Ron
Regular Diver
Posts: 41
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Post by Ron on Feb 19, 2006 11:52:51 GMT -8
Were would all the tumblers, tumbling media, tumble juice and flash reducing liquids go if we get rid of all our steel tanks.
Ron
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Post by duckbill on Feb 19, 2006 16:08:03 GMT -8
Though rare, under some circumstances aluminum can develope a very insidious type of corrosion with deep pits and all I am told. Sea water in an aluminum tank is just as harmful as in a steel tank although the aluminum may give you a little more time to correct the problem without mishap. Aluminum is certainly not corrosion-free. Chances are, you pulled the valve on a nice warm, humid day and the rust formed when the relative humidity in the now closed cylinder rose drastically on a cold day, forming condensation. Who knows. Only your tank knows for sure!
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Post by SeaRat on Feb 19, 2006 22:31:11 GMT -8
Nemrod,
Leaving a tank at ambient pressure with the valve off allows the air to exchange and water vapor to be introduced into the tank. Then the normal temperature fluctuations can cause condensation to form inside the tank, and rust starts immediately. It is normal precaution with a steel tank to always leave the valve on it, and at least a few hundred psi in the tank to ensure that this does not happen.
Concerning the aluminum tank, from what I know the corrosion process is the opposite of a steel tank, in that aluminum oxide forms a protective coating that inhibits further corrosion. In steel, the corrosion process is an accelerating process. Corrosion should not be a huge problem for aluminum tanks, even with exposure to sea water. But aluminum tanks can develop a "sustained loading crack" in the neck by the threads if pressure is left in them all the time. This is very rare, but apparently can happen (especially to one type of aluminum alloy), which is why the aluminum tanks also need the VIP.
John
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Post by luis on Feb 20, 2006 6:13:13 GMT -8
Aluminum oxide will normally protect from further corrosion, but I have seen tanks with deep corrosion pits on the inside. I am not certain of all the conditions involved to cause this type of corrosion. I imagine salt water and high O2 partial pressure were definitely involved. A LDS has an aluminum tank that the exterior is heavily pitted. It looks like the craters of the moon. The tank was brought up by a fishing net from a few hundred feet deep. The tank was full of air, but the outside was so pitted it was probably about to start leaking through the wall. This obviously a very rare occurrence. I am just mentioning it because I thought it was kind of interesting. I bet it was pretty scary when the fishermen realized that the tank was full. Sustained load cracking has been found to affect aluminum made of alloy 6351. Newer tanks made of 6061 have not shown any signs that they are affected by SLC. In the US aluminum alloy 6351 was used until 1988. The link to the Luxfer web site below has good information: www.luxfercylinders.com/support/faq/sustainedloadcracking.shtmlSustained load cracking is relatively uncommon, but it does happen. As a favor to two of my LDS I have cut (using a band saw at work) three tanks that had visible cracks in the neck. I cut the top off the tank so they could use it for show and tell. Three tanks out of the hundreds of tanks they deal with is a small number, but it does happen. The only way to protect against it is to store your tanks with just a couple hundred psi of air. Another un-usual type of corrosion on aluminum Scuba cylinders is galvanic corrosion. This happens when dissimilar metals are in contact and there is an electrolyte present (as salt water). Many years ago we were removing a valve from an aluminum tank and the aluminum threads came out with the valve. The chrome normally isolates the bronze/ brass valve from the aluminum, but in the presence of a little salt water the dissimilar metals involved just destroyed does threads. I have only seen this happen once to this extent. We used to put Teflon tape between the threads to isolate the different metals and to seal the thread so no salt water could get into them (the air pressure is sealed by the O-ring). You have to be careful with this procedure because you don’t want any loose Teflon tape falling in your air passages. Another place were galvanic corrosion can occur is between stainless steel tank bands and the surface of the tank. The bottom line is that all high pressure cylinders are kind of a pain to deal with, but without them we are just left with a snorkel. Well in the case of Nemrod… free-diving
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Ron
Regular Diver
Posts: 41
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Post by Ron on Feb 20, 2006 10:18:29 GMT -8
Hi
I'm over whelmed by all this information guys are we now into one upping each other over cylinder data. Its so easy to copy and past but another thing to do the work so leave it to the technicians to determine your tanks health and safety.
Ron
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Post by Captain on Feb 20, 2006 10:39:52 GMT -8
Ron, no disrespect intented but I worked in 31 years in a industry that used and maintained high pressure vessels plus 40+ years diving and filling my own tanks. I was involved and trained in their inspection and operation of these systems by my employer. Luis is a trained mechanical engineer who has also been involved in desigin of pressure vessels. To imply that that we should blindly follow what someone who may or may not know or follow what is correct and put aside our years of experience is condesending.
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Post by Captain on Feb 20, 2006 12:01:34 GMT -8
A hydro test is a quantifiable test, either the cylinder passes or fails no personal judgement is involved. I visual test is not quantifiable, test results depend on the knowledge and experience of the inspector. A cylinder that one inspector may fail may be passed by another inspector who might possibly have more knowledge and experience. Which inspector would be right. If I feel that the inspector doesn't have the skills or experience I have every right to challenge him or go elsewhere but I don't have to blindly accept his judgment. The more knowledge you have the better you will be able to know if what you are being told is correct. I thought the object of this forum is to increase knowledge.
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Ron
Regular Diver
Posts: 41
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Post by Ron on Feb 20, 2006 12:25:50 GMT -8
Hi
Sorry guys you can have 40 years doing this or years doing that, but what counts is the person who has the DOT issued certificate that says they and the equipment they are using to test cylinders with are certified to persform cylinders testing and not the back yard diver who has developed a negative attitude towards licenced facilities because he thinks he knows better because he's and engineer or has read data from Yahoo. If I'm correct wasn't it engineers that built the levies in New Orleans and said they will hold hell back. as stated above and I quote: A hydro test is a quantifiable test, either the cylinder passes or fails no personal judgement is involved so if you choose to not believe in the results of a test go ahead and use the tank but don't come back crying bloody murder to the test facility if your tank blows and ends up in your neighbours living room. Now lets get back to Vintage diving.
Ron
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Post by Captain on Feb 20, 2006 12:53:10 GMT -8
Your analogy that the levies were designed by engineers and they failed proves my point. Mistakes are made by everyone or is it only trained DOT inspectors who don't make mistakes.
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Ron
Regular Diver
Posts: 41
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Post by Ron on Feb 20, 2006 13:14:36 GMT -8
Hi
DOT inspectors use machines that are calibrated to specific measurements and when set up correctly they don't make mistakes. A visual inspection on the other hand is left to the discretion of the DOT certified technician. If he or she finds pitting, distortion, or excess corrosion beyond the limits of a tanks integrity they are instructed to remove the tank from service for your safety by not stamping or attaching a visual sticker. If you choose to continue to use this tank afterwords YOUR AN IDIOT. For the cost of a steel tank just chuck it and find another in optimal condition. be safe, be smart and dive safe.
Ron
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Post by luis on Feb 20, 2006 13:49:46 GMT -8
Ron
Hydro test procedures and regulations were not given to us by God. They were developed by engineers with the same kind of education as people like me (and perhaps yeah the engineers who design the levies).
DOT license is carried by the facility. I worked in a DOT licensed facility for several years doing hundreds of hydro tests before I became an engineer (over 25 years ago).
If you want to pull badges, certifications, etc., I carry a Professional Engineering License. This is the kind of license required to perform and sign (stamp) the calculations behind ASME and DOT pressure vessel design. The design may even include the testing procedure, etc.
I strongly promote that every consumer should be an educated consumer.
You may have not read that I lost three tanks last summer because a hydro test facility decided to play it safe and condemned my tanks. Well they ran into the wrong customer or maybe the right customer. After reviewing their test data, they did replace all three tanks (the residual expansion was below 10% of the maximum expansion). They were also not following the round out (“pre-test”) procedure recommended by steel tank manufacturers now a days. With their data, I could have reported them to the DOT. That could have compromised their license. Instead I hope they learned something that day.
Hydro test is a quantifiable test and judgment should not be involved, but the operator is still human.
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Post by sea.explorer on Feb 20, 2006 13:57:55 GMT -8
The value of this post in my opinion is to empower the individual against being taken advantage of by ignorant or agenda driven shop employees. Vintage diving and equipment is outside of the experience of the vast majority of shop personnel around the country. I suspect that almost every vintage diver has experienced discrimination based on ignorance or people wanting to sell new gear. Educating yourself is important in order to avoid being taken advantage of. I have had dive shop owners and employees tell me that I was going to die diving with a perfectly maintained Royal Aqua Master but we all know that is an absurd assumption made out of ignorance. I dive with tanks with current vis. and hydro but I have still benefited from this discussion as well as the numerous conversations I have had with my trusted LDS and Hydro tester. I don’t think anyone here wants to dive with an unsafe cylinder but it is a big waste to throw away a perfectly good tank as the result of ignorance and not safety. -Ryan
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Post by luis on Feb 20, 2006 14:02:27 GMT -8
Hi DOT inspectors use machines that are calibrated to specific measurements and when set up correctly they don't make mistakes. A visual inspection on the other hand is left to the discretion of the DOT certified technician. Ron The DOT does not certify VIP technicians. The DOT does recognize PSI's certification. The equipment is run by an operator that can make many mistakes. Would you like a partial list? I will totally agree that high pressure cylinders contain a lot of energy that can be destructive. They have to be treated with respect not with blind ignorance.
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Ron
Regular Diver
Posts: 41
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Post by Ron on Feb 20, 2006 14:26:58 GMT -8
I did not say Hydro station technicians are certified by DOT to slap visual sticker on tanks, any idiot can go out and have stickers made and put them on there own tanks. VIP course are given out like candy by scuba clubs and shops. I said they do the hydro test and if you ask they to can do a visual for you and put there sticker on. Man this htread is idiotic we can go on and on on this topic with no practical out come.
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Post by RMannix on Feb 20, 2006 15:27:13 GMT -8
My 72's are better than yours
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