How To...
Ensure Accurate DFT Readings: 10 Important Things to Know

From JPCL, July 2016

By David Beamish, DeFelsko Corporation


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Tagged categories: Coating/Film Thickness; David Beamish; DeFelsko; Dry Film Thickness (DFT); Instruments; Quality Control

Comment from Robert Audlee, (8/29/2016, 7:36 AM)

David, we manufacture a coating that is HEAVILY loaded with 316L stainless steel, which acquires some magnetic properties in processing (some conversion from martensitic to austenitic form. Our customers sometimes report that "no matter how much paint they put on, the meter still says X". Our guess is that the instruments confuse the steel in our coating with the substrate itself, and continue to 'think' their practically in contact with the substrate. Any ideas?

Comment from Robert Audlee, (8/29/2016, 7:38 AM)

(I should have worn my glasses when I typed that! sorry about the "their" instead of they're, and other typos!)

Comment from David Beamish, (8/29/2016, 10:55 AM)

Your guess is good. A DFT gage that uses a magnetic principle is calibrated to measure non-magnetic coatings on carbon steel. It assumes the weaker the magnetic field, the further it is from the substrate, ergo, the thicker the coating. Your customer’s gage is not registering much of a change as your partially magnetic coating gets thicker, so it “thinks” thickness has not changed much. A 2-pt calibration adjustment will not fix the problem, nor will a gage using the eddy currently principle work. We could explore ultrasonic means, a ferrous probe with a magnet and extended measurement range, or destructive tests. Contact us directly for further discussion.

Comment from Robert Audlee, (8/30/2016, 1:39 PM)

Thanks! I will!

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