How Magnets Are Tested & Measured

Confusion in magnet quality almost always traces back to mixing up four different quantities:

The hysteresisgraph (permeameter) is the reference instrument for material grade verification. A sample is clamped in a closed magnetic circuit between electromagnet poles and driven through its full magnetization cycle while B and H are recorded — producing the demagnetization (second-quadrant) curve from which Br, Hcb, Hcj, and BHmax are read directly.

• What it proves: the material in this production lot actually meets the claimed grade — e.g., N42SH minimums of Br and Hcj per the applicable standard (IEC 60404-5 method; grade values per supplier datasheet or IEC 63348 / GB/T 13560 tables).
• How it's used: lot-level sampling at the factory, on standard-geometry samples from the same sintering lot. It is not a 100% or incoming-dock test — the instrument is a lab asset and the test consumes time.
• What to ask for: the per-lot material certificate reporting measured Br, Hcb, Hcj, BHmax against grade minimums — this is the document that connects your parts to the grade you paid for.
• Temperature data: for elevated-temperature applications, curves can be run hot to verify the knee stays out of your operating region — worth requesting for critical high-temp programs.

For the theory behind the curve — load lines, the knee, permeance coefficients — see the BH curve chapter in Magnets 101.

The helmholtz coil is the workhorse of part-level magnet inspection. The magnet is placed at the center of a pair of matched coils and withdrawn (or flipped); the induced voltage, integrated by a fluxmeter, gives the part's total magnetic moment — a single number characterizing the whole magnet.

Why it's the preferred acceptance test

• Robust: the reading is nearly insensitive to exact positioning within the coil's uniform region — unlike surface probes.
• Fast: seconds per part; suits 100% testing or large samples at both the supplier's final inspection and your receiving dock.
• Meaningful: total moment captures material grade, part volume, and magnetization state together — a low reading flags undersize, wrong grade, or incomplete saturation.
• Comparable: supplier and customer measurements correlate well when coil constants are calibrated, making it a clean basis for accept/reject agreement.

Limitation to note: the helmholtz measurement gives one aggregate number. It confirms how much magnetization exists, not how it is distributed — a multipole pattern error or a locally weak zone needs the scanning methods in section 06.

A gaussmeter reads flux density at the location of a small Hall-effect sensor in the probe tip. It is indispensable for polarity checks, field mapping, air-gap measurements, and troubleshooting — and notoriously unreliable as an acceptance criterion when used casually.

Why surface readings scatter

• Distance sensitivity: near a magnet face, field falls steeply — tenths of a millimeter of probe lift-off change the reading by several percent. The Hall element also sits recessed inside the probe tip by an amount that varies by probe model.
• Position sensitivity: the field varies across the face (typically higher at edges of an axial disc than at center). "Measure the surface field" without defining where is not a test.
• Angle sensitivity: Hall probes read the field component normal to the element; tilt reads low.
• Probe variability: different probe models on the same magnet can read meaningfully different values for all the above reasons.

Pull testing measures the force to detach a magnet from a defined steel plate (or through a defined gap) using a force gauge. It answers the customer-language question — "how strong is it?" — and is legitimate for holding-application verification, but it is a system test, not a magnet test:

• Results depend on plate thickness, steel alloy and surface finish, alignment, pull direction and rate — a published "pull force" is only reproducible against the same fixture and procedure.
• Pull force is insensitive to some real defects (a mildly under-saturated magnet loses more flux than pull force) and oversensitive to surface condition.
• Values scatter more than flux measurements; typical use is design verification and marketing ratings rather than production acceptance.

Use pull tests to validate your holding design (with your fixture, documented); use helmholtz flux to accept magnets. Estimate expected values with our pull force calculator — then verify on hardware, since calculators assume ideal conditions.

Beyond "how much," many applications must verify "which way" and "in what pattern":

• Polarity identification: a pole indicator or referenced Hall probe confirms which face is N. Trivial — and still worth 100% checking on parts where reversed polarity assembles invisibly and fails in the field.
• Orientation angle: for diametric parts clocked to a feature, a rotary fixture with a Hall probe locates the pole axis; the angle to the keyway or flat is compared against the drawing tolerance.
• Multipole scanning: a Hall probe on a precision rotary stage traces the field around the part at a defined radius/standoff, producing a field-vs-angle plot. From it: pole count, peak field per pole, pole-to-pole balance, and transition angle errors — the quantities that determine encoder accuracy.
• True radial uniformity: the same circumferential scan on a radial ring verifies field level and uniformity through 360° — and immediately exposes a segmented assembly masquerading as a true radial ring, since every glue joint appears as a periodic dip in the trace. See Magnetization Directions & Pole Configurations.

Magnetic tests don't replace the physical ones:

The principles that keep supplier and customer measuring the same thing:

• Every requirement measurable, with a method attached. "N42SH per lot material certificate; total moment ≥ X µWb·cm per helmholtz/fluxmeter" is enforceable. "Strong N42SH magnet" is not.
• Pick the property that matches the risk. Grade fraud risk → material cert (hysteresisgraph data). Saturation/size risk → helmholtz moment. Pattern risk → pole scan. Corrosion risk → coating thickness + salt spray.
• One primary magnetic acceptance number. Multiple overlapping magnetic specs (moment AND surface field AND pull force) guarantee that good parts fail one of them. Choose the primary; make the rest reference.
• Set limits from measured hardware. Anchor minimums to first-article measurements plus tolerance analysis, not to ideal calculator values.
• Do the MSA. A gauge R&R on the actual fixture and instrument is what makes limits defensible — and is expected content in a PPAP submission for automotive programs.
• Define the sampling plan — 100%, AQL sampling, or per-lot — and the temperature of test if your product is temperature-sensitive.

A supplier who runs these tests routinely will happily put the numbers in writing. Every Radial Magnets production lot ships with material and plating certifications, and we support helmholtz moment limits, pole scanning, and full PPAP documentation for programs that require them.