I looked up some details on hydro testing of scuba tanks. Interesting how they measure expansion of the tank by the amount of water displaced. I had no idea...
Yep. I've tested literally thousands of high pressure tanks; the vast majority were CO2 tanks for Fire Extinguishers. At the time (late 70's), I worked for a FL fire equipment company. We tested in a pressure testing tank that was ~80% or so underground - or in this case - gravel. See pic below. Ours was buried to about 6" above the large port on the lower left of the pic. See pic at the link - it won't load for some reason:
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The number of tanks I had fail had to be low single digits - it simply didn't happen all that often. Most of them were scuba tanks (we could test them, but not fill them, unlike their CO2 counterparts). I never had one blow, although I did have one scuba tank that refused to stabilize, and I released the high pressure before it could blow. All failed tanks were retested to confirm failure. Scuba guys hated hearing their tanks had failed - and you had to tell them to their face, because they were almost all walk-ins.
As stated above, the test criteria was determined by the difference between the zero pressure volume in the test equipment - before high pressure is applied to the tank under test, then the zero pressure volume after the test pressure was applied and sustained for a certain time - usually when test volume stabilized and stopped increasing. If the difference between the two volumes exceeded a certain %, it was failed. Basically, you're measuring the ductility of the tank.
The procedure for indicating a failed tank was simple: You simply overwrote the test date on the tank, like
this. Test dates were always stamped prior to test.
The most interesting tanks were those from WWII and before, especially the Nazi ones.
