Understanding key influences in coating thickness measurement and how to avoid errors
Geometrical and material influences on measurement accuracy – what they are and how to compensate for them.
Modern coating thickness measuring instruments using the magnetic induction method (DIN ISO 2178) or the amplitude-sensitive eddy current method (DIN ISO 2360) are designed to be simple and quick for all users: apply the probe, read the value, done. However, the ease of use should not distract from the fact that both measuring methods are based on a comparative measuring principle – the signal from the measured object is compared with a calibration standard via the characteristic curve stored in the instrument. This means that a measurement result is only accurate if the calibration and measuring conditions are the same. Any deviation – for example, in geometry, surface roughness, or curvature – leads to a change in the signal and thus to systematic measurement errors.
How does the geometry of the component affect measurement results?
The most frequent influences are caused by the size and shape – the geometry – of the components to be measured. The measuring field passes through air before reaching the surface. On an outside diameter or convex curvature, this air path is longer than on a flat surface; on an inside diameter or concave surface, it is shorter.
The result: If the instrument is calibrated on a flat surface, readings on a convex outside diameter will be too high, and readings on a concave inside diameter will be too low.
Fig 1: Influence: Curvature, convex and concave curvature of object to be measured
What role does the substrate thickness play?
If the base material is not thick enough, the measuring field of the probe will fully penetrate the part being measured, distorting the result. The direction of the error depends on the mismatch between calibration and measuring conditions:
Calibrated on thin substrate, measuring on thick substrate → readings will be too low.
Calibrated on thick substrate, measuring on thin substrate → readings will be too high.
Users can avoid these errors by calibrating on a base material of the same thickness as the parts to be measured.
Fig 2: Influence: Base material thickness
What other geometrical factors need to be considered?
Further geometrical influences include the roughness of the base material surface, the size of the measuring surface, and the distance of the probe to the edges of the part. The effect of these influences can be reduced by selecting the appropriate FISCHER probe. We are glad to advise you on this.
Fig 3: Influence: Base material roughness
Fig 4: Influence: Size of measuring surface and distance to edge
How should I calibrate to compensate for these influences?
To compensate for geometrical and material influences, the following golden rule applies to magnetic induction and eddy current instruments:
Calibration should take place on the uncoated part of the measuring surface, then on the coated part of which the coating thickness is to be measured.
There are exceptions to this rule in individual cases, which should be given careful consideration and confirmed by trial measurements. The patented FISCHER eddy current probes ETD3.3 and FTD3.3 with curvature compensation are one such exception: if calibrated on a flat, non-magnetic object, measurements can be made without curvature influence down to a minimum diameter of 4 mm.
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