It is important that incoming inspection of magnetic characteristics be clearly and properly specified. End point characteristics (such as B_r or H_c) cannot be directly observed; therefore inspection personnel should not expect to measure 8,500 Gauss on a SmCo 18 magnet even though the B_r is specified at 8,500 Gauss.

A test method or combination of test methods should be based upon the criticality of the requirement, and the cost and ease of performing tests. Ideally, the test results should be able to be directly translated into functional performance of the magnet. A sampling plan should be specified which inspects the parameters which are critical to the application. A brief description of some common test methods follows below.

B-H Curves
B-H curves may be plotted with the use of a permeameter. These curves completely characterize the magnetic properties of the material at a specific temperature. In order to plot a B-H curve, a sample of specific size must be used, then cycled through a magnetization/demagnetization cycle. This test is expensive to perform due to the length of time required to complete. The test is destructive to the sample piece in many cases, and is not practical to perform on a large sample of finished magnets. However, when magnets are machined from a larger block, the supplier may be requested to provide B-H curves for the starting raw stock of magnet material.

Total Flux
Using a test set up consisting of a Helmholtz coil pair connected to a fluxmeter, total flux measurements can be made to obtain total dipole moments, and interpolated to obtain close estimates of B_r, HC, and BH_max. The angle of orientation of the magnet can also be determined using this method. This is a quick and reliable test, and one that is not overly sensitive to magnet placement within the coil.

Flux Density
Flux density measurements are made using a gaussmeter and an appropriate probe. The probe contains a Hall Effect device whose voltage output is proportional to the flux density encountered. Two types of probe construction (axial, where the lines of flux traveling parallel to the probe holder, and transverse where the lines of flux traveling perpendicular to the probe holder, are measured) allow the measurement of flux density of magnets in various configurations. The placement of the probe with respect to the magnet is critical in order to obtain comparable measurements from magnet to magnet. This is accomplished by building a holding fixture for the magnet and probe, so that their positions are fixed relative to each other.

Flux Maps
Using special scanners equipped with 3-axis Hall probes, magnetic arrays can be mapped, to capture flux densities in x, y, and z directions with a specified number of data points across the entire array. The resulting data can then be output as a flux contour map, as flux vectors, or as a data table for further analysis.

Pull Tests
This is a commonly used test for magnets. The pull of the magnet is proportional to B², and is therefore very sensitive to the value of B. Variations in B occur due to variations in the inherent properties of the magnet itself, as well as environmental effects such as temperature, composition and condition of the material that the magnet is being tested on, measurement equipment, and operator. Since B decays exponentially from a zero air gap, small inadvertently introduced air gaps between the magnet and the test material can have a large effect on the measured pull. It is therefore recommended that pull be tested at a positive air gap. Performing pull tests at a number of air gaps, and plotting results as air gap vs. (pull)^1/2 , provides a more accurate description of the pull characteristics of the magnet. Extrapolating from this pull at zero air-gap may be calculated.

Other Functional Tests
These should be determined according to the application and after discussion with the supplier. They may involve complex tests such as a profile of flux density along a specified axis, flux uniformity requirements within a defined volume, or relatively simple tests such as a torque test.