Bubble-tight is a relevant term if applied to a certain set of conditions.  It must be explained that way.  For example, with Check Valves, if you do an seat leakage test with air as permitted by API 598, ASME B16.34, MSS SP-110, and other standards, you can place a layer of water over the top of the seat being tested and look for free flowing bubbles.  No bubbles during the test duration means it was "bubble-tight" and the valve passes the test.  You will waste a lot of time testing tight sealing valves if you try to measure minute amounts of leakage with flow meters.
It is important to recognize that the factory test is strictly for quality.  Are all the parts installed properly, are they manufactured correctly, etc.  Never try to correlate the factory test with proof of performance.  there are way to many variable and assumptions involved that the results are not vaild.
Bubble tight is a valid term provided it applies to specific, defined conditions.

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