For a quick and cheap calibration just buy a cheap starret or mitiyoto 1/4x3" block. Heck I think I have something that would work for you. PM me your address and next week I'll send it to you. Free. I have lots of engineering goodies.
That's very much appreciated! Engineering goodies are great to have. But like on brian6pac's unit, this sonic gauge comes with a disc of calibrated thickness built into the side of the case.
And I checked this gauge against a brake rotor because it is cast iron, and I wanted to make sure I knew how to set the gauge up for the different material velocity. The sonic velocity through cast iron is about 20% lower than steel so you have to be sure to set it up right, or the results will be waaay off.
