I think I already know I can't touch these... but just wondering if anyone has any experience either to confirm or tell me how they can be done. I believe this is NDT (Non-Destructive Testing) stuff, and uses eddy current associated with pulsed ultrasonic transmitter/receiver. I think the NDT people usually have reference samples they use to check them(??). I looked around on GIDEP and couldn't really find anything (I found a nice NDT handbook - whole book). If anyone else is eternally curious about everything like I am, I'll be happy to share the title if any GIDEP participants want to go in and get it. Have a great week all, and don't forget WORLD METROLOGY DAY is soon.
this might help...
http://www.ndt-ed.org/EducationResources/HighSchool/Sound/immersion.htm