Why only 30psi? When I tested my system, I intended to go up to 90psi (50% over working maximum). It held fine at 30 psi, but blew out at 45psi after a couple of minutes -- when a CPVC joint I had only primed, but not cemented, let go. Once that was fixed, the system held at 90psi for a couple of weeks. It was interesting to watch the pressure rise during the day and fall at night (the system was overhead in a Florida attic) as the air in the pipes warmed and cooled. I was going to see if the pressure differential actually obeyed the gas laws, but haven't gotten around to that yet
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To answer your question, though, if you're really paranoid about leaks, you can buy or rent very sophisticated "sniffer" leak detectors that depend on a rare gas (usually helium) leaking from the vessel under test. Unfortunately, it doesn't take much of a leak to let helium out -- I don't know if average plumbing, particularly plastic, would contain it or not. I've seen silver-soldered joints in hard copper lines leak helium. Weird stuff. Stick with the Dawn & Windex spritzer test.