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Post by luis on Oct 3, 2015 16:29:43 GMT -8
Luis, I'll let you and Bill High duke it out on this one. I was merely going by what Bill wrote years ago: Believe it or not, I have my notes from the U.S. Navy School for Underwater Swimmers, the I took in 1967. I have not looked at them before for this question. This corroborates what you've said about them being tested. Maybe I should have helped myself to two of those cylinders when I saw a pallet of them. John Ok, I am impressed. I save stuff like that, but I have a hard time finding stuff that is not even that old. Nice hand writing. You may notice that the hydro test periodicity is every 3 years. That is not typical DOT requirements. I just looked at MIL-C-24316A (SHIPS) 4.4.3 Hydrostatic test. Each cylinder in the lot shall be hydrostatically tested to 5000 psi by the water jacket method of CGA Pamphlet C-1. The total volumetric, elastic permanent expansions shall be determined in accordance with 3.2.7. Cylinders that leak or fail to meet these requirements shall be rejected. This is a purchase spec, so there is no periodicity for re-qualification. Re-qualification are covered on a different type documents. I have older revisions of this Mil-spec, but they all read about the same. Note: What is now called: NAVSEA used to be call SHIPS
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Post by duckbill on Oct 3, 2015 21:49:55 GMT -8
We know the load was sustained for thirty-five (or so) years, but have no idea what kind of load we are talking about. Am I the only one who is even remotely curious how much load it took to lead to Sustained Load Cracking after being sustained for thirty-five years?
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Post by luis on Oct 4, 2015 4:56:19 GMT -8
At this point, the magnitude of the stress and the time-stress history becomes very interesting from an academic stand point, but I am not sure why it matters from a practical end user point of view.
Having more information is always good, but having time-stress history can actually give you a false sense of security if you are using it to make a decision.
It is interesting that the so called Sustain Load Cracking (SLC) has been found to accelerate more due to fill cycles, not just from holding a sustained static load for extended periods of time. The fill cyclic crack propagation behavior, actually follows more closely with the classical crack propagation theory.
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Post by duckbill on Oct 4, 2015 15:29:07 GMT -8
At this point, the magnitude of the stress and the time-stress history becomes very interesting from an academic stand point. Exactly why I'm curious, as I'm sure other scientifically minded viewers are.
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Post by luis on Oct 4, 2015 17:31:59 GMT -8
We know the load was sustained for thirty-five (or so) years, but have no idea what kind of load we are talking about. Am I the only one who is even remotely curious how much load it took to lead to Sustained Load Cracking after being sustained for thirty-five years? At this point, the magnitude of the stress and the time-stress history becomes very interesting from an academic stand point. Exactly why I'm curious, as I'm sure other scientifically minded viewers are. You are contradicting yourself… But you are very funny. BTW, I would not call that information by itself that scientific... a curiosity... yes.
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Post by red on Oct 4, 2015 20:23:34 GMT -8
I can't speak to those specific tanks, but somewhere around 1990, when "VIP PLUS" and eddy current were all the new rage, my friend and I dropped off some tanks for inspection, directly at a hydro shop. They condemned and drilled out THREE of our Alu80's (mine was Luxfer, his were either Luxfer or Dacor) that were all fairly new and lightly used.
Needless to say this resulted in some loud words. And eventually intervention from someone at the CGA and the tank makers, who re-examined the tanks and explained to the hydro shop that there will normally be manufacturing "marks" in the neck of the tanks, which LOOK DAMNED SIMILAR TO STRRESS CRACKS.
The shop bought us three brand spanking new tanks and had to get their staff retrained on using the VIP PLUS equipment.
So before you take anyone's word on "cracks" in the neck of a tank...Make Real Damn Sure they are an expert, and get a second opinion from an expert from the real expert side of things.
Ours were apparently not the only tanks to be falsely condemned--but we were among the few people that raised a loud ruckus and got the confirmed truth out of it.
FWIW.
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Post by duckbill on Oct 4, 2015 21:05:00 GMT -8
You are contradicting yourself… I can see how it looks that way, but it's irrelevant. My question was not directed to you.
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Post by luis on Oct 5, 2015 3:46:52 GMT -8
I can't speak to those specific tanks, but somewhere around 1990, when "VIP PLUS" and eddy current were all the new rage, my friend and I dropped off some tanks for inspection, directly at a hydro shop. They condemned and drilled out THREE of our Alu80's (mine was Luxfer, his were either Luxfer or Dacor) that were all fairly new and lightly used. Needless to say this resulted in some loud words. And eventually intervention from someone at the CGA and the tank makers, who re-examined the tanks and explained to the hydro shop that there will normally be manufacturing "marks" in the neck of the tanks, which LOOK DAMNED SIMILAR TO STRRESS CRACKS. The shop bought us three brand spanking new tanks and had to get their staff retrained on using the VIP PLUS equipment. So before you take anyone's word on "cracks" in the neck of a tank...Make Real Damn Sure they are an expert, and get a second opinion from an expert from the real expert side of things. Ours were apparently not the only tanks to be falsely condemned--but we were among the few people that raised a loud ruckus and got the confirmed truth out of it. FWIW. That was a very common issue in the early days of detecting SLC cracks. It is still an issue due to the lack of training, or knowledge, of many of the VIP inspectors. It is not hard to differentiate a tool mark left behind by the thread cutting tap. But the inspector needs to be aware of the difference. The tool marks do look suspicious at first, but it is not that hard to identify them. I can easily tell you that the tool mark is obviously an artificially straight mark (a crack is not), but if you have not being trained or are not truly aware of the differences they could get confused. Many cracks are extremely obvious, but not all. There are other indicators of the difference, but it much easier to view them than to explain them.
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Post by nikeajax on Oct 5, 2015 11:04:25 GMT -8
It took me a while to figure out that most dive shops don't do their own hydro's. It took this happening before I got too PO'ed and sought out where nearly all of Alameda County takes their tanks and fire extinguishers: The test facility I go to is pretty much exclusively a testing place: they know what they're doing and up to date safety certified The best part is I don't get a lecture about how my Rene tank is so old fashioned and that I should buy a new modern steel tank Jaybird
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Post by surflung on Oct 5, 2015 13:43:46 GMT -8
Failure is Liberating... - I have to say, the Eddy test failure of my SP6498 USD Aluminum tank is quite liberating. I no longer feel like I'm wasting a good tank by retiring both the bad one AND the good one. As for the sentimental value because these USD 50s were a gift from my Dad... Well heck, my Dad wouldn't want me to keep screwing around with these things when the questionable safety of them is what it is.
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Post by red on Oct 5, 2015 14:03:03 GMT -8
Inquiring minds just have to ask: Has anyone done cryogenic treatments on either steel or alu tanks? Dip 'em in liquid nitrogen, bring 'em down to around -300F, cover 'em up and let them slowly come back to room temperature, and the crystal structure of the metal changes. Stresses usually get relaxed, and all sorts of good things have been known to happen.
Just wondering...would that also increase the long-term conformance of a steel tank? (And can we apply for some government grant money to study the question, if they haven't got an answer yet?(G)
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Post by SeaRat on Oct 5, 2015 14:47:48 GMT -8
Red, I haven't done anything like dipping a tank in liquid nitrogen, and don't know whether that is a good thing or not. I'd have to do a lot of research, talk to some metallurgists, etc. before even considering it. I took my AL 80 into the river with my Dacor Nautilus CVS, and had a great dive with the system. Saying that, this may be the last dive of this AL80, as it is pretty easy to use my single steel 72s for this system. The only thing lost is the ability to stand it vertical, due to the flat bottom of the AL 80. I will use it that way (it's now at about 1300 psig) for a talk on buoyancy compensation this Saturday in Olympia. Then, it probably is headed back to the mothball fleet. The twin AL 50s that I have and have mothballed had an interesting history. I got them from the wife of a diver killed in a construction accident. He was on scaffolding using fall protection when the portion of the bridge they were working on gave way, and dragged him to his death. They are nicely painted, and as such make a nice display. I've dived them since the 1980s, I think, but now they are out-of-service. Here's a photo of them: Here's a photo of my Nautilus CVS with a single 72 mounted on it. Note that the tank boot is off, and this is because a tank boot would interfere with dropping the weights held between the tank and the Nautilus CVS in the weight chute. John
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Post by red on Nov 2, 2015 11:44:59 GMT -8
"the Eddy Current process is fool proof " Always remember the Gods have more time than you do, they are constantly building Improved Fools.
When all this stuff was new, my friend and I had some tanks hydro'd and three Alu80's were condemned on the basis of the eddy current test. And after some discussions with the manufacturer and equipment maker, THE SHOP BOUGHT US THREE NEW TANKS BECAUSE THE IDIOT DIDN'T KNOW HOW TO INTERPRET THE TEST and he'd condemned three perfectly good tanks.
One hopes by now that any operator would be better trained and experienced, but that's one reason they now just stamp "CONDEMNED" instead of cutting the tank up, so you can get a second opinion and make sure the cracks are really stress cracks and not someone's imagination.
Just saying, a second opinion from the most qualified source you can find, might not be unreasonable.
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Post by SeaRat on Nov 2, 2015 22:23:39 GMT -8
My AL 80 is now in storage with about 200 psig in it. I doubt I will have it filled again until the Spring, as the rains have now descended on Oregon. It was a good dive, with some of the clearest water of the year. I will probably dive a few more times this fall and winter, but even if I use the Nautilus CVS, it will be with a single steel 72.
John
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