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					Originally Posted by gobytron  but wouldn't blasting the glass put pressure on the seams?
 meaning blasting the assembled tank, from the inside, with a predetermined PSI?
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 If you pressurized the whole assembled tank then yes but pressurizing with air alone would not be advisable. Water or fluid is used because it's incompressible, if a leak forms during testing it basically just leaks while using air or gas will expand and potentially cause more damage if a leak forms. Basically without fluid you could be building a bomb which is not something you want in your shop. Also this test wouldn't be something you would do on normal or average tanks, really just large/specialty custom tanks that would have a large investment behind them. The cost of doing such a test would be high as the builder would probably spend more on producing a stronger tank to ensure it passes a hydrotest. For example spending more time on sealing (stages), machining all edges including non- exposed and/or using thicker glass.