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August 19 - August 25, 2013

Where are errors most likely to occur when measuring dry film thickness on steel, and how can they be avoided?

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Selected Answers

From Christian Favennec of DCNS on April 23, 2014:
When a dry film thickness (DFT) is specified, it corresponds to the thickness of the paint over the peaks of the profile. DFT measurements on steel are generally performed with electromagnetic gauges. Parameters which can influence on the accuracy of the measurement include the profile of the substrate. If the calibration is done on a smooth plate, the profile of the surface s not taken into account in the measurement. ISO 19840 recommends a correction value (25 µm for medium profile, 40 µm for coarse profile). It means that if the NDFT is 100µm, the measurement must be done for 125 µm for a medium profile. The other method is to perform the calibration directly on the rough surface before paint application with the 2-points method, as also defined in the annex of ISO 19840. The profile is then correctly taken into account.  The magnetic properties of the steel can also influence the measurement. It is then recommended to perform the calibration on the same quality of steel, and a representative thickness. Edge effect cans also influence the measurement. This is why measurements are generally not performed less than 20 mm from an edge.  Ferritic or magnetic pigments in the paint can also influence the measurement, as the calibration is done on non-magnetic foils.

From PARVEZ SHAIKH of SANDVIK ASIA PVT. LTD. on August 2, 2013:
When calculating the DFT of a primer over a blasted surface, one has to ensure that the DFT meter is calibrated on the blasted surface itself to compensate for the BMR over the blasted surface. Also, the thickness measurement of coatings containing  micaceous iron oxide are sometimes not accurate.

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Tagged categories: Dry Film Thickness (DFT); Steel


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