How Magnets Are Tested: Flux, Pull Force, Salt Spray, Declination

Every magnet datasheet is a promise; testing is where promises meet instruments. Knowing how magnets are actually measured — and what each instrument can and cannot see — turns you from a spec reader into a spec auditor. Here is the test floor, machine by machine.

Magnetic tests

• Flux measurement (Helmholtz coil + fluxmeter). The workhorse: pull the magnet through a calibrated coil pair and integrate the induced voltage into total flux. Fast, repeatable, geometry-independent — ideal for 100 % sorting and the per-lot consistency data serious buyers request.
• Surface field mapping (gauss meter). A Hall probe reads flux density point by point. Perfect for verifying working-point fields and pole patterns; meaningless as a single unlocated number, as we argued in gauss vs pull force.
• Declination testing. Measures the angle between the magnetic axis and the geometric axis. A part can pass flux yet be magnetized two degrees off-axis — invisible until an encoder wanders or a rotor vibrates. Precision motor and sensor lots get this check.
• B–H curve tracing (hysteresisgraph). The full material fingerprint — Br, HcJ, BHmax and the knee — measured on samples per batch. This instrument is what stands behind grade claims; ask any supplier whether they own one.

Mechanical and surface tests

• Dimensions. Micrometers for the routine; 2D vision measurement for profiles, chamfers and thin arcs at scale.
• Pull force. Calibrated rigs with defined plates and gaps — a system test, quoted with its conditions.
• Coating: thickness, adhesion, salt spray. X-ray fluorescence reads plating microns; cross-hatch and thermal-shock check adhesion; salt-spray chambers turn the corrosion promises of the coatings guide into logged hours.
• Vision sorting. Automated optical inspection catches chips, cracks and plating defects at production speed — the difference between sampled quality and delivered quality.

Reading a test report like a buyer

1. Match the method to the claim. Grade claims need hysteresisgraph data; consistency claims need flux distributions (mean and spread, not one line).
2. Check conditions. Pull force without plate spec, or salt spray without hours and standard, is decoration.
3. Ask for the distribution. Ten measured values tell you more than one “typical” — an honest supplier sends histograms without flinching.

Our lots ship with flux, dimensional and coating reports as standard; declination and full curves on request. Ask for a sample report before you ever place an order — it is the cheapest supplier audit available.

FAQ

Is 100 % testing realistic?

For flux, appearance and dimensions at automated speed — yes, and common. Destructive or slow tests (salt spray, full curves) are sampled by plan. The right question is which tests run at 100 % and which are sampled.

What is an acceptable flux tolerance?

General applications live happily at ±3–5 %; precision motors and sensors specify tighter windows per drawing. Tighter costs more — specify what the design needs, not what sounds impressive.

Can I test incoming magnets without a lab?

Meaningfully, yes: a decent gauss meter with a positioning jig plus a pull-force rig catches gross problems. For material-level disputes, third-party labs and our own instruments settle the question.