What exactly is the eddy test?

Discussion in 'Tanks, Valves and Bands' started by grouchyturtle, May 27, 2004.

  1. grouchyturtle

    grouchyturtle Photographer

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    First of all what exactly is it?

    Second I thought this was an additional test done on the older aluminum tanks made from the 6351 alloy?

    I have a 30cf Catilina that is only about 2 years old and one of the VIP stickers on it says Eddy Current. See the top sticker.

    Is it done on all tanks now?

    Is this required on all tanks now? Just wondering, because the 2004 sticker doesn't say this. Can I remove the top sticker? It doesn't make any sense having a 2003 and 2004 sticker.

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  2. Delta_P

    Delta_P Nassau Grouper

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    Magic.

    Er, actually it has something to do with testing for flaws in aluminum by inducing a current in the material and measuring the induced magnetic field. The electrical current will "eddy" around defects and the probe will sense this. Technology stolen from the aircraft industry.

    Hopefully an engineer will chime in here and point out how off base this explanation is.


    Correct.


    Probably. The shops that paid $1500 for an eddy current tester seem real keen to use them.

    No problem until they get a false positive on the 6061 alloy and fail your tank. Then you get to hump it around town for a second opinion or ship it back to Luxfer (on your $) so they can tell you there's no problem.


    No. Luxfer has advised against its use on 6061 tanks since 2000. Latest memo is here:

    http://www.luxfercylinders.com/news/releases/20040204a.shtml



    You just need the current VIP sticker. If nothing else, the kid at the filling vault will appreciate not having to waste five minutes reading your tank trying find the valid sticker before he can fill it.
     
  3. Drew Sailbum

    Drew Sailbum Scuba Instructor

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    Go ahead and remove the 2003 sticker. That should have been done during the visual inspection.

    Eddy Current testing is a supplement to a Visual Cylinder Inspection (VCI). It is specifically designed to detect cracks in the neck threads. It does not test any of the rest of the cylinder. The device uses a sensor which detects alterations in an induced magnetic field caused by cracks in the threads.

    The various eddy testing devices were introduced as an enhanced means of examining threads after the old 6351 alloy tanks began going BOOM. This alloy is susceptible to Sustained Load Cracking (SLC). The visual eddy machines were specifically designed to work in 6351 aluminum alloy and are not designed to be used in 6061 alloy cylinders. Allegedly newer designs are in development which would work in either alloy.

    SLC did not become apparent as a means of cylinder failure until the 6351 alloy cylinders had been in service for some time. Metallurgists allege that the differing properties of the 6061 alloy mean that SLC cannot happen in the newer alloy. Time will tell, and there are some pretty old 6061 cylinders out there. As of yet, there have been no confirmed cases of SLC in 6061 alloy cylinders. However, cracks can develop in threads in 6061 cylinders, they are just caused by other means.


    The widespread use of Visual Eddy devices in 6061 cylinders has led to a high rate of false positives. A contributing factor is that neck threads must be very clean for the device to work properly. Debris left in the threads can read as a false positive. Thus Eddy testing should only be done as a supplement to VCI. A trained technician should be able to visually verify the prescence of a cracks detected by eddy current testing.
     
  4. Scottri

    Scottri Divemaster

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    What Drew Said!
     
  5. scubatoys

    scubatoys ScubaBoard Business Sponsor ScubaBoard Business Sponsor

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    We test them all with it... but we don't trust the machine by its self. We get very few "hits", and when we do, we mark the spot with a grease pencil, so we can take a good look with the light and mirror. If used as an aid to visuals - and not a replacement of them, I think they are a good tool.

    It's like my dad told me... "If the train track lights are flashing it means a train is coming. If they're not - It means one might be."

    Same kind of theory in reverse, and I'd rather get a false positive and rule it out with the mirror, then miss something that eddy testing might find.
     

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