Sonic checkers

Ok I did some more testing to day at work. I took a 3.375 ID pipe with .250 thick walls and hit it with hammer just enought to oval it very slightly. Then I turned it down smooth on a lathe. I also turned it down so it would have steps in it. The thicknesses ranged from .185-.112 which I used a point mic to measure then calibrated my tester to it. The readings were very stable....I calibrated it to the middle step and it was reading .003 low on the thicker parts and .008 on the thin side. I was alittle disappoint but tried something I've read. I turned the tip to where it was side ways in the pipe and the reading got alot better. .001 on the high side and .002 on the low side. I'm very satified with the readings. I will post a pic after I get off work. My next step will be to put salt on the outside of the pipe and let it rust up outside and see what happens then.