Understanding Surface Gauss: Why Probe Placement is Critical
Surface Gauss (Gs) is the magnetic flux density measured at a specific point on the magnet's surface using a Hall effect sensor. The reading varies dramatically with:
Probe position relative to the magnetic pole center: at the pole center, reading is maximum; at the pole edge, reading drops 20-50%.
Distance from surface: Gauss decreases approximately with the square of distance (e.g., a 10mm N42 disc reads 5,000 Gs at contact, 3,000 Gs at 1mm gap, 1,200 Gs at 2mm gap).
Probe orientation: the Hall sensor must be aligned with the magnetic field direction; a 10° tilt causes 3-5% reading error.
Therefore, surface Gauss is only repeatable if the probe is held in a fixed jig with precise contact pressure and position. For QC, we use a motorized stage that positions the probe at the exact geometric center of each magnet face with ±0.1mm repeatability. Gauss readings are compared to the specification tolerance: for a batch of 500 magnets, we accept if the average reading is within ±5% of nominal and the range (max-min) is within ±8%.
Surface Gauss is useful for:
Quick sorting of magnet strength at incoming inspection.
Detecting coating defects (bubbles or scratches affect magnetic field locally).
Verifying magnetization uniformity across a batch.
Helmholtz Coils and Fluxmeters for Total Magnetic Moment
Total magnetic moment (m, in A·m² or μT·m³) is the integral of the magnetization over the magnet volume. It determines the torque a motor develops or the force an actuator exerts. Unlike surface Gauss, total magnetic moment is independent of probe position and does not vary with the measurement location.
Fluxmeter measurement principle: Place the magnet inside or against a Helmholtz coil (a pair of identical coils arranged coaxially with a separation equal to the coil radius). The magnet's field induces a voltage in the coil when the magnet is moved in/out or rotated. The fluxmeter integrates this voltage over time, giving the total flux linkage, which is directly proportional to the magnetic moment.
Advantages of fluxmeter measurement:
Highly repeatable: ±0.5% variation between measurements.
Independent of magnet shape (as long as the magnet fits within the coil's uniform field region).
Provides a single number (total flux) that correlates directly to motor torque.
Enables comparison between magnet batches with high confidence.
Disadvantages: The measurement fixture costs $5,000-$15,000 for a Helmholtz coil system (compared to $300-$1,000 for a Hall probe). Measurement takes 10-15 seconds per magnet (vs. 2-3 seconds for Gauss). Hence, fluxmeters are used for batch sampling (e.g., 10% of each batch) rather than 100% inspection.
Gauss Meter vs. Flux Meter: Comparison
| Property | Gauss Meter (Hall Probe) | Flux Meter (Helmholtz Coil) |
|---|---|---|
| What it measures | Surface flux density at a point (Gs, T) | Total magnetic moment (Φ, A·m²) |
| Dependency on probe position | Critical (center vs. edge) | None (integrated over whole magnet) |
| Dependency on magnet orientation | Critical (must align with field) | Low (magnet is rotated/dropped) |
| Repeatability (same magnet) | ±3-5% (with manual probe) / ±1% (with jig) | ±0.5% |
| Measurement time per magnet | 2-5 seconds | 10-30 seconds |
| Equipment cost | $300-$3,000 | $5,000-$20,000 |
| Best use | 100% incoming/outgoing sorting, quick checks | Batch qualification, NPI validation, customer certification |
| Correlates to motor torque | Poor (surface field ≠ total flux) | Excellent (directly proportional) |
How We Guarantee Consistency in Bulk OEM Orders
Our quality assurance protocol for bulk orders (≥500 pcs):
Raw material receiving: Sample 5% of incoming sintered blocks, measure Br and Hcj on a permagraph (hysteresisgraph). Reject batch if Br < specified value or Hcj < min.
Machining: After grinding to final dimensions, 100% dimensional inspection with CMM (coordinate measuring machine) or laser micrometer. Reject parts outside ±0.05mm tolerance.
Coating: Eddy current testing for coating defects on 100% of parts. Ni-Cu-Ni thickness measured by XRF on 10% of parts.
Magnetization: Magnetize all magnets in a pulsed magnetizer. After magnetization, 100% surface Gauss check using a motorized stage (2 measurement points per magnet). Accept if within ±5% of specified value.
Flux verification: From each batch, select 10 samples (or 5% for large batches) and measure total flux on a Helmholtz coil fluxmeter. Calculate average and standard deviation. Accept if flux average is within ±2% of the nominal value and CPK ≥1.33 (process capability index).
Shipping: Include a QC certificate showing the batch average Gauss, flux, and dimensional measurements.
If a sample fails flux verification (±2%), the entire batch is held for 100% flux measurement. The failed units are segregated and remagnetized or scrapped.
laboratory showing Gauss meter with Hall probe measuring surface field of a neodymium magnet and Helmholtz coil fluxmeter measuring total magnetic moment for batch consistency verification.)
For detailed quality protocols for your bulk magnet order – including measurement methods, pass/fail criteria, and sample sizes – please visit our Quality Assurance page on our website.
To request a QC package with flux verification and CPK data, please specify your requirement in the purchase order. We provide full inspection reports for every lot.
Frequently Asked Questions
Q: Can I measure the magnetic moment of a magnet already assembled in a motor?
A: No, the motor's back iron and rotor alter the flux path. You must measure the magnet before assembly. For assembled rotors, we can measure torque constant (Kt) to infer magnet strength indirectly, but this requires a motor test bench.
Q: What is the difference between a fluxmeter and a permagraph (hysteresisgraph)?
A: A fluxmeter measures total magnetic moment (a single number). A permagraph measures the entire demagnetization curve (Br, Hcj, BHmax) by applying a varying external field and measuring the magnet's response. Permagraphs are used for material verification, not production QC.
Q: Do you provide calibrated Gauss meters for customers to verify magnets at their facility?
A: Yes. We sell NIST-traceable Gauss meters with probe and calibration certificate. Price: $850-$1,500 depending on range (0-20,000 Gs, 0-30,000 Gs). Calibration recommended annually by an accredited lab.
