Measuring non-conducting thickness on conductors - using eddy current coil with frequency varied to adjust phase

A measuring method for the thickness of non-conducting coatings on electrically conducting and magnetic materials is for producing measurement results independent of the coated material characteristics or tilting of the measurement probe. The eddy current principle is employed in which a coil carrying a.c. is placed on the test surface. The phase of the measurement signal is measured and adjusted to a defined value by regulating the coil test frequency. The amplitude or the real or imaginary component of the impedance vector is then measured. The probe is made insensitive to orientation by providing it with a conical or pyramid-shaped testing surface. By use of two coils an operating frequency range of 10 KHz to 10 MHz is achieved. This covers the required material range.