Realcent.org

anyone do this?

http://about.ag/ultrasonicthicknessgauge.htm

Not personally for this purpose, but I know UT technology in general, and it could be helpful, but it's not a be-all, end-all technique. Basically it is just measuring time-of-flight of a shock wave, and two things affect that. Speed of sound (as noted in the article) and thickness. Someone could still game the tester by (for example) making a silver plated zinc bar that is slightly thicker than the silver bar is supposed to be, that would return the same time of flight. So in the end you'll still have to do some other checks such as thickness, weight, density etc to confirm things. But it could be a useful addition to the arsenal for some people.

(grumble)

We are going to need to drag around a foot locker worth of gear to test the darn coins, the way things are going.