Optimising measurement performance

Optimising measurement performance

All roads lead to Rome. In the case of tactile coating thickness measurement, that sentence could be adapted to read: all measurements lead to accurate readings. However, it is not that simple. Instead, it should be stated that there is at least one road that leads to Rome – but only if you use a reliable measuring method and device relevant to the application, explains Dr Martin Leibfritz, CEO of Helmut Fischer Global.

A coating thickness gauge provides precise and correct measurement results if it has been adequately normalised and calibrated. While normalisation represents an adjustment of the coating thickness gauge to the base material, calibration describes a process in which the measured value is compared with the actual known value – usually of a standard with certified thickness. If these values differ, the device must be adjusted accordingly.

In practice, a measurement task often cannot be solved in a textbook manner. Instead, the circumstances must be considered, and the measurement method that is most suitable for the present situation must be selected. The coating thickness gauge must therefore be specifically adjusted to the existing measurement task.

The ideal solution

The process for the normalisation and calibration of a hand-held coating thickness gauge requires the following:

First, measurements are taken five to 10 times on an uncoated base material. Now the coating thickness gauge knows the zero point of the coating and is thus normalised. Next, a certified calibration foil of known thickness is measured five to 10 times on the uncoated work piece. Afterwards, the target value of the foil is entered in the coating thickness gauge. The film thickness should correspond as closely as possible to the coating on the work piece. If the layer on the work piece does not extend over a more extensive thickness range, measuring one foil is sufficient for a successful calibration.

Following this procedure ensures that precise, accurate and, above all, correct measurement results are achieved. However, the prerequisite is that the base material is uncoated and homogeneous.

Optimising measurement performance

If it is known that the measurement result lies within a specific range, measurement precision and accuracy can be optimised by corrective calibration. Also, in this case, uncoated material is mandatory. In addition to the base material, one or two foils can be selected for the corrective calibration.

If this procedure is performed under the given conditions, a significantly better measurement performance is achieved within the calibration range. Measurements outside this range result in less precise readings.

Best measurement performance with an existing interlayer

The magic word here is offset. This setting is used if a middle layer of known thickness is given and the thickness value of a second layer is to be measured. The procedure is as follows:-

First, normalisation is performed on the uncoated base material. Next, the first or middle layer is applied, and its thickness is measured. This measured value – the so-called offset value – is set in the menu of the hand-held gauge. The second coating process follows. During the second coating thickness measurement, the previously-entered offset value is subtracted directly from the total thickness value (the sum of the first and second coating thickness). Thus, the exact thickness value of the second layer is obtained immediately, without any other calculations being necessary.

Normalisation and calibration on a coated material

If there is no uncoated work piece available when normalising and calibrating a coating thickness gauge, some Fischer coating thickness gauges offer a ‘Calibration on coating’ feature. This feature is available when using the magnetic induction method with Fe probes.

First, the coated work piece is measured. Next, a foil of known thickness is added and measured. Based on these values, the first measured value can be determined by interpolation. The best results can be achieved if the coating on the material is close to zero.

Note: although interpolation does not offer the high measurement performance of calibration, sufficiently good measurement accuracy and precision is often achieved in practice.

Conclusion
Fischer can solve highly complex metrological challenges and help you achieve the optimum measurement performance. We supply high-quality gauges combined with the knowledge and experience about suitable measurement methods we‘ve gathered in the last 68 years. The hand-held Fischer coating thickness gauges offer the features to achieve accurate, precise and correct measurement results regardless of condition, using corrective calibration, offset, calibration on coating (please refer to the table below) as well as many other features.

Measurement methods at a glance: