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Morris
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Exactly. As I said you can sometimes see the pressure gauge reading not doing what is expected but otherwise it's a none event.
Agroshield
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JB Weld not good in tension. Supplement with old trainer laces around the outside. Double bow is the safety factor.
With the ultrasonic, it gives a resolution of 0.1mm. I do not know what the nominal wall thickness of that tank is, but, for instance 0.1mm is 10% of a 1mm wall or 5% of a 2mm wall so it might not be sensitive enough to tell you anything useful.
I guess you would have to investigate how these things are determined to be safe. If the document says "local thickness reduction of 3% is acceptable", you need something that can measure to that figure.
Resolution of 0.1mm but also have to consider accuracy. I don't know wherever the claimed accuracy is reliable, but I'm not considering buying one anyway.
You are correct about needing to know what is acceptable, and that's something I don't know.
That's a great idea, but I have a better one:Cut it in half, measure thickness and if acceptable, weld back together - simples!
Cut both ends off and make into metal lampshades?
cheb
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The ends make good funnels.
Two anecdotes from me:
I asked the makers of my air receiver whether it needed tested. He said no, it'll pinhole first. I'm a home user, not trade.
A mate was picking at the base of one his filled 300bar dive cylinders. A good sized rust flake came off and it started to hiss. He stripped the kit off it and lobbed it over the side of the boat.
Make of that what you will.
Two anecdotes from me:
I asked the makers of my air receiver whether it needed tested. He said no, it'll pinhole first. I'm a home user, not trade.
A mate was picking at the base of one his filled 300bar dive cylinders. A good sized rust flake came off and it started to hiss. He stripped the kit off it and lobbed it over the side of the boat.
Make of that what you will.
Truth is I paid very little for the compressor. It was missing a few parts (easily replaced) but I knew it might have issues. That's why I was originally considering doing a pressure test.I would use it but build a cage around it from steel box and 18mm plywood with vents each end for air flow. Your comp,your choice.
As it stands, I have a working 3HP motor (repaired at no cost) and compressor pump (probably working, but not yet fully tested). If I decide to scrap the tank, it's no big problem. I have a much smaller and older compressor that it might have replaced, but I'm now considering my options.
My first compressor was a big old 3 phase Broomwade with 300 liter tank, I had a look inside through the two manhole doors & it was very rusty. But after cleaning out it didnt look too bad, I derated it a bit & used it for years. But the tank was a good half inch thick so had some redundancy! Plenty of vids on you tube, they tend to fail along the bottom or seams & open up, anyone who ever put a tin of beans on a campfire will know how it goes!
I do have an old AIE compressor but it hasnt run in years, it has a kohler petrol engine coupled to it with an unloader that blows off at 120psi, to be honest I dont feel happy about running the thing again unless its tested.
As an aside to the power in air vessels, My first job was at Thames marine a boat factory, one lunchtime one of the lads took an elbow length rubber glove as used by the moulders, they were very strong. He taped it to the end of an airline with blower nozzle in the end. Upon plugging the airline in the glove rapidly swelled to an impossible size, It was over 4ft long & 3ft across, the fingers stuck out like cucumbers. We expected it to burst but it didnt & the air stopped flowing into it. The pressure gauge on the air line regulator was set at 100Psi & it held it. There was some discussion over what to do now & one of the lads fired a pop rivet at it out of a blowpipe. The resulting bang shook the building & the blast of air actually moved the 28ft yacht hull standing next to it with the blast. Nobody was hurt but it was a salutary lesson.
Pete.
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I would simply up-end the tank and go around it with a ball pein hammer. If it's rotten then it will be rotten in the bottom or on the seam so if the hammer bounces everywhere but those places consider it dodgy. If there's no difference in the way the hammer reacts then I'd be satisfied that the metal is not significantly thinned by rust.
brightspark
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same here if the tank did go it would only pinhole at the bottom . i had a tank that was 80 years old and was solid but rust pitted and thats all it did
only thing ive had happen is rubber air hoses blister and balloon up and burst .@Pete. I can see it's not rotten in the bottom, and the seams (that I can see) look good. It's the roof that appears most affected by the rust, presumably from condensation.
I've emailed the tank manufacturer. I don't know whether they will reply, but it will be interesting if they do.
Thanks for all the replies, including the funny anecdotes.
Maybe so, but I reckon an 80 year old tank would be considerably heavier construction / over engineered than this one.same here if the tank did go it would only pinhole at the bottom . i had a tank that was 80 years old and was solid but rust pitted and thats all it did
brightspark
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ive got a large tank on a twin pump and motor compressor thats over 20 year old it aint that thick but solid just turned it over and batted all along with the ball pein hammer and it never dented thats good enough for me there all a bit rusty inside . a 1 gallon oil can with crimped square ends will stand up to 50 psi before the crimped ends give
and there thin . i posted a picture of it blowing up on here over 10 years ago 
I had another poke around and clean out this evening. It's very difficult to show a good photo (phone won't fit in the hole) but from what I can see the welded seams are in excellent condition. I had a poke and scrape at the rusty roof and the rust doesn't seem particularly deep.
JOEPRO
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Why don't you go ahead and follow the advice provided way back in post #3 if you insist on testing it. Go ahead - test it. If it passes then good stuff job done.
Pete.
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The thing is in order for the tank to be significantly thinned by rusting there would be evidence - a lot of it - inside the tank.
I'd also be considering historical evidence here too. How many compressor tanks are there in use across the country (probably millions) and how often do you hear of them failing (never actually heard of one myself nor seen one in the news)? the odds of my compressor being the first one I ever heard of are pretty damn slim, and orders of magnitude slimmer if I do what no-one else does and actually inspect the condition.
I'd also be considering historical evidence here too. How many compressor tanks are there in use across the country (probably millions) and how often do you hear of them failing (never actually heard of one myself nor seen one in the news)? the odds of my compressor being the first one I ever heard of are pretty damn slim, and orders of magnitude slimmer if I do what no-one else does and actually inspect the condition.
brightspark
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just get it used god i was told making air tanks out of calor gas bottles and was told would explode 50 years ago
made 4 comps and 7 shot blasting pots out of em since then
god i was told making air tanks out of calor gas bottles and was told would explode 50 years agomade 4 comps and 7 shot blasting pots out of em since then 