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Choosing the Right Magnetic Measurement Equipment for Your ApplicationTeslameters vs. Fluxmeters, Explained

Testing magnets, assemblies, and sensors is a critical step throughout the entire manufacturing process to maintain quality and avoid costly stalls in production. Understanding the capabilities of different magnetic measurement tools can help ensure you’re getting the data you need. In this paper, we’ll focus on two commonly used instruments: the teslameter and the fluxmeter.

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TeslameterA teslameter, or gaussmeter, is used to measure flux density at a specific location. This instrument is ideal for magnetic measurement in industrial and scientific research applications.

In combination with Hall effect sensors and probes, a teslameter enables:

� Precise measurement of field strength at a point in space

� Measurement of field in small spaces or gaps

� Rapid individual static measurements

� AC and DC field measurement

� High-resolution field mapping

How it works

The teslameter pairs with a sensor or probe (a sensor mounted to a stem) that exhibits the “Hall effect.” The Hall sensor’s active element is a square or rectangular piece of semiconducting material, which, when a fixed current is passed through it in one direction, generates a voltage across the element in a perpendicular direction. The “Hall voltage (VH)” generated is proportional to the magnitude and direction of the magnetic field orthogonal to the plane of the sensor element, as shown in Figure 1.)

Figure 1. The Hall effect — VH is proportional to B for a fixed I

The teslameter provides a current to the sensor, measures VH and compensates for any offsets, then calculates and displays a calibrated value for the field strength seen by the sensor. The sign of the displayed value denotes the field direction relative to the sensor, and the magnitude represents the component of the field orthogonal to the sensor’s “flat” surface.

Choosing a Hall probe for your teslameter

In selecting a teslameter, proper selection of a Hall probe is probably the most difficult and important decision to make. Using an improper probe could lead to less than optimal accuracy, or even worse, costly damage.

Single-axis probes

For single-axis measurements, there are two main probe or sensor configurations, referred to as “transverse” and “axial,” giving you the flexibility to choose the best configuration for your given application:

Transverse sensors and probes are best for measuring fields between magnet gaps where north and south poles face each other to create a strong uniform field. They are also useful when recessed into a surface to verify magnets that pass over the top of the sensor or probe.

Figure 2a. A transverse Hall sensor

Figure 2b. A transverse Hall probe

KEY TERMINOLOGY:Flux density: The magnitude of a magnetic, electric, or other flux passing through a unit area.

Magnetic field strength: A measure of magnetic force at a given point.

INSIGHT:VH is a result of the “right hand rule.” A force results from the perpendicular current and field and impacts electrons in the sensor element. This “electromotive force” results in a potential difference across the element.

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Axial sensors and probes are designed for measurements where the field runs parallel to the probe stem or wires. This is particularly relevant when measuring inside solenoid-based magnets such as magnetizers.

Figure 3a. An axial Hall sensor

Figure 3b. An axial Hall probe

3-axis probes

In single-axis measurements, it’s critical to align the probe or sensor to ensure orthogonality between the field and the plane of the sensor element. For greater accuracy and simplicity, 3-axis sensors/probes and teslameters can be used, eliminating the need for precise, orthogonal positioning. The 3-axis probe can be inserted into the field from any orientation, and the gaussmeter calculates the overall magnitude and displays the measured x, y, and z components.

Figure 4a. A 3-axis Hall sensor

Figure 4b. A 3-axis Hall probe

While an entire encapsulated body of a sensor can have dimensions of several millimeters or more, the “active area” of the sensor’s internal Hall effect element is generally made as small as possible to provide accurate field measurement at a point, while enabling sensors and probes to be inserted into very small measurement spaces. This also facilitates high resolution field mapping by indexing a probe across the measurement area.

Figure 5. Active area of a Hall sensor. The smaller the active area, the more accurate the measurements.

SetupWhile many setup options exist, some basic settings common among teslameters include:

� AC/DC mode selection: DC mode is best suited for measuring static or slowly changing fields. AC mode is for measurement where there are periodic AC fields. AC is also specified with selectable narrow and wide band frequency modes.

� Field range: While some high-end teslameters offer an auto-ranging option that automatically senses and configures the device for the needed measurement range, specifying the range manually may sometimes be preferred. The range selection, typically in units of gauss (G), kilogauss (kG) or tesla (T), is also affected by the probe selection. Ensure the teslameter and probe combination selected includes ranges large enough to measure your expected signals, but not so large that you will lack measurement resolution. Generally, you should avoid measuring in ranges where the field value would be less than 10% of the full range value.

� Hold function: A few more capable teslameters offer a display hold setting that shows the highest reading obtained during the current measurement operation. This is quite valuable when the operator must look away from the front panel display to make a difficult probe placement. Still other types of mode settings, including reset instructions, keypad lock/unlock, alarm and pass/fail settings, and other operator instructions can be entered, depending again on the teslameter type.

+B

0.35±0.25 mm (0.014±0.01 in)

3.5±0.06 mm (0.138±0.002 in)

0.75±0.25 mm (0.03±0.01 in)

O set of active area from centerline 0.1 mm (0.004 in) maximum

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Features to look forAccuracy: While teslameters offer accurate measurements at an affordable price, some discrepancies between devices do exist. A reduced level of accuracy might suffice for some users but not for others, depending on the application.

Field magnitude range: Though teslameters can offer a wide range of measurable magnetic fields, not all instruments do. General-purpose teslameters will measure fields in the 2 G to 20 kG range. If you need to make measurements below or above this range, then you must take more care in instrument selection. More sophisticated teslameters may offer the ability to measure fields as low as a half-milligauss up to 350 kG. Generally, DC field ranges are wider than AC, and the probe must have the necessary field-magnitude capability.

Probe selection: The more probe types and sizes available, the wider range of testing you can conduct. Various versions of transverse and axial versions compatible with the teslameter are a must when selecting a sophisticated instrument.

Ease of use: If many members of your team need to conduct magnetic measurement, simplicity is key to ensure easy adoption and accurate results. Today’s teslameters feature touchscreen technology with intuitive navigation so a user can follow onscreen instructions instead of consulting a printed user manual to set up measurement parameters.

Fluxmeter

A fluxmeter, together with a Helmholtz coil, measures overall magnetization by characterizing flux, flex density, magnetic field strength, dipole movement, and magnetic potential. A fluxmeter is ideal for magnet testing and sorting in industrial and measurement system settings. This instrument serves as the main component in BH loop or hysteresis graph measurement applications.

Modern fluxmeters are stable electronic integrators calibrated to accept magnetically-induced voltages from coils. They are slightly more difficult to use and navigate than teslameters due to the wide variety of measurement choices and units they offer.

In general, a fluxmeter is used to:

� Assess a magnet’s overall magnetization, including internal to the magnet

� Measure flux inside of solid objects such as transformer cores

� Perform single-dipole permanent magnet characterization when paired with a Helmholtz coil

� Conduct straightforward in-house coil design and calibration

� Conduct high-frequency field measurements

How it works

A fluxmeter uses a loop or coil of wire as a sensor to determine the amount of magnetic flux passing orthogonally through the coil. What this measurement method reveals depends on the size of the coil used. Fluxmeters only respond to changes in flux, meaning that something has to be moved during the measurement.

The fluxmeter’s magnetic measurements take advantage of the physical relationship between the number of turns of wire in a coil and the rate of change of magnetic flux across the coils. Because the sensor for a fluxmeter is a coil with an open center volume, a single dipole item under test can be characterized simply by passing through the center of the coil.

Using search coils

A small search coil with a diameter of just a few millimeters can be mounted at the end of a probe, as shown in Figure 6, and used to assess local field strength.

Figure 6. A search coil probe

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The search coil probe and fluxmeter can provide similar insights as a gaussmeter, with certain limitations:

� The measurement corresponds to the total flux that is perpendicular to the area enclosed by the search coil. Dividing by the coil’s area gives a value for magnetic field strength. If the magnetic field is not uniform across the search coil, this measurement will give an average value for field strength.

� In order to generate a reading, the search coil probe must first be placed at the desired point of measurement, and then rapidly removed from the field’s influence. This triggers a momentary voltage in the search coil, the magnitude of which corresponds to the density of flux lines enclosed. The fluxmeter rapidly samples and integrates this voltage to determine the flux density at the start of the measurement.

� Rather than moving the probe in a static field, it is also possible to generate readings by placing the probe in an AC field.

Generally, a teslameter and Hall probe are superior to search coils for measuring magnetic fields at a specified point. Search coils are usually only chosen by users who already own a fluxmeter with a Helmholtz coil as an inexpensive way to deliver teslameter-style measurements without having to purchase separate instrumentation.

Using Helmholtz coils

The unique capability of the fluxmeter becomes apparent when the search coil probe is replaced with a pair of larger, open Helmholtz coils (see Figure 7). This configuration enables the overall magnetic quality of products, such as permanent magnets, to be readily assessed. When the diameters of the Helmholtz coils exceed the dimensions of the product to be tested, the product can be placed between the coils in alignment with the coil axis and then rotated 180 degrees or withdrawn. The resulting measurement gives a value from which flux density per unit volume can be determined. This essentially indicates how magnetically “strong” the overall product is.

Figure 7. Helmholtz coils

In the Helmholtz coil configuration, a fluxmeter is also capable of providing more insight into the characteristics of the magnetic product by determining parameters like permeability and permeance.

Features to look for

Due to the fluxmeter’s ability to measure different parameters and units, user-friendly software is a particularly desirable feature. Look for software that includes: � Conversion between SI and cgs: Both cgs and SI units are used

extensively by engineers throughout the world, but converting values between the two is quite complex. Look for an instrument with a user interface that converts these values for you.

� Automatic recognition of coil constants: Calibration data for the simple coil sensor is normally: total resistance, number of turns, and effective internal area. Helmholtz coils and potential coils have an additional constant that needs to be entered. Modern systems have memory components in the coil connectors that are preloaded with calibration data, which is automatically transferred to the fluxmeter.

� Automatic range conversion: The full-scale range is difficult to determine for many users, as it changes with coil data. Automatic software conversion of coil parameters to the proper range for a selected unit of measure is very convenient.

� Effective zero-drift compensation: Drifting integrators can affect reading accuracy. When manually testing over a long period, easy-to-use software with long lasting and adaptive drift compensation will be especially helpful.

� Peak pulse capture: For speed of testing, many functions are measured by rapid peak field measurements (the item under test moves rapidly past or through the search coil). Accurate, high-speed response and rapid, stable reset are mandatory.

� Computer interface: Most modern industrial test instruments are interfaced with a network or system through GPIB, USB, RS-232, or Ethernet.

� Accuracy: Due to the complexity of the manufacturer’s specifications, there are several factors that affect overall accuracy. The operator should understand the total accuracy of the measurement performed.

Interested in learning more? Read the application note, “Measuring Permanent Magnet Characteristics

with a Fluxmeter and Helmholtz Coil.”

How to evaluate product specs with a critical eye: Read the article.

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Teslameters/Hall sensors Fluxmeters/coilsSensing technology Hall effect sensor/probe — Hall voltage

proportional to perpendicular field.Search coil or Helmholtz coils — change in flux induces a voltage in the coil.

Strengths � Measuring field at a specific location

� Making fast measurements that are easily repeatable

� Generally easier to operate

� Generally lower-priced technology

� Characterizing overall magnetization using a Helmholtz coil

� Advanced users capable of winding and characterizing their own coils can create custom coils capable of measuring field inside a magnet

Positioning of probes/coils Probes must be positioned close to the magnet for highest field values. Single-axis probes should be orthogonal to field. 3-axis probes require no special orientation.

Field measurement occurs at the location of the Hall sensor active area.

Helmholtz coils require 180-degree rotation or withdrawal of item. Search coils initial orientation should be orthogonal to field as field measurement occurs inside the space contained by the coils.

Magnet quality control assessments Measure field strengths at specified positions around the item. Fixturing is recommended to ensure repeatable placement.

Measure overall flux/flux density to ensure proper magnetization or construction using Helmholtz coil. Fixturing simplifies the movement of the item through the coils, but is not strictly needed. Caution: This measurement gives overall magnetization as a magnitude value. It may not detect whether magnetization occurred in the correct direction.

Field vector measurement Inherently provide vector measurements. 3-axis probes are ideal, though it is possible to use a single-axis probe and hunt for a maximum field reading.

Not recommended for vector measurements.

AC and DC field measurement at a location Inherently support both AC and DC measurements, though AC measurements become more difficult with increasing frequency due to inductive pickup in the sensor wiring (an unintended error component that is not part of the Hall voltage signal).

DC measurements are possible but tedious. Users must position the probe at the measurement location, reset the fluxmeter, then move the probe out of the field.

AC measurements are a strength of the fluxmeter with a search coil given that the measurement relies on inductive coupling, which increases with frequency.

Field mapping Well-suited to the teslameter. 3-axis probes provide fast measurements.

Not well-suited to a fluxmeter.

Measurement in free space vs. internal Only measures in free space. Measures both. Use search coils for free space. Use Helmholtz or custom search coils for internal measurements.

Pick the right magnetic measurement instrument for the job There may be clear reasons to select one type of meter over the other, depending on what you need to measure. Or, perhaps you will need one of each. The following table provides a summary comparison of teslameters and fluxmeters.