How to measure magnets with magnetic encoder?

Content

1 Magnetic Measurement Units You Actually Need to Know
- 1.1 Tesla (T)
- 1.2 Gauss (G)
2 Measuring the Physical Dimensions of a Magnet
3 Using a Gaussmeter to Measure Magnetic Field Strength
4 Fluxmeters and Total Flux Measurement
5 How Magnetic Encoders Measure Magnets in Real Time
6 Field Mapping: The Complete Picture of a Magnet's Behavior
7 Measuring Different Types of Encoder Magnets
8 Practical Tips for Accurate Magnet Measurement
9 Comparing Measurement Methods at a Glance
10 Matching Magnet Grade to Measurement Requirements

Measuring a magnet properly means looking at two distinct things: its physical dimensions (diameter, thickness, and shape tolerances) and its magnetic field properties (flux density, surface field, and polarity distribution). Most people only check one when they need both—and that gap becomes critical the moment a magnet goes into a precision device like a magnetic encoder.

This guide walks through every practical measurement method: the tools involved, the units that matter, when each approach is appropriate, and what good numbers actually look like for encoder-grade magnets specifically. Whether you are sourcing magnets for a motor control system, qualifying a batch from a supplier, or troubleshooting a drifting position signal, the answers are below.

Magnetic Measurement Units You Actually Need to Know

Before picking up any instrument, it helps to lock down the unit system. Two coexist in most datasheets and you will see both on any gaussmeter display.

Tesla (T)

The SI unit for magnetic flux density. One tesla is the field strength that produces one volt of electromotive force in a one-metre conductor moving at one metre per second perpendicular to the field. High-performance neodymium magnets reach surface fields of 1.0–1.4 T. This is the unit used in engineering specifications and encoder IC datasheets.

Gauss (G)

The CGS equivalent. 1 T = 10,000 G—that conversion is all you ever need. Gauss is still common in manufacturing specs, supplier certificates, and North American industrial documents. A fridge magnet runs around 50 G; a neodymium disc magnet used in a magnetic encoder system will typically sit between 2,000 G and 5,000 G at its surface depending on grade and geometry.

Two other quantities appear regularly in magnet datasheets:

Key magnetic quantities and their practical relevance in measurement and encoder design
Quantity	Symbol	Unit (SI)	What It Tells You
Magnetic flux density	B	Tesla (T) / Gauss (G)	How concentrated the field is at a point in space—the primary measurement from a gaussmeter
Magnetic field strength	H	A/m	The applied field independent of the material; related to B through permeability (B = μH)
Magnetic flux	Φ	Weber (Wb)	Total field passing through an area (B × A); measured by a fluxmeter for total output assessment
Coercivity (Hci)	Hci	kA/m	Resistance to demagnetization; high Hci is essential for encoder magnets in motor environments

Measuring the Physical Dimensions of a Magnet

Physical measurement comes first in any incoming inspection workflow because a magnet that is out of tolerance geometrically will cause problems regardless of how strong its field is. For a disc magnet used in a magnetic encoder, even a 0.1 mm deviation in diameter or thickness changes the air gap—and the air gap directly controls the field strength the sensor IC actually sees.

Tools for Dimensional Measurement

Vernier Caliper

Adequate for general-purpose magnet inspection where tolerances are ±0.1 mm or wider. Measure diameter across at least two perpendicular axes and average the readings. Measure thickness at the center and near each edge. Standard machine tolerances for off-the-shelf neodymium magnets typically run ±0.076 mm to ±0.127 mm (±0.003 to ±0.005 inches).

Digital Micrometer

Required when tolerances tighten below ±0.05 mm, which is common for encoder magnets. A micrometer gives resolution down to 0.001 mm and is far less sensitive to operator technique than a caliper. Measure thickness at multiple points across the face to detect taper or grinding variation.

Laser Measurement System

Used in manufacturing quality control for high-volume encoder magnet production. Laser gauges verify dimensions without contact, which matters when a magnet's surface finish or coating (nickel plating on neodymium, for example) must not be disturbed. This method is also faster and less prone to operator variability on small disc geometries.

CMM (Coordinate Measuring Machine)

For prototype qualification or tight-tolerance custom magnets, a CMM provides full geometric inspection including cylindricity, flatness, and perpendicularity of the pole face. These measurements matter when a magnet designed with a nominal 10 mm diameter must stay between 9.95 mm and 10.05 mm across an entire production run.

What Dimensions Matter Most for Encoder Applications

Not all dimensions carry equal weight. For a two-pole diametrically magnetized disc magnet mounted above an encoder IC, three dimensions dominate encoder performance:

Diameter — determines the lateral field coverage relative to the sensor array. Oversized magnets can saturate sensors; undersized magnets reduce field amplitude below the IC's minimum operating range.
Thickness — directly controls the axial field strength at the encoder's air gap distance. A thicker magnet generally produces a stronger axial field, but can also create interference with nearby ferrous components.
Pole center offset — the polarization boundary should be exactly centered in the magnet. Offset greater than a few tenths of a millimeter introduces integral nonlinearity error in the angular reading. This cannot be corrected by dimensional measurement alone—it requires magnetic field mapping.

Using a Gaussmeter to Measure Magnetic Field Strength

A gaussmeter (also called a teslameter or magnetometer) is the standard instrument for quantifying the magnetic field a permanent magnet produces at any given point. It works on the Hall effect: when a current flows through a semiconductor placed in a magnetic field, a voltage develops perpendicular to both the current and the field. That voltage is proportional to the field strength, making it a direct and accurate readout of flux density.

Practical starting point: position the gaussmeter probe flat against the pole face of the magnet, perpendicular to the surface. Record the surface field value. For a sintered NdFeB N42 disc magnet of typical encoder dimensions (6 mm diameter × 2.5 mm thick), expect a surface field in the range of 3,500–5,000 gauss (0.35–0.50 T). Values significantly outside that range indicate a substandard grade, partial demagnetization, or the wrong magnet specification.

Probe Types and When to Use Each

Gaussmeters come with two common probe orientations that serve different purposes:

Axial probe — sensitive along the probe's long axis. Used to measure the field coming off a flat pole face, which is the standard measurement for disc and block magnets. Best for checking the surface field of an encoder magnet before installation.
Transverse probe — sensitive perpendicular to the probe's axis. Used when the field is oriented sideways relative to the measurement direction, such as measuring the radial field on the edge of a ring magnet or along the pole boundary of a diametrically magnetized cylinder.

For a 3D field map—measuring X, Y, and Z components simultaneously—a three-axis probe is necessary. High-end gaussmeters like the 3-axis teslameter (such as the TM-4300 class of instruments) perform simultaneous tri-axial measurement and can display the resultant vector angle, which is directly useful when characterizing encoder magnets where angular field error is the spec that matters.

Understanding Probe-to-Surface Distance Error

One of the most underappreciated sources of gaussmeter error is probe standoff distance. Because magnetic fields drop off rapidly with distance, even a probe that sits 0.9 mm from the surface instead of the nominal 0.035 inches (~0.89 mm) produces a measurable deviation. For a D82 disc magnet, a probe positioned at the standard standoff reads approximately 10% less than the theoretical surface field. For small magnets under 6 mm in diameter, this error grows sharply. As a practical rule, gaussmeter measurements are not reliable for field verification on magnets smaller than about 6 mm (1/4 inch) without a precision-controlled fixture that sets probe-to-surface distance repeatably.

Step-by-Step Gaussmeter Procedure

Zero the gaussmeter in an area free of ferrous material and external magnetic fields (move at least 300 mm from any steel surface or strong magnet).
Select the correct probe type for your measurement orientation (axial for pole face, transverse for edge/radial field).
Place the probe perpendicular to the magnet surface with consistent contact or a fixed standoff fixture.
Record the center reading, then take readings at four additional points (12, 3, 6, and 9 o'clock positions on the face) to assess field uniformity.
Flip the magnet and repeat on the opposite pole. The south pole reading should be the same magnitude as the north, within the instrument's stated accuracy (typically ±1–2%).
Compare all readings against the supplier's specification or your drawing's field value requirement.

Fluxmeters and Total Flux Measurement

A fluxmeter measures the total magnetic flux passing through a defined area rather than the field at a single point. Where a gaussmeter gives you a pointwise snapshot, a fluxmeter gives you the integrated output of the entire magnet. This makes fluxmeters better suited for batch acceptance testing and for comparing magnets of the same specification across production lots.

The measurement involves a search coil: a wound coil with a known area is placed over the magnet and then rapidly removed (or the magnet is withdrawn through the coil). The changing flux induces a voltage, which the fluxmeter integrates over time to produce a total flux value in Webers or Maxwell. Because the result captures the whole magnet rather than a probe spot, it is less sensitive to probe placement errors and better reflects the magnet's total energy output.

For encoder magnet qualification, a fluxmeter is particularly useful for detecting partial demagnetization that a single-point gaussmeter reading might miss—a magnet can show a normal surface field at its center while having degraded sections near the edges if exposure to elevated temperatures or opposing fields has been uneven.

How Magnetic Encoders Measure Magnets in Real Time

A magnetic encoder is not just a user of magnets—it is also, in a functional sense, a continuous field measurement device. Understanding how a magnetic encoder reads its magnet helps clarify what magnetic properties matter most during the selection and verification process.

Permanent Magnet Generates a Rotating Field

A diametrically polarized magnet (poles oriented across the diameter, not through the thickness) is mounted on the shaft. As the shaft rotates, the direction of the magnetic field at the sensor's location sweeps through 360°. This rotation of field direction is what carries the position information.

Hall Sensors Detect Field Components

The magnetic encoder IC contains two or more Hall effect sensors oriented in orthogonal directions. Each sensor measures the field strength along its own axis. As the magnet rotates, one sensor produces a sine-wave output and the other a cosine-wave output, each varying with field strength from near-zero to peak as the north pole sweeps past.

On-Board CORDIC Computes Angle

The encoder IC uses a CORDIC (or equivalent trigonometric) algorithm to compute atan2(sin, cos) from the two sensor outputs. The result is the absolute angular position within one electrical cycle. For a two-pole magnet, one mechanical revolution equals one electrical cycle—so the output is absolute shaft angle from 0° to 360°.

Signal Conditioning and Output

The angular result is passed through a signal conditioning circuit and output as a digital word (SPI or I2C), analog voltage, or incremental quadrature pulses (A/B channels), depending on the encoder IC's configuration. The resolution of a magnetic encoder is ultimately determined by the bit depth of the on-chip ADC and the angular interpolation algorithm—modern encoder ICs achieve resolutions down to 12 or 14 bits (4,096 to 16,384 counts per revolution).

What the Magnetic Encoder Requires from Its Magnet

Because a magnetic encoder continuously measures its own magnet in real time, the magnet's properties define the system's accuracy ceiling. The specifications that matter most are not always the ones listed most prominently on magnet supplier pages:

Magnet specifications critical to magnetic encoder accuracy and their acceptable ranges
Specification	Why It Matters	Typical Requirement
Field amplitude at gap distance	Must fall within the encoder IC's operating window (too weak = noise; too strong = saturation)	Varies by IC—commonly 15–75 mT at 0.5–3 mm gap
Pole center offset	Off-center polarization adds DC offset to the sensor's sine/cosine outputs, causing nonlinearity error	<0.1 mm from geometric center preferred
Axial mounting misalignment	Lateral offset between magnet center and sensor center generates harmonics in the angle output	Maximum ±0.5 mm recommended
Temperature stability (Br)	Remanence drops with temperature; if Br falls below IC's minimum field threshold, readings become unreliable	NdFeB N42+: usable to 80–120°C; SmCo for higher
Diametric polarization purity	Multipole contamination in a two-pole magnet distorts the sinusoidal field and creates periodic angle errors	Verified by 2D field mapping, not a gaussmeter point reading

Field Mapping: The Complete Picture of a Magnet's Behavior

A single gaussmeter reading tells you the field at one point. For general-purpose magnets in non-critical applications, that may be all you need. For encoder magnets, it is rarely sufficient. Field mapping—measuring the field distribution across the full surface area or in a 3D volume—reveals problems that point measurements hide: asymmetric polarization, flux leakage, pole boundary displacement, and high-order harmonic content.

2D Surface Field Mapping

A magnetic field camera or scanning hall probe system moves a sensor (or an array of sensors) across the magnet surface in a fine grid, recording field values at each point. The result is a 2D map of flux density that clearly shows the north and south pole regions, the location of the neutral boundary line, and any asymmetry in the field distribution. For a two-pole disc magnet intended for a magnetic encoder, the neutral boundary should bisect the disc exactly through the center. Any shift from center introduces an offset in the encoder's angle output.

3D Extrapolation and the Distance Filter Method

A more advanced technique involves measuring the 3-axis magnetic field distribution close to the magnet surface and then mathematically extrapolating that data to predict the field at larger distances. Using algorithms like the Distance Filter method, this approach can compute the field distribution at the exact air gap where an encoder IC will sit—and do so with resolution finer than a physical probe could achieve at that distance. This method is particularly powerful for characterizing the intrinsic angular error of an encoder magnet, identifying whether the magnet's field distribution will produce sub-degree or sub-arc-minute linearity performance once installed.

Visual Inspection with Magnetic Viewer Film

For rapid qualitative assessment, magnetic viewer film (sometimes called ferrofluid film or magnetic field viewer sheet) provides an immediate visual display of the pole pattern. The film contains microcapsules filled with ferromagnetic particles that align with the field lines, producing a visible image of the magnetization pattern without any electronics. While this method cannot quantify field strength, it is a fast and effective check for: correct pole orientation, gross asymmetry in a two-pole magnet, and multipole contamination in what should be a two-pole part. In manufacturing environments, it is often used as a rapid pass/fail check before more detailed gaussmeter testing.

Measuring Different Types of Encoder Magnets

Not all encoder applications use the same magnet geometry. The measurement approach adjusts depending on the form factor.

Two-Pole Disc Magnet (Rotary Encoder)

The most common form for angular position measurement. Measure surface field with an axial probe at the pole face center. Key check: field strength at the intended air gap distance (typically 0.5–3 mm), and pole center offset using a 2D scan. Grade N42 or above in neodymium, or SmCo for elevated temperature environments. Mounting air gap tolerance is typically ±0.5 mm maximum.

Multipole Ring Magnet (Motor Feedback)

Ring magnets with 32, 64, or more poles are used in motor-integrated magnetic encoder systems where the ring mounts directly on the rotor. Measurement focuses on pole pitch uniformity and radial field strength consistency. A scanning gaussmeter traverses the inner or outer diameter surface to verify that each pole has matching field amplitude and correct angular spacing. Pole pitch errors directly translate to commutation errors in the motor drive.

Linear Scale (Linear Magnetic Encoder)

A linear magnetic encoder uses a magnetic scale—a strip of alternately magnetized segments bonded to a substrate. The read head traverses the scale and detects alternating north-south pole transitions. Measuring a linear scale involves checking the pole pitch (spacing between poles, typically 1 mm or 2 mm), the peak radial field strength at the read head's standoff distance, and the consistency of that field along the full scale length. Deviations in pole pitch directly produce position error.

Practical Tips for Accurate Magnet Measurement

Measurement errors in magnet characterization are common and often systematic rather than random. Eliminating them is largely a matter of controlling the environment and the procedure.

Keep ferrous material away from the measurement area. Steel tables, tool cabinets, and nearby motors all distort the field. Work on a non-ferrous surface—granite or aluminum—and maintain at least 300 mm clearance from structural steel.
Zero the gaussmeter in the measurement position. Walk the probe to the spot where measurements will be taken, and zero there—not at a different location where the background field is different.
Control probe-to-surface distance. For small magnets (under 10 mm), use a fixture that sets the probe standoff to a fixed, documented distance. A 0.5 mm variation in standoff on a 6 mm disc magnet changes the reading by several hundred gauss.
Measure multiple points, not just the center. Field density typically peaks at the poles and drops near the neutral boundary. A single center reading misses field uniformity and polarity distribution information that matters for encoder applications.
Check both poles. North and south pole readings should be equal in magnitude. A significant asymmetry (more than 3–5%) indicates manufacturing defects, demagnetization, or contamination with a second magnet nearby.
Temperature-condition the magnets before measuring. Neodymium magnets lose roughly 0.1% of their field strength per degree Celsius of temperature rise. If magnets arrived after shipping in heat, let them equilibrate to room temperature (20–23°C) before recording baseline measurements.
Verify polarization direction for encoder magnets before mounting. A diametrically magnetized magnet installed with its polarization axis rotated 90° relative to the encoder IC will produce only noise. A viewer film or axial gaussmeter sweep around the magnet's circumference confirms the correct orientation in seconds.

Comparing Measurement Methods at a Glance

Summary of magnet measurement methods with their strengths, limitations, and best application contexts
Method	Measures	Strengths	Limitations	Best Used For
Caliper / Micrometer	Physical dimensions	Fast, low cost, universal	No field information	Incoming dimensional inspection
Gaussmeter (1D)	Point flux density	Direct, quick, portable	Single-point only; probe standoff error	Batch field strength verification
Gaussmeter (3D)	Vector field at a point	Shows field direction and resultant angle	Still single-point; higher cost	Encoder magnet air gap characterization
Fluxmeter	Total magnetic flux (Wb)	Captures whole magnet output; less probe-position sensitive	Requires search coil; no spatial detail	Lot acceptance testing, demagnetization detection
2D Field Camera	Surface field distribution map	Shows pole symmetry, offset, uniformity	Equipment cost; slower than spot measurement	Encoder magnet qualification, angular error analysis
Viewer Film	Qualitative pole pattern	Instant, no electronics needed	No quantitative data	Rapid pass/fail polarization check

Matching Magnet Grade to Measurement Requirements

Magnet grade affects which measurements are critical and what values you should expect. The grade designation directly determines remanence (Br), which sets the surface field; and coercivity (Hci), which determines how stable that field remains under thermal and demagnetizing stress.

Sintered NdFeB (Neodymium)

The most widely used material for encoder disc magnets. Grades N35 to N52 offer Br values from approximately 1.17 T to 1.48 T. Surface fields on typical encoder-sized discs (5–8 mm diameter, 2–3 mm thick) range from 3,000 to 5,500 gauss. Maximum operating temperature without significant field loss is 80°C for standard grades; high-temperature grades (N42SH, N42UH) extend this to 120–180°C. Always verify temperature rating when the encoder operates near motor windings or in industrial heat environments.

Sintered SmCo (Samarium Cobalt)

SmCo magnets offer lower Br than comparable NdFeB grades, but significantly better temperature stability and corrosion resistance. The temperature coefficient of Br is roughly -0.03% to -0.04% per °C for SmCo versus -0.10% to -0.12% per °C for NdFeB. In high-temperature encoder applications (above 120°C), SmCo is often the correct choice despite its higher cost, because the field delivered at operating temperature remains within the encoder IC's specified window.

Ferrite (Hard Ferrite)

Used in multipole ring magnets for motor-integrated magnetic encoder rings. Ferrite offers very high coercivity—excellent resistance to demagnetization from the motor's stray fields—and good corrosion resistance without surface coating. The trade-off is low Br (typically 0.35–0.43 T), which means a ferrite encoder ring must be measured with appropriate sensitivity settings on the gaussmeter, and field uniformity between poles requires tight process control during magnetization.