Though, as Mick says, we'll try to be less technical, please ask, as you did here.
Sometimes the terminology many of us use is used often only in a few specific trades. We get used to the jargon we depend on daily, and it's easy to forget that not everyone will know what a particular word or abbreviation means in the context of a post.
I'll try to clarify the context of his topic:
Most of my NDT work is specific to high-vacuum service (think Thermos bottle on steroids), and I use four main methods, in a sometimes complicated sequence. I begin with Helium leak detection, using a Helium-specific mass-spectrometer attached to a vacuum pump, and intoduce helium to the entire outer surface of the vessel, typically a semi-trailer (tanker, for liquid hyrogen, nitrogen, oxygen, argon, etc.) to prove whether a leak exists in the outer vessel. If no leak is found, helium is introduced to the inner vessel, under pressure, to determine whether a leak exists there.
The procedures differ for inner and outer leaks so I'll stick with "outer", which are more common. I'll divide the trailer into halves, and repeat the test for each half. (A huge tarp is used to contain the helium, for both the full- and half vessel tests.) Once narrowed by 1/2, assuming there aren't two leaks, which has happened, I isolate individual areas, such as cabinets and substructure and test again. Once I'm narrowed down to a "region", I begin visual inspection in that region, looking for previous repairs, excessive rust, and obvious damage. I then introduce helium to the small suspect areas, ruling out one at a time until I have a test area of a few sqaure feet, or sometimes a weld zone of a few linear feet. This continues untill I've narrowed the leak to a few square inches, then I usually break vacuum, buff off all paint or section out a welded structure in the way and buff to clean metal, and move to dye-penetrant. If I get a dye indication, I then pressure the space between the inner and outer vessel (a precarious operation... There must be more pressure in the inner vessel than around it, or it will crush like a beer can), and do a soap-bubble test to prove beyond doubt that's where the leak is. Failing a soap-bubble test (some leaks are simply too small), I can introduce helium to a very small area around the dye indication.
I hope this wasn't too long winded an explanation of the process I go through before I weld the stuff I take pictures of...
Steve S
Did you need to dye pen after back grinding?
).
But that would have been accompanied by many more of these 
