Coating Thickness Gauges

Coating thickness gauges are specialized instruments for measuring coatings on metal objects. They are divided into 3 groups. Ferrous measure coating/paint/galvanizing on ferrous steel substrates. Non Ferrous measure coating / anodizing on non-ferrous substrates (e.g. Aluminum). Finally, Ferrous & Non Ferrous, which cover both cases, are widely used for checking car paint, in metal structures, etc.

An excitation coil wound around an iron core, when a low-frequency alternating current is applied to its ends, creates an alternating magnetic field in the air gap around its poles. This arrangement is used to measure the thickness of a coating surface according to the magnetic induction method.

Figure 1: Typical applications for surface thickness gauges using the principles of Magnetic Induction or Eddy current.

When a pole approaches a piece of iron, the magnetic field will be strengthened. It then generates a voltage in a second coil (measuring coil). This voltage depends on the distance from the iron. By placing the iron core or the measuring sensor on the coating, a defined distance results between the pole and the iron and therefore a defined coil voltage. This is evaluated electronically and displayed digitally as coating thickness.

Figure 2: The operating principle of magnetic induction on an insulating coating with a ferromagnetic substrate.Ferrous

Operating principle of Eddy eddy current on a Non- Ferrous substrate

Measurement according to the Eddy current method uses only one coil through which a high-frequency alternating current flows. This produces an electromagnetic alternating field around the coil. When the coil with the alternating field approaches a non-ferrous metal, a varying current—called eddy current—is induced in that metal. This produces another electromagnetic alternating field in the opposite direction, weakening the original field. The effect on the coil is shown by the change in inductance, a characteristic property of each coil. By placing the measuring sensor on the coating, a defined distance results between the coil and the non-ferrous metal, thus leading to a defined inductance value in the coil. This will be evaluated electronically and displayed digitally as coating thickness.

Figure 3: The operating principle of Eddy current on an insulating coating with a non-magnetic substrate.Non-Ferrous

A device that is compatible with both types of applications Ferrous & Non-Ferrous

Coating thickness gauges that operate in both of the above cases have a combination of F and N type sensors, as well as both coil systems mounted in the measuring sensor so that the alternating field of both coils is affected—obtaining results in both low-frequency and high-frequency alternating fields—when the sensor is placed on the surface to be measured. The measurement signals of the two coils are automatically evaluated electronically by a microprocessor, so that it can select the measurement process corresponding to the substrate material and display the correct layer thickness, regardless of whether the measurement takes place on iron or non-ferrous metal.

Conversion Table of coating thickness to galvanizing weight