The physical principles and the most important influencing factors
The thickness of coatings can be measured non-destructively using the magnetic induction method. The prerequisite for this is a magnetizable base material, for example steel or iron. The coating, on the other hand, must be non-magnetic. This process is therefore suitable for measuring galvanic coatings such as zinc and chrome, as well as paints and plastics.
HERE’S HOW THE MEASUREMENT WORKS
The probe for magnetic-inductive measuring consists of an iron core around which an exciter coil is wound. A low-frequency alternating current flows through this coil (typically in the Hz range). This creates an alternating magnetic field around the poles of the iron core.
Now, when the pole of the probe approaches a magnetizable object, such as a part made of iron, the iron strengthens the alternating magnetic field. A measuring coil registers this increase as voltage. The difference in voltage depends on the distance between the pole and the iron part. For coated parts, this distance corresponds to the layer thickness.
HERE’S WHAT YOU NEED TO PAY ATTENTION TO DURING THE MEASUREMENT
All electro-magnetic test methods are comparative. This means that the measured signal is compared with a characteristic curve that’s stored in the device. In order for the result to be correct, the characteristic curve must be adapted to the current conditions. This is achieved through calibration.
Correct calibration makes all the difference!
Factors that greatly affect the results of a magnetic measurement include: the magnetic permeability of the base material, the shape of the sample and the roughness of the surface. Furthermore, the operator can also influence the result.
The magnetic permeability indicates how well a material adapts to a magnetic field. Substances such as iron or nickel have high permeability. They become magnetized themselves and strengthen the magnetic field.
Since the permeability is different for the metals and their alloys, the measuring device has to be recalibrated when the materials change.
In practice, most magnetic coating measurement errors occur due to the shape of the sample. With curved surfaces, the proportion of the magnetic field that passes through the air is different. For example, if a measuring device was calibrated on a flat sheet, measuring on a concave surface would lead to a lower result, whereas measuring on a convex one would lead to a higher result. The errors that occur in this way can be many times the actual value!
Small, flat parts
A similar effect can occur if the sample is small or very thin. Also in this case, the magnetic field extends beyond the sample and into the air, which systematically distorts the measurement results. To avoid these errors, you should always calibrate on an uncoated part that corresponds to the end product.
For rough surfaces, the result can be distorted depending on whether the probe pole is placed in a ‘valley’ or on a ‘peak’ of the roughness profile. With such measurements, the results vary widely and it is advisable to repeat the measurements several times in order to accumulate a stable mean. In general, magnetic coating thickness measurements on rough surfaces only make sense if the coating is at least twice as thick as the roughness peaks are high.
For better accuracy, Fischer offers probes with particularly large poles, as well as 2-pole probes. These probes integrate the roughness profile and thus reduce the scatter in the measured values.
Heavy scattering of readings, because the probe tip is positioned on “hills” and in “valleys”
Probe with a wide pole is positioned on several “hills”, thus reducing the scattering
Dual-pole probes integrate via the roughness profile and reduce the scattering
Last but not least, the way the measuring device is operated also plays a major role. Always make sure that the probe is set vertically on the surface and without pressure. For better accuracy, a stand can be used to automatically lower the probe onto the sample. Helmut Fischer offers both Handheld Coating Thickness devices and Automated Measurement Systems.