Tesla meters (gaussmeters), based on the Hall effect principle, have wide applications in magnetic field measurement. These instruments consist of two parts: a Hall probe (as a sensor) and the main instrument unit. The size, performance, and packaging structure of the Hall element within the probe play a crucial role in the accuracy of the magnetic field measurement.
The accuracy of Hall effect teslameters for measuring uniform, constant magnetic fields is generally between 0.5% and 5%, with high-precision measurements reaching 0.05%. However, accuracy is questionable when measuring non-uniform magnetic fields on magnet surfaces. Often, different instruments, or instruments of the same model but with different probes, or even different sides of the same probe, will produce vastly different results when measuring the magnetic field at the same location (which should appear to be the same) on the same magnet surface. Errors can exceed 20%, or even reach 50%.
There are two reasons for the above differences: First, the Hall element is packaged in different positions within different probes, or the element is not located in the middle of the sides of the probe. In a uniform magnetic field, these probes will not sense changes in the magnetic field due to changes in position, and the measurement data will not be affected by positional errors. When different probes are used to measure the divergent, non-uniform magnetic field on the surface of a magnet, although the surface appears to be placed in the same position, the internal Hall element senses a different magnetic field. Different sensed field values lead to different measurement results. Generally, for radial probes, the smaller the thickness, the closer the internal Hall element is to the surface, and the larger the reading of the surface magnetic field measurement. Readings using ultra-thin probes to measure the surface magnetic field can be more than 20% higher than those using conventional probes (the smaller the size of the magnet being measured, the greater the curvature of the magnet surface, and the more non-uniform the surface magnetic field distribution, the greater the difference in measurement data). However, no matter how thin the probe is, there is always a gap between the magnetic field-sensitive part inside and the magnet surface; it cannot be zero distance. Therefore, it is impossible to measure the true surface magnetic field. It can only be said that the thinner the probe used, the more accurately the reading reflects the surface magnetic field of the magnet.
The second reason is that the sensitive area of the Hall element encapsulated within different models of Hall probes varies in size. Early bulk Hall elements, such as germanium and silicon Hall elements, typically had dimensions of 4×2 mm², but some were 6×3 mm², 8×4 mm², and as small as 1.5×1.5 mm². The effective sensitive area was essentially the size of the element itself, resulting in a large area. If such a Hall element were used to measure the magnetic field of a small magnet with diverging magnetic field lines, the corner of a magnet, or a multi-pole magnetized surface, it would only reflect the average value of the magnetic flux density passing through the element's surface. This value would inevitably be less than the maximum value in that area. If a smaller Hall element with a smaller sensitive area, such as a gallium arsenide Hall element, were used, its effective sensitive area would be approximately 0.1×0.1 to 0.2×0.2 mm², far smaller than the area of a bulk element. This type of element would better reflect the field distribution of the surface magnetic field, and the measured maximum value would be closer to the actual maximum magnetic flux density in that area.
As the preceding analysis shows, the actual value (i.e., the true value) of the surface magnetic field cannot be measured using the Hall effect method. In other words, it is impossible to find or establish a unified, common standard for measuring the surface magnetic field. The only option is to seek methods that can more closely approximate the actual value of the surface magnetic field.
A Discussion on Measuring the Magnetic Field of a Magnet Surface Using the Hall Effect Method -- How to Select a Probe for a Gaussmeter
I. Permanent Magnet Measuring Instruments
Permanent magnet measuring instruments are specialized instruments used to measure the magnetic properties of various permanent magnet materials. Commonly used instruments include: gaussmeters (Tesla meters), BH hysteresis loop meters, and fluxmeters.
1. Gauss meter (Tesla meter): Used to measure the surface magnetic field strength and air gap magnetic field strength of various permanent magnets.
2. Magnetometer: Used to measure the induced magnetic flux of a permanent magnet.
3. BH Hysteresis Loop Meter: Used to measure magnetic properties parameters such as Br, Hcb, Hcj, and BHmax of permanent magnet materials, and can automatically plot hysteresis loops and demagnetization curves.
II. Gaussian Meter (Tesla Meter)
1. Types of Gaussmeters (Tesla Meters): Analog, Digital, Portable.
2. Applications of Gaussmeters (Teslameters)
(a) Measurement of the surface magnetic field of permanent magnets: The magnetic field strength of the surface of permanent magnet products is measured using a gaussmeter (Tesla meter). This is mainly to evaluate the quality of permanent magnet products and the consistency of their magnetic properties after magnetization. Typically, the magnetic field strength at the center point of the magnet surface is measured. By comparing the data with standard sample data, it is possible to determine whether the product is qualified, and at the same time, the consistency of the materials can be guaranteed.
(b) Measurement of air gap magnetic field: The use of a gaussmeter (teslameter) to measure air magnetic field is widely applied in scientific research, electronics manufacturing, machinery and other fields. Currently, the most typical industries for application are motors and electroacoustics.
(c) Residual magnetism measurement: such as detecting the demagnetization effect after demagnetizing the workpiece.
(d) Magnetic leakage measurement: such as magnetic leakage measurement of a horn.
(e) Measurement of ambient magnetic field
3. Selection of Gaussmeter (Tesla Meter): The selection of a gaussmeter (Tesla meter) should begin with the object being measured, considering the following aspects:
a. Magnetic field type: Magnetic fields are divided into two types: DC magnetic fields and AC magnetic fields. The magnetic field strength of permanent magnet materials should be measured using a DC gaussmeter.
b. Instrument range: Determine the approximate magnetic field range of the object being measured, and select an instrument whose range is greater than the magnetic field being measured;
c. Measurement accuracy: refers to the instrument's resolution, such as 1 Gs or 0.1 Gs, etc.
d. Probe selection: Instrument manufacturers usually offer a variety of test probes in different specifications to meet various testing requirements. When measuring surface magnetic field strength, probe specifications are usually not a concern.
① Air gap magnetic field measurement: The size of the probe should be considered. If the probe size is larger than the air gap to be measured, it will not be able to enter the air gap and therefore cannot be used.
② Probe orientation selection: Probe orientation is divided into two types: transverse and axial. Users should select a suitable probe based on the object being measured.
③ Probe connection cable: The length of the probe cable is usually fixed by the instrument manufacturer. If there are special measurement requirements that require extending or shortening the probe cable, this should be requested from the manufacturer. Gaussmeter
e. Power supply: Desktop gaussmeters are usually powered by AC 220V, while portable gaussmeters are powered by batteries.
f. Function Selection
①Standard functions: polarity determination, maximum value locking, etc.;
② Portability: For outdoor operation or on-site measurement, a portable handheld gaussmeter can be selected. These instruments are small, lightweight, and battery-powered.
③ Rapid measurement on the production line: The instrument has upper and lower limit settings and alarm functions;
④ AC magnetic field measurement: Used to measure the magnitude of low-frequency (1-400Hz) alternating magnetic field strength;
III. Magnetometer
A fluxmeter typically measures the magnetic flux through the probe coil directly. It is often used in conjunction with a Helmholtz coil. This method is mostly used to compare with a standard sample in order to determine the product's conformity.
Before using the fluxmeter, it must be preheated as required. During use, the integration drift must be adjusted to ensure that the drift is within the specified range. Before each measurement, the meter must be reset to zero to release any residual charge or drift charge from the integrating capacitor.
When the magnetic circuit of a magnet is closed, a fluxmeter can be used to measure and calculate the remanence. The specific calculation method is: Br=Φ/N/S where: Φ-- magnetic flux; N-- number of coil turns; S-- cross-sectional area of the magnet.
When using a fluxmeter to inspect the conformity of a product, the relative positions of the sample being tested and the coil must be the same as those of the standard sample and the coil. If the product has strict requirements for its performance range, samples with the upper and lower performance limits should be kept for calibration and inspection.
IV. Permanent Magnet B-H Hysteresis Loop Measuring Instrument
The permanent magnet BH hysteresis loop measuring instrument can measure the hysteresis loop and demagnetization curve of permanent magnet materials, and accurately measure magnetic characteristic parameters such as remanence Br, coercivity HcB, intrinsic coercivity HcJ, and maximum energy product (BH)max.
With the rapid development and application of computer system integration technology, magnetic measurement systems based on computer operating platforms have also emerged.
V. Magnetizing
When magnetizing a magnet whose length is close to that of the magnetizing coil, the vertical center of the magnet should coincide with the vertical center of the magnetizing coil. This ensures that the magnetic field strength at both ends of the magnet is equal, thus reducing the symmetry of magnetization and minimizing the impact of magnetization methods on the equal magnetic field strength at both ends of the magnet.
Theoretically, the magnetic field strength at both ends of the magnetizing coil is half that at the center of the coil. When the magnet is close to the length of the magnetizing coil, it may not be possible to achieve saturation magnetization for products in the H and SH series and above. Even for products in the M and N series, saturation magnetization may not be possible if the magnetic field strength is not sufficiently high. Generally, the length of the magnet should ideally be less than 2/3 of the length of the magnetizing coil.
VI. Determining the direction of easy magnetization of a magnet
For square blocks and cylinders oriented perpendicular to the axis, there is a problem in identifying the orientation (direction of easy magnetization). This can be done using magnetized products or by borrowing instruments. The specific methods are as follows:
(1) Identification using magnetized products: For square blocks, due to the anisotropy of the material, the magnetic bars are arranged according to the orientation direction, so the orientation direction is easy to magnetize. After magnetization, the attraction between opposite poles is stronger, while the attraction between non-orientation directions is weaker. This is used to identify and determine the orientation direction. The magnet used for testing should be slightly larger, as it is difficult to distinguish the difference in attraction force when the magnet is too small. For cylinders with a vertical axial orientation, generally only magnetized magnets can be used for testing: Use a magnet to attract the surface of the cylinder, lift the cylinder, and the direction perpendicular to the ground is the orientation magnetization direction.
(2) Identification using a magnetometer: A magnet can be attracted to a square material. The direction of the magnet is perpendicular to the magnetic flux coil. The side with a relatively large magnetic flux value is the orientation side, and the direction perpendicular to this side is the orientation direction.
Gauss (G) is not an internationally accepted unit of magnetic flux density. It is named in honor of the German physicist and mathematician Gauss.
Single-unit conversion: 1T (Tesla) = 1000MT (millitalas) = 10000GS (gauss)
1MT = 10GS
What is the Hall effect?
A semiconductor wafer is placed in a magnetic field with magnetic induction intensity B, the direction of which is perpendicular to the wafer, as shown in the figure. When a current I flows through the wafer, an electromotive force EH is generated in the direction perpendicular to both the current and the magnetic field. This phenomenon is called the Hall effect, the electromotive force is called the Hall potential, and the semiconductor wafer is called a Hall element.
The principle is briefly described as follows: An excitation current I flows into the sheet from ends a and b, while a magnetic field B acts on the sheet from directly above. At this time, the electrons e move in the opposite direction to the current and are subjected to a Lorentz force FL, causing them to deflect inwards. Electrons accumulate on this side, thus generating an electric field E in the c and d directions of the sheet. The more electrons accumulate, the larger FE becomes. The electromotive force EH established between the c and d end faces of the semiconductor sheet is the Hall potential.
Experiments show that the larger the current I flowing into the excitation current terminal and the stronger the magnetic field B acting on the thin plate, the higher the Hall potential. When the magnetic field direction is opposite, the direction of the Hall potential also changes. Therefore, Hall sensors can be used to measure static or alternating magnetic fields.
