ASTM-B568:98(R2021) pdf free download – Standard Test Method for Measurement of Coating Thickness by X-Ray Spectrometry.
6.8 Surface cleanliness—Foreign materials such as dirt, grease, or corrosion products will lead to inaccurate thickness determinations. Protective coatings such as lacquer or chromate conversion coatings over the coating to be measured will also affect the results.
6.9 Specimen Curvature—Thickness measurements should be made on flat surfaces if practical. In those cases where the measurement of thickness on curved surfaces cannot be avoided, a collimator should he used on the excitation beam, reducing the measurement area to a size that will minimize the effects of curvature. Spatial relationships between the curved surface, the excitation beam, and the detector are particularly important, and variations in these relationships can introduce errors in measurement. Calibration standards having the same radius of curvature as that of the test specimens can also he used to eliminate curvature effects.
6.10 Excitation Energy—The intensity of the characteristic secondary radiation from either the coating or the substrate is strongly affected by any variation in the excitation energy. that is, by changes in potential applied to the X-ray tube or changes in the tube current, or both. In general, the radiation intensity varies directly with the current and the square of the potential. Therefore, in any method based on a simple relationship between intensity and thickness, the final adjustment of excitation energy must he made with reference to the observed intensity from a standard sample used to construct the working curve. However, if the method is based on intensity ratios rather than absolute intensities, minor variations in excitation energy are compensated for.
6.11 Detector—Errors can be introduced by erratic operation of the detector system which includes the associated scaling circuitry as well as the detector tube itself. If instability is suspected, a series of twenty or more Count measurements should be made on the same specimen without moving the specimen and the standard deviation of the series calculated. Most modern industrial X-ray instrumentation will perform this calculation automatically. The value should not be significantly greater than the square root of one measurement, Some forms of instability become evident if the same specimen is measured periodically.
6. 11 . 1 All radiation-detectionlpulsc-processing systems have limitations with respect to reliable count-rate capability. Operation of the gas-filled and scintillation types above their count-rate capabilities will result in loss of counts and erroneously low readings. Operation of an energy-dispersive system at high-input pulse rates will require an excessively long time to obtain a statistically valid total, even with ‘dead-time” compensation (see 6.1).
6.12 Any extrapolation beyond the thickness range covered by the calibration standards excluding infinite thickness can result in serious measurement errors: therefore, it is necessary to take additional steps for measurements outside this range.
6.12.1 When making measurements in the range between the highest thickness standard and the saturation (or infinite thickness) standard, especially in the so-called hyperbolic range, one must always use additional thickness standards with values slightly above and below the presumed thickness of the test specimen. Instrument measurement precision will rapidly decrease with increasing thickness in the hyperbolic range. For this reason, significantly longer measurements times are usuaiiy required for measurement applications using the hyperbolic range.