ASTM B567-98 pdf free.Standard Test Method for Measurement of Coating Thickness by the Beta Backscatter Method.
In general,instructions for selecting the proper iso-tope are furnished by the manufacturer.
Measurement Time —Too short a measurement timewill yield a poor measurement precision.The selection of themeasurement time will, therefore, depend on the measurementprecision that is required.Each time the measurement time isincreased by a factor of n, the counting measurement precisionwill improve by a factor of approximately 1/vn.Activity of Radioactive SourceThe count rate isdependent on the activity of the source. An old source mayhave a low activity,requiring excessive time to make a goodmeasurement (see 6.1).As a practical guide, the source shouldbe replaced before its half-life has elapsed.Coating-Substrate Combination-The measurementprecision depends on the difference between the atomic num-bers of the coating and substrate materials. The greater thisdifference, the better the precision (see also 7.2).
Surface Roughness—Measurement accuracy can besignificantly influenced by the roughness of the coating sur-face, but the effect is minimized if the energy of the betaparticles is high and the atomic number of the coating is low. Detector—Errors can be introduced by erratic opera-tion of the detector. If instability or drift is suspected, the useris advised to consult the manufacturer.Wear of Calibration Standards:Coating thickness standards used to calibrate betabackscatter instruments are subject to wear when used and thusto a decrease in thickness.The thickness of a calibration standard should bechecked from time to time by comparing it with anothercalibration standard or reference sample that has not been usedsince the last check.Calibration of Instruments:Beta backscatter instruments shall be calibrated withstandards before measurements are made and also each timethe measuring conditions are changed. To obtain the bestpossible measurement precision, the largest possible aperturesuitable for the area to be measured should be selected. Selectthe calibration measuring time,the number of calibrationmeasurements to be made on each calibration standard, and thetest piece measuring time in accordance with the manufacturer’s instructions to obtain the required measurement precision (see 8.8) with the measuring time, aperture, isotope, and number of readings to be used for measuring the test piece. For certain types of measurement application, this may require unusually long measuring times (greater than 80 s).ASTM B567 pdf download.