industry was using a different alloy, known as 6061-T6. There are no indications that the 6061-T6 alloy has problems with SLC, and no steel cylinder has ever shown problems with SLC.

Why will you only fill 6351-T6 cylinders that Fill Express has inspected?
According to Luxfer, your 6351-T6 cylinders are safe for us to fill only "as long as your 6351-alloy scuba tank is properly inspected". The problem is some aluminum 6351-T6 SCUBA cylinders are NOT properly inspected, and therefore may be unsafe to fill because of SLC. According to Bill High, President of Professional Scuba Inspectors, Inc. (PSI) only about 10% of the people performing SCUBA cylinder inspections in the US are trained inspectors. Proper inspection of 6351-T6 alloy cylinders requires special tools and additional specialized training. Because we have no way of being certain the cylinder has been properly inspected, and the increased risk to our fill station operators if it hasn't, we choose to require all 6351-T6 alloy cylinders be inspected by our PSI trained staff before we will fill them.

Aren't 6351-T6 cylinders properly inspected when they are hydrostaticly tested?
The DOT now requires 6351-T6 cylinders receive a combined visual and eddy current examination when they are hydrostaticly tested. However, the DOT has not clearly established a training and inspection standard for combined visual and eddy current examination; they leave it to each facility to determine their own specific procedures. If you have a 6351-T6 cylinder that has been hydrostaticly tested within the last 12 months, AND you can provide documentation that the DOT testing facility also performed a combined visual and eddy current examination to PSI standards, our Fill Station Operators at their personal discretion may elect to fill the cylinder. As always, Fill Express reserves the right to refuse to fill any cylinder for any reason.

Why do you recommend 6351-T6 SCUBA cylinders be removed from service?
Because our customers tell us it's becoming increasingly difficult to find facilities that are willing to fill the 6351-T6 SCUBA cylinders. In southeast Florida, and particularly the Keys, some dive shops will not fill 6351-T6 alloy cylinders unless they have performed the visual inspection and some shops will not fill them under any circumstances. We feel it's a disservice to our customers to charge to perform an inspection or hydro when they may have difficulty getting the cylinder filled elsewhere.