Coating Thickness Measuring

What is coating thickness measurement?

The thickness of coatings on metallic substrates can be measured with simple methods when the coating and the substrate have significantly different magnetic or electrical properties. The thickness of non-ferromagnetic coatings on ferromagnetic substrates is most easily measured with instruments based on magnetic or electromagnetic principles.

Magnetic methods are mainly used to measure coating thicknesses of paint, plastic, and non-magnetic metal coatings on "soft" steel, meaning steel hardness up to 550-600 HV. More accurate and better reproducible measurements are achieved with electromagnetic inductive methods. The non-ferromagnetic layer forms an air gap in the magnetic circuit between the base material and the probe. The inductance in an iron-cored coil or in a transformer core varies with the size of the air gap. Changes in inductance can, via suitable electronics, be converted into a measure of coating thickness, which is read on a display.

Historically:

Corrosion protection or decorative coatings have always needed a certain thickness to fulfill their purpose. Many years ago, this created a demand for instruments that allowed quick and relatively accurate measurements of coating thickness on metals.

With steel as the most important structural material, it was natural from the beginning to focus on the magnetic attraction force, which clearly decreases with distance as a magnet is moved away from a steel component. The spring-suspended magnet as a thickness gauge almost invented itself.

Unfortunately, the accuracy was not outstanding. As semiconductor technology advanced and lightweight, stable amplifiers were developed that could detect very small changes in electromagnetic circuits, a series of increasingly improved instruments emerged, using magnetic inductive principles. Already in the early 1960s there were instruments as good as today’s, but they were rather bulky and not particularly inexpensive. Since then, development has mainly been aimed at producing more user-friendly instruments.