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Significance and Use
General—No presently available thickness gauge is applicable to all combinations of coating-substrate thicknesses and materials. The limitations of a particular instrument are generally delineated by its manufacturer.
Magnetic—Magnetic-type gauges measure either magnetic attraction between a magnet and a coating or its substrate, or reluctance of a magnetic flux path passing through the coating and substrate. These gauges are designed to measure thickness of a nonmagnetic coating on a magnetic substrate. Some of them will also measure thickness of nickel coatings on a magnetic or nonmagnetic substrate.
Eddy Current—Eddy-current-type thickness gauges are electronic instruments that measure variations in impedance of an eddy-current inducing coil caused by coating thickness variations. They can only be used if the electrical conductivity of the coating differs significantly from that of the substrate.
Accuracy—The accuracy of a measurement depends on the instrument, its calibration and standardization, and its operating conditions. The accuracy is also affected by the interferences listed in Section 5, such as part geometry (curvature), magnetic permeability, and surface roughness.
Note 2—This practice allows the coating thickness to be determined within ±10 % of its true thickness or to within ±2.5 μm (or ±0.0001-in.), whichever is the greater. (See exceptions in Appendix X2.)
1.1 This practice covers the use of magnetic- and eddy-current-type thickness instruments (gauges) for nondestructive thickness measurement of a coating on a metal substrate.
1.3 Units—The values stated in SI units are to be regarded as standard. The values given in parentheses are mathematical conversions to inch-pound units that are provided for information only and are not considered standard.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.
B244 Test Method for Measurement of Thickness of Anodic Coatings on Aluminum and of Other Nonconductive Coatings on Nonmagnetic Basis Metals with Eddy-Current Instruments
B499 Test Method for Measurement of Coating Thicknesses by the Magnetic Method: Nonmagnetic Coatings on Magnetic Basis Metals
B530 Test Method for Measurement of Coating Thicknesses by the Magnetic Method: Electrodeposited Nickel Coatings on Magnetic and Nonmagnetic Substrates
D7091 Practice for Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings Applied to Ferrous Metals and Nonmagnetic, Nonconductive Coatings Applied to Non-Ferrous Metals
E543 Specification for Agencies Performing Nondestructive Testing
E1316 Terminology for Nondestructive Examinations
G12 Test Method for Nondestructive Measurement of Film Thickness of Pipeline Coatings on Steel
AIA StandardNAS-410 Certification and Qualification of Nondestructive Testing Personnel Available from Aerospace Industries Association of America, Inc. (AIA), 1000 Wilson Blvd., Suite 1700, Arlington, VA 22209-3928, http://www.aia-aerospace.org. (Replacement standard for MIL-STD-410.)
ASNT StandardsANSI/ASNT-CP-189 Standard for Qualification and Certification of NDT Personnel
International StandardISO2178 Nonmagnetic Coatings on Magnetic Substrate--Measurement of Coating Thickness--MagneticMethod Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
ICS Number Code 17.040.20 (Properties of surfaces)
UNSPSC Code 41111801(Eddy current examination equipment)