Absolute and comparative methods provide a means for sorting large quantities of ferrous parts of stock with regard to composition, condition, structure, or processing, or a combination thereof.

The comparative or two-coil method is used when high-sensitivity testing is required. The advantage of this method is that it almost completely suppresses all internal or external disturbances such as temperature variations or stray magnetic fields. The two-coil method is normally used when harmonic evaluation is employed for sorting.

The ability to accomplish satisfactorily these types of separations is dependent upon the relation of the magnetic characteristics of the ferromagnetic parts to their physical condition.

These methods may be used for high-speed sorting in a fully automated setup where the speed of testing may approach ten specimens per second depending on their size and shape.

The success of sorting ferromagnetic material depends mainly on the proper selection of magnetic field strength and frequency of signal in the test coil, fill factor, and variables present in the sample.

The degree of accuracy of a sort will be affected greatly by the coupling between the test coil field and the test specimen and the accuracy with which the specimen is held in the test coil field during the measuring period.

When high currents are used in the test coil, a means should be provided to maintain a constant temperature of the reference specimen in order to minimize measurement drift.

1.1 This practice covers the procedure for sorting ferrous metals using the electromagnetic (eddy-current) method. The procedure relates to instruments using absolute or comparator-type coils for distinguishing variations in mass, shape, conductivity, permeability, and other variables such as hardness and alloy that affect the electromagnetic or magnetic properties of the material. The selection of specimens to determine sorting feasibility and to establish standards is also included.

1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.