ASTM B499-96 pdf free.Standard Test Method for Measurement of Coating Thicknesses by the Magnetic Method: Nonmagnetic Coatings on Magnetic Basis Metals.
Do not make readings close to an edge,hole, inside corner, etc., of a specimen unless the validity of thecalibration for such a measurement has been demonstrated.Do not make readings on a curved surfaceof a specimen unless the validity of the calibration for such ameasurement has been demonstrated.Because of normal instrumentvariability and in order to minimize surface roughness effects,a measurement shall be the mean value of several readings.For each measurement，make at least 3 readings,removing the probe after each reading,and average thereadings. If any 2 of the readings differ from each other bymore than 5 % of the average reading or 2 um, whichever is thegreater, then the measurement shall be discarded and repeated.The substrate or coating, or both may be too roughto meet this criterion. In such a case it may be possible toobtain a valid measurement by averaging a number of readings.To be valid under this test method, the validity of such aprocedure must be demonstrated (see Appendix X1).
Instruments of the attractive force type are sensitiveto vibrations, and readings that are obviously erroneous shouldbe rejected.
Direction of Mechanical Working-If the direction ofmechanical working has a pronounced effect on the reading,make the measurement on the test specimen with the probe inthe same orientation as that used during calibration. If this isimpossible, make four measurements in various orientations byrotating the probe in increments of 90°.When residual magnetism ispresent in the basis metal， when using twopole instrumentsemploying a stationary magnetic field make measurements intwo orientations differing by 180°. With single-pole instru-ments employing a stationary magnetic field,it may benecessary to demagnetize the test specimen to get valid results, and this may also be advisable with two-pole instruments.Before making measurements,clean any foreign matter such as dirt，grease， and corrosionproducts from the surface without removing any coatingmaterial. Avoid any areas having visible defects，such aswelding or soldering flux， acid spots，dross，or oxide whenmaking measurements.The magnet of an instrument of theattractive force type may stick to lead and lead alloy coatings.Apply a very thin film of oil to improve the reproducibility ofreadings and correct the measurement for the thickness of theoil film. Excess oil shall be wiped off so that the surface isvirtually dry. The correction may be determined by measuringthe coating thickness of a nonsticking coating of appropriatethickness with and without the oil film and taking the differ-ence between the two measurements. Do not use this procedurewith other coatings.The readings obtained may depend onthe technique of the operator.For example, the pressure applied to a probe,or the rate of applying a balancing force to amagnet， will vary from one individual to another. Reduce orminimize such effects either by having the instrument calibrated by the same operator who will make the measurement orby using constant-pressure probes. In appropriate cases when aconstant pressure probe is not being used,the use of ameasuring stand is strongly recommended.ASTM B499 pdf download.