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March 19 - March 23, 2018

What errors are most likely to occur when measuring dry film thickness on steel, and how can I avoid them?

Selected Answers

From Tom Schwerdt of TxDOT on April 3, 2018:
Tom Swan is correct.Your zero plate should be as similar to the actual coated surface as possible.

From Tom Swan of M-TEST on April 2, 2018:
A slight correction to the above answer is the DFT meter should be calibrated to the surface being measured and not a smooth zero plate. Make sure you zero on a geometry similar to the one being measured and with the same surface profile. This will eliminate many of the problems mentioned above.

From Zenith Czora of Master Builders Solutions on March 29, 2018:
The correct dry film thickness gauge to use and the accuracy and precision of measurements depend on the thickness range of the coating, the calibration of the instruments, the shape and curvature of the substrate, substrate thickness, the edge effect, surface profile and roughness of the substrate and how critical it is to get an accurate measurement. Other factors that may cause some error readings are foreign particles on the surface or  embedded in the coating, soft coatings, probe pressure and temperature effects. With the use of ultrasonic coating thickness measuring instrument, ensure the operation of the instrument is in accordance to the operating manual and that it is operating satisfactorily. The battery level must be sufficient to complete the testing session. The instrument or thickness gauge must be zeroed and calibrated on a smooth reference surface. A specified dft of the coating should be provided for each project. Depending on the coated area to be measured, approximately 20 readings shall be taken. Both probe and paint surfaces should be checked for cleanliness before taking measurement. Ensure the film of the coating to be measured is fully cured. Impose no excessive pressure on the probe that may damage a coating and give erroneous reading. Avoid taking readings during extreme temperature, which may influence both readings obtained from the instrument and the deformation of the coating being measured. Measurement should not be taken close to an edge, a hole or inside corner of the test surface area. ASTM D6132 is the standard test method for non-destructive measurement of dry film thickness of applied organic coating using an ultrasonic gauge.

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Tagged categories: Asia Pacific; Dry Film Thickness (DFT); EMEA (Europe, Middle East and Africa); Good Technical Practice; Latin America; North America; Steel

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