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What are Magnetometers?

Magnetometers are scientific instruments that measure the strength and/or direction of a magnetic field. Typically, magnetometers measure a magnetic field or flux density in metric units of gauss (G) or the international system (IS) unit tesla (T).

Magnetometers and Gaussmeters are sometimes interchangeable names describing the same magnetic field instrument used to sense or measure a magnetic field. The two can be differentiated, however, based on their level of sensitivity. Gaussmeters are considered high strength measuring devices for fields above 1 milliTesla (mT), while magnetometers are used for high sensitivity applications below 1 mT.

Magnetism and Magnetometers

Magnetism describes how, on a microscopic level, materials respond to an applied magnetic field. These magnetic fields surround magnetic materials and are detected by the force they exert on other magnetic materials and moving electric charges. Types of magnetism include ferromagnetism, ferrimagnetism, antiferromagnetism, paramagnetism, and diamagnetism.

Because magnetism varies by location and with differences in the Earth's magnetic field or magnetosphere, magnetometers are used to provide orientation, navigation, and leveling information. For example, combination compasses and magnetometers are used on satellites and aboard airplanes that map the local structure of the Earth's magnetic field. Navies use compasses and magnetometers to detect submarines under water. Surveyors can use these magnetometers to locate boundary stakes buried in the ground or hidden by vegetation.

Selection

When selecting magnetometers, buyers should consider the specific type of magnetometer technology, its form, its outputs and interfaces, and its design specifications.

Type

There are two basic types of magnetometers: scalar and vector. Scalar magnetometers are used to measure the total strength of a magnetic field. Vector magnetometers are used to measure a component of a magnetic field in a particular direction. Both types of scientific instruments incorporate a number of technologies.

Scalar

Scalar magnetometers include Overhauser, optically-pumped, and proton precession devices.

  • Overhauser magnetometers incorporate an electron-rich liquid combined with hydrogen and subjected to a radio frequency (RF) signal.In the presence of this signal, the unbound electrons in the liquid transfer to the protons of the hydrogen nuclei; the resultant energy transfer polarizes the liquid.The precession frequency is linear with the magnetic flux density and thus can be used to measure it.
  • Hall Effect magnetometers are devices that convert the energy stored in a magnetic field to an electrical signal by means of proportional voltage in a current carrying conductor. These devices also sense polarity.
  • Proton precession magnetometers or proton magnetometers use nuclear magnetic resonance (NMR) to measure the resonance frequency of protons in the magnetic field. This incorporates liquids with high densities of hydrogen atoms such as kerosene and methanol.A polarizing DC current is passed through a coil surrounding the sample, creating a high magnetic flux.When the polarizing flux is released, the frequency of the precession of the protons to normal realignment can be used to measure the ambient magnetic field.

Vector

Vector magnetometers use orthogonal vector technology to determine magnetic field strength as well as inclination and declination. Device categories include flux gates, Magnetoinductive devices, superconducting quantum interference devices (SQUIDs), atomic spin exchange relaxation-free (SERF) magnetometers, and Magnetoresistive devices.

  • Flux gate instruments perform a continuous measurement of the differences in a magnetic field at the ends of a vertical rod and plots these on a grid of the area.They are widely used in magnetic compass-based navigation applications. They consist of a small, magnetically-susceptible core with two coils of wire. They may be arranged in a gradiometer configuration.
  • SQUID consists of two superconductors separated by thin insulating layers to form two parallel Josephson junctions. They are very sensitive to low magnetic fields and can measure fields as low as the femtoTesla range. They have a very wide range of magnetic field measurement and are used in medical, particularly neuroscientific, applications.
  • 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.
  • Magneto-inductive sensors incorporate a coil surrounding a ferromagnetic core whose permeability changes within the earth's magnetic field.The coil serves as the inductive element in an oscillator, whose frequency is proportional to the magnetic field being measured.A DC current is provided to set the oscillator into its linear region. Directional changes cause a shift in the oscillator frequency, enabling the magnetometer to provide directional and navigational feedback.
  • Magnetoresistive magnetometers are semiconductor devices in which the electrical resistance is a function of the applied or ambient magnetic field.

Form

Form can be an important factor in selecting a magnetometer, as it determines where a device can operate. Handheld devices are those that are portable or could be carried or transported for field work. Desktop or module devices are standalone modules normally used in a fixed location.

Outputs and Interfaces

Outputs and interfaces determine how a magnetometer displays and transfers its information. Outputs include analog current, analog voltage, analog frequency, or digital. Interfaces include parallel, serial, switch, or USB.

Design Specifications

Magnetometers include a large array of design specifications which can be important to finding a device suitable for a given application.

  • Sensing accuracy defines the reading accuracy that is required of the magnetometer.
  • Resolution is the smallest increment of measurement possible with the device. A higher resolution denotes a device that can take more sensitive readings or can measure on a smaller scale.
  • Bandwidth is the frequency range over which the device meets its accuracy specifications. Accuracy is degraded at lower and lower frequencies unless the device is capable of dc response, and at higher frequencies near resonance and beyond, where its output response rolls off. Frequencies in the database are usually the 3dB roll-off frequencies.
  • 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. Magnetic flux density is also known as magnetic field. It is measured in units of Gauss or Tesla (1 T = 1000 G).
  • Operating temperature indicates the temperature range over which the device must operate.

References

NIST - Magnetometer Principles