ASTM-B244:09(R2021) pdf free download – Standard Test Method for Measurement of Thickness of Anodic Coatings on Aluminum and of Other Nonconductive Coatings on Nonmagnetic Basis Metals with Eddy-Current Instruments.
8.3 Basis-Metal Thickness—For each measurement, there is a critical thickness of the basis metal above which the measurements will not be affected by an increase in that thickness. Its value should be determined experimentally, if not supplied by the manufacturer of the measuring instrument, since it depends on both the measuring frequency of the probe system and the electrical conductivity of the basis metal.
8.3.1 General Rule—For a given measuring frequency, the higher the conductivity of the basis metal, the smaller its critical thickness. For a given basis metal, the higher the measuring frequency, the smaller the critical thickness of the basis metal.
8.4 Edge Effect—This test method is sensitive to abrupt changes in the surface contour of the test specimen. Therefore, measurements made too near an edge or inside corner will not be valid unless the instrument is specifically adjusted for such a measurement.
9.2 Verify the accuracy of the instrument at the test site each time the instrument is put into service and at frequent intervals during use to assure proper performance.
9.3 Many instruments can be adjusted in order to improve their accuracy on a specific surface or within a specific portion of its measurement range. The effects of properties of the substrate (composition. shape, roughness, edge effects, electrical properties) and coating (composition, mass, surface roughness), as well as ambient and surface temperatures, may require adjustments to be made to the instrument. Follow the manufacturer’s instructions.
9.4 Observe the following precautions:
9.4.1 Basis Metal Thickness—Check whether the basis metal thickness exceeds the critical thickness. If not, either use the back-up method in 7.5, or make sure that the calibration adjustment has been made on a reference standard having the same thickness and electrical properties as the test specimen.
9.4.2 Edge Effects—Do not make readings close to an edge, hole, inside corner, etc., of a specimen, unless the validity of the calibration adjustment for such a measurement has been demonstrated.
9.4.3 Curvature—Do not make readings on a curved surface of a specimen unless the validity of the calibration adjustment for such a measurement has been demonstrated.
9.4.4 Number of Readings—Because of normal instrument variability, it is necessary to make several readings at each position. Local variations in coating thickness may also require that a number of measurements he made in any given area: this applies particularly to a rough surface (see 8.9).
9.4.5 Suiflice Cleanliness—Before making measurements, clean any foreign matter such as dirt, grease, and corrosion products from the surface without removing any coating material.
10. Accuracy
10.1 The instrument, its calibration, and its operation shall be such that the coating thickness can be determined within 10 % or 1 tim, whichever is greater, of the true thickness.
11. Report
11.1 The report shall include the following information:
11.1.1 Type of instrument used including manufacturer, model number, principle of operation. and date of calibration,
11.1 .2 Size and description of test specimen,
11.1.3 Whether special jigs were used,
11.1.4 Type of coating thickness standard and/or reference standard and the method used for accuracy verification and any calibration adjustment
11.1.5 The number of measurements taken and the value of each measurement,
11.1.6 Operator identification, and
11.1.7 Date.
NOTE I—Although. theoretically, this test method can be used for measuring nonconductive coatings on a magnetic basis metal, its use for coatings below 25 pm (I mil) is not recommended, and the magnetic method outlined in Test Method B499 shall be used.