ASTM-A977/A977M:07(R2020) pdf free download – Standard Test Method for Magnetic Properties of High-Coercivity Permanent Magnet Materials Using Hysteresigraphs.
4.3 Tests performed in general conformity to this test method and even on the same specimen, hut using different test systems, may not yield identical results. The main source of discrepancies are variations between the different test systems in the geometry of the region surrounding the sample, such as, size and shape of the electromagnet pole caps (see Annex Al and Appendix X1), air gaps at the specimen end faces, and especially the size and location of the measuring devices for H and B or for their corresponding flux values (Hall-died probes, inductive sensing coils). Also important is the method of B calibration, for example. a volt-second calibration of the fluxrneter alone versus an overall system calibration using a physical reference sample. The method of B and H sensing should be indicated in test reports (see Section 9).
5. Measuring Methods and Apparatus
5. 1 Measuring Flux and Induction (Flux Density).
5. 1 . I In the preferred B-measuring method, the total flux is measured with a sensing coil (search coil) that surrounds the test specimen and is wound as closely as possible to the specimen surface. Its winding length should be no more than a third of the specimen length, preferably less than one fifth, and must he centered on the specimen. The leads shall be twisted tightly. As the flux changes in response to sweeping the applied field. H. the total flux is measured by taking the time integral of the voltage induced in this coil. This measurement is taken with a fluxmeter. Modern hystcresigraphs use electronic integrating fluxmeters that allow convenient continuous integration and direct graphic recording of magnetization curves. If the signal is large enough. high-speed voltage sampling at the coil and digital integration is also possible.
5.1.2 The magnetic induction B is determined by dividing the total flux by the area-turns product NA of the B-sensing coil. For permanent magnets in general, and especially for high-coercivily materials, an air-flux correction is required (see 5.1.5).
5.1.3 The total error of measuring B shall be not greater than ±2%.
5.2 Deiermining Iiiiriiisic Induction
5.2.1 For high-coercivity magnets. it is more convenient to sense directly an electrical signal proportional to the intrinsic induction, derive the average B by dividing this flux by the area-turns product of the surrounding B coil, and to plot B, versus H. B then is obtained by mathematical or electronic addition of H to B.
5.3.2 To determine the magnetic field strength, a flat surface coil, a tightly fitted annular coil, a magnetic potentiometer, or a Hall probe is used together with suitable instruments. The dimensions of the magnetic field sensor and its location shall hc such that it is within an area of limited diameter around the test specimen (see Annex AI).