When I was a remodeling contractor I would quite often run threaded iron gas lines and have to test them, and if I had a slow leak I would use soapy water to try to locate it. If that didn't work
, I would hook up my nail gun compressor
to the line and crank it up to 110 lbs of pressure and than you could hear the leak from across the street, all six of them.
Also a easy way to make your small dinghy a much larger dinghy.