MIL-STD-1537C
the probe and the standard. The difference in the two readings shall not exceed 0.5 %IACS. If it does exceed 0.5 %IACS, return the instrument to the calibration facility for repair and recertification. This test shall be performed, immediately prior to use, or at a frequency based upon objective evidence that the required accuracy is consistently maintained. On indirect reading instruments, lift-off shall be checked immediately prior to use.
4.3.2 Laboratory calibration.
The eddy current instrument shall be returned to the calibration laboratory on a periodic schedule for a complete calibration and certification including scale tracking (if analog type), lift-off, drift and battery replacement (if required). Periodic recalibration of eddy current instruments used for conductivity testing shall be conducted every 120 days or at an interval based upon objective evidence that the required accuracy is consistently maintained. The calibration interval shall not exceed 12 months.
4.4 Test article.
4.4.1 Surface.
The areas to be tested shall be free of dirt, grease, oil or other contaminants. The surface finish of the areas to be tested shall be 150RHR (Roughness Height Rating) maximum, RMS (Root Mean Squared) 165 maximum or less than a maximum established by objective evidence, for the instrument in use, that the required accuracy is maintained. A nonconductive coating up to
0.003 inch (0.076 mm) thick is permissible on parts without determining a lift-off correction factor for eddy current readings. If the nonconductive coating thickness is greater than 0.003 inch (0.076 mm), the effects of the coating on instrument readings shall be determined. Effects of coating thickness shall be established by measuring the conductivity at individual locations before and after removal of the nonconductive coating or by simulating the coating thickness on bare metal by the use of plastic or paper shim stock (see 5.1.4). If the difference between the
two readings from coated and uncoated aluminum is greater than 0.5 %IACS, further
measurements and instrument recalibration shall be made to further verify the coating thickness and establish the magnitude of the difference in instrument readings. A lift-off correction factor shall be applied as determined from the difference in readings.
4.4.2 Edge effect.
To insure readings are not distorted from edge effect, the specimen area to which the eddy current instrument probe is applied shall be at least as large as the outside diameter of the test probe or per manufacturer's specifications. The coil shall be centered on all parts whose dimensions approach this limitation.
4.4.2.1 Curvature correction factor.
The limitations on the minimum radius of curvature of concave and convex surfaces on which conductivity measurements can be performed accurately shall be determined by user's tests or shall be in accordance with the instrument manufacturer's specifications. Round stock having a radius of less than the instrument manufacturer's specifications may be tested, provided a
fixture is used to insure that the probe is placed in the same position on each piece and a
curvature correction factor is used. An acceptable alternate method for round stock with a radius of less than four inches (100 mm) would be to move the probe slowly and gently across the surface at right angles to the length, read the maximum value indicated on the instrument and then apply the appropriate correction factor.
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