Magnetic field instruments are used to measure the magnetic field or flux around permanent magnets, coils, and electrical devices. They use several types of sensing technologies. Magnetoresistive instruments measure electrical resistance as a function of the applied or ambient magnetic field. 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. 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. Magnetoinductive devices consist of a coil that surrounds a ferromagnetic core whose permeability changes within the earth's magnetic field. Proton precession devices use liquids such as kerosene and methanol that have high densities of hydrogen atoms. Overhauser or nuclear precession devices combine an electron-rich liquid with hydrogen and then subject the mixture to a radio frequency (RF) signal. Optically-pumped instruments polarize a gaseous alkali with a specific wavelength of light. Superconducting quantum interference devices (SQUID) are very sensitive to low magnetic fields.
Selecting magnetic field instruments requires an analysis of performance specifications. Flux density, the total measurement range in gauss (G), often corresponds to the linear output region of the sensing technology. Resolution, another important specification, is the smallest measurable increment. Accuracy is a percentage of full-scale. Bandwidth is the frequency range over which magnetic field instruments meet their accuracy specifications. The number of axes indicates the number of possible, simultaneous measurements for magnetic field instruments. Single-axis devices are common, but two-axis and three-axis devices are also available. Directional magnetic field instruments measure the direction of the magnetic field and may provide bipolar outputs. Magnitude instruments measure the size of the magnetic field.
Magnetic field instruments are used to measure the magnetic field or flux around permanent magnets, coils, and electrical devices. They use several types of sensing technologies. Magnetoresistive instruments measure electrical resistance as a function of the applied or ambient magnetic field. 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. 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. Magnetoinductive devices consist of a coil that surrounds a ferromagnetic core whose permeability changes within the earth's magnetic field. Proton precession devices use liquids such as kerosene and methanol that have high densities of hydrogen atoms. Overhauser or nuclear precession devices combine an electron-rich liquid with hydrogen and then subject the mixture to a radio frequency (RF) signal. Optically-pumped instruments polarize a gaseous alkali with a specific wavelength of light. Superconducting quantum interference devices (SQUID) are very sensitive to low magnetic fields.
Selecting magnetic field instruments requires an analysis of performance specifications. Flux density, the total measurement range in gauss (G), often corresponds to the linear output region of the sensing technology. Resolution, another important specification, is the smallest measurable increment. Accuracy is a percentage of full-scale. Bandwidth is the frequency range over which magnetic field instruments meet their accuracy specifications. The number of axes indicates the number of possible, simultaneous measurements for magnetic field instruments. Single-axis devices are common, but two-axis and three-axis devices are also available. Directional magnetic field instruments measure the direction of the magnetic field and may provide bipolar outputs. Magnitude instruments measure the size of the magnetic field.
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.
