Magnetic Field Instruments Information
Magnetic field instruments are devices used to measure the magnetic field or flux around permanent magnets, coils, and electrical devices. They include meters, gauges, sensors, recorders, and other instrumentation. Selecting a specific magnetic field instrument depends upon the type of device needed, the technology it implements, its form, its outputs and interfaces, and various performance specifications.
Magnetic field instruments include magnetometers and Gaussmeters (Tesla meters). Magnetometers and Gaussmeters are sometimes used interchangeably for describing devices used to measure magnetic field strength. However, the two can be differentiated based on the type of field strength they measure. Gaussmeters are considered devices for high field strengths while magnetometers are used for low field strengths. Specifically, Gaussmeters are said to take magnetic field measurements above 1 mT (milliTesla), while devices measuring fields below this value are considered magnetometers.
Magnetic field instruments include several types of sensing technologies. Depending on the range of sensitivities the devices can be designed for, they can be considered Gaussmeters, magnetometers, or both.
Hall Effect devices convert the energy stored in a magnetic field to an electrical signal by developing a voltage between the two edges of a current-carrying conductor whose faces are perpendicular to a magnetic field.
Magnetodiodes are two-terminal Hall effect devices similar to a conventional bipolar diode. The voltage-current characteristic of a magnetodiode is sensitive to a magnetic field.
Magnetotransistors consist of a bipolar transistor implemented on a semiconductor surface. They are three-pronged devices consisting of an emitter region, an elongated base region, and a collector region. The presence of a magnetic field in the base region creates a Hall effect voltage which produces a pulse on the transmission line.
Magnetometers are magnetic field instruments for high-sensitivity applications detecting low-strength fields. They can be classified as vector or scalar devices based on their ability to sense field direction in addition to field strength.
Scalar magnetometers measure magnitude only.
Proton precession devices use liquids such as kerosene and methanol that have high densities of hydrogen atoms.
Optically-pumped instruments polarize a gaseous alkali with a specific wavelength of light. An RF signal is modulated to determine its optimum depolarization frequency - this depolarization frequency varies with the ambient magnetic field.
Overhauser or nuclear precession devices combine an electron-rich liquid with hydrogen and then subject the mixture to a radio frequency (RF) signal.
Vector magnetometers measure both magnitude and direction.
SQUIDs or superconducting quantum interference devices consists of two superconductors separated by thin insulating layers to form two parallel Josephson junctions. They are most commonly used to measure the magnetic fields produced by brain or heart activity.
Atomic SERF magnetometers achieve very high magnetic field sensitivity by monitoring a high density vapor of alkali metal atoms precessing in a near-zero magnetic field. They are among the most sensitive magnetic field sensors available.
Flux gate or coil instruments measure differences in the magnetic field at the ends of a vertical rod and plot this information on a grid.
Magnetoinductive devices consist of a coil that surrounds a ferromagnetic core whose permeability changes within the earth's magnetic field.
Magnetoresistive instruments measure electrical resistance as a function of the applied or ambient magnetic field. They can be built as magnetometers for more sensitive applications, or as Gaussmeters for stronger magnetic fields.
Magnetic field instruments can either be in handheld or mounted form. For field applications and those requiring portability, handheld form may be necessary. Mounted forms are usually bigger devices incorporated into a larger transportable unit or vehicle, or are used in fixed lab or building environments.
Outputs and Interfaces
It is important for a magnetic field instrument to have outputs and interfaces that are usable for the operator and compatible with other incorporated systems. Magnetic field instruments differ in terms of electrical outputs. Analog current levels such as 4 - 20 mA are suitable for sending signals over long distances. Analog voltages are simple, usually linear functions. Modulated analog output signals are encoded, but still analog in nature. Examples include sine wave, pulse wave, amplitude modulation (AM), and frequency modulation (FM) signals. Several digital outputs are available. RS232, RS422, and RS485 are common serial, digital protocols. Popular parallel protocols include the general-purpose interface bus (GPIB), a standard which is also known as IEEE 488. Other digital outputs for magnetic field instruments include transistor-transistor logic (TTL) signals. Outputs that change the state of a switch or alarm are also available.
Magnetic field instruments can be selected based on a number of different specifications related to device performance.
- Flux density measurement is the range through which the sensor or instrument is designed to measure, often corresponding to the linear output region of the sensing technology.
- Sensing accuracy is the required measuring accuracy of the device.
- Resolution is the smallest increment of measurement possible with the device. An instrument with higher resolution can make smaller measurements.
- Bandwidth is the frequency range over which the device meets its accuracy specifications. Accuracy degrades with lower frequencies unless the device is capable of dc response. Accuracy also degrades near and above resonance frequencies, where its output response rolls off.
- Operating temperature is the temperature range over which the device must operate
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