Testing Tanks........I am lucky to have a place where I can test tanks free. I help the scuba repair facility when they have staff shortages and that is my pay. I have too many tanks or perhaps just too wide a variety and that has been a problem.
Here in Australia we do not have VIP stickers and inspections. We have to test scuba tanks every year and vintage tanks usually have a large number of ugly stamp marks on them. Surface breathing appliances are every 5 years. When I moved here from Salt Lake City I was not to pleased about the annual requirement. However, I have learned that test and inspections are a good idea. Most tanks pass their inspections, but not always. The aluminum tanks that everyone has worried about were not banned in Australia and still aren't. However, one blew up and a man lost an arm last year. The tank had a current test and was being filled. While the government has not taken action on the old aluminum tanks here, getting one tested is pointless because the dive shops won't touch them. Some say that the divers have had 30 years of service from those tanks and asking them to buy a new tank is not unreasonable. That seems fair, but when you have a collector's interest it, is annoying at best.
Today tanks a hydraulically pressed into shape from a disc. Steel tanks are then heated at the neck, formed and drilled. There are videos on You Tube if you want to watch how it it done.
The early German IKW tanks we used here were almost a Damascus forming, which meant rust could occur between layers. They we very strong and a test tank didn't come apart until about 17,000 psi.
I found a great 50 cu ft tank and wanted it tested. I sent it out to be sand blasted and it passed the test when it returned. I was a bit concerned about it's appearance though and we decided to retire it. It had deep pitting where a sticker had been and the paint had hidden it. The repair factory owner said he has seen tanks leak through the walls. The spun non-mag tanks can leak through their base plugs.
I dived with a friend when I was young and I think he let water into his tank. He ran out of air and dived down to retrieve a dropped snorkel holding his breath. His one year old tanks failed their annual test and were full of rust. It was so bad you could shake it an hear the rust sliding inside. That was in just 6 months. Expanding compressed air has a similar expansion rate to detonated TNT. It's nothing to fool around with. If your tank fails a test, they are doing you a favor. Steve