You may be over thinking this. Unless you are just looking. To the check the box for a Customer, think about what you really want to know about the leak testing process. There are two critical components to the effectiveness of the test: the inspector’s ability to ‘observe’ a leak, and the test method’s effectiveness of initiating a leak without creating a leak that wouldn’t happen naturally in use.
Leak testing is often not destructive but it is a ‘functional’ Test and so we must use different approaches than the standard AIAG approach. Just in general I would gather several pieces that leak and dont’ leak. Then test them TWICE with each of 2-3 inspectors. Then simply look at cross tabs for each inspector. Even a graph at this point is probably overkill. Things to look for: does the test method repeat leaks and non leaks. Does each inspector repeat within themselves and then are the results replicated across the inspectors? How many pieces you use are flexible. Try to get enough pieces and a good proportion of leakers to non leakers that the inspectors won’t remember what they saw. You can help this by testing the second round a couple of days after the first round...
You might find this paper on
MSA for Functional Tests helpful. There is a section on water leak testing for cars