Fastener Testing

Here Fischer Instrumentation (GB) Ltd looks at the different ways to achieve valid measurement values and optimise measurement performance when it comes to the measurement of coating thicknesses, using reliable measuring methods and relevant devices to specific applications.

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, with the measurement method most suitable for the present situation selected. The coating thickness gauge must therefore be specifically adjusted to the existing measurement task. 

Normalisation and calibration for best measurement performance

The process for the normalisation and calibration of a handheld coating thickness gauge requires several steps. First, measurements are taken five to ten times on an uncoated base material. Then, 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 ten times on the uncoated workpiece. 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 workpiece. If the layer on the workpiece does not extend over a more extensive thickness range, measuring one foil is sufficient for a successful calibration. If this procedure is followed, it is ensured that precise, accurate and, above all, correct measurement results are achieved. However, the prerequisite is that the base material is uncoated and homogeneous. 

Corrective calibration to optimise performance

If it is known that the measurement result lies within an even, if no uncoated material is available, a measuring device can be calibrated –
if you know how it works. 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. On the other hand, measurements outside this range result in less precise readings. 

Best measurement performance with existing interlayer

The magic word 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. After, 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 handheld 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 coated material

If there is no uncoated workpiece available when normalising and calibrating a coating thickness gauge, some Fischer coating thickness gauges offer the feature ‘calibration on coating’. This feature is available when using the magnetic induction method with Fe probes. First, the coated workpiece 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. Whilst interpolation does not offer the high measurement performance of calibration, sufficiently good measurement accuracy and precision is often achieved in practice. 

At Fischer Instrumentation we can solve highly complex metrological challenges and help customers achieve the optimum measurement performance. We supply high-quality gauges combined with the knowledge and experience on the suitable measurement methods we have gathered in the last 68 years. Handheld Fischer coating thickness gauges offer the features to achieve accurate, precise and correct measurement results, regardless of the conditions – using corrective calibration, offset, calibration on coating, as well as many other features.”