I did some NDT but we had a $5k test unit with and oscilloscope. The o scope was to look at the signal so you could get an idea if you were reading a Good signal or junk.
I find the success with the less expensive units is all with the operator and obtaining a good cal. Then realise the limits, most less expensive units have poor accuracy when you get to less than 0.10 wall thickness. Sometimes you can get a spot that the unit reads as a thin spot, but it could also be that 'yes' it is thinner than the surrounding area but may not be accurately read by the meter. Since you are relying on the meter and can't monitor the signal to see what the signal is you have to make a judgement call. On and old block with water jacket corrosion you may read 0.08, but is it real? You get into that gray area, could be 0.065 or 0.095 wall thickness. So what I am saying is a lot of the success is in developing your technique and that applies even to high dollar equipment.
