Fluxmeters Magnetic Field -- FIELD MEASURING FLUXMETERS -- instrumentation and Characterization

Fluxmeters

The magnitude of the magnetic field H created in a certain region can be also determined by measuring the magnetic flux change through a moving coil (usually called ‘searching coil’ or ‘search coil sensor’). With such a purpose a Fluxmeter can be used. The fluxmeter is an electronic voltage integrator that integrates any voltage signal applied to its input terminals. So, in conjunction with a search coil can be used to measure the magnetic field, but this is a versatile instrument that can be used as fluxmeter, gaussmeter or voltage integrator as it is described in the following. Fluxmeters are therefore general purpose laboratory instruments used in both, research and industry.

Fluxmeter – measuring of magnetic fields using a moving coil. Lets consider a plane coil, made by winding fine insulated copper wire, with its plane perpendicular to a certain uniform magnetic field H. If the area of the coil is A, the magnetic flux change through it is HA. If the coil is suddenly moved from the region where H is to be measured to a region where the field is known to be essentially zero the magnetic flux change will be HA. By Faraday’s law the voltage induced by the change in flux (or emf) produces a current if the circuit is closed.

The sensing or search coils used by a fluxmeter are easily constructed and must be designed for each application. These coils are usually very small and are fixed to the end of a thin rod connected by a light cable to the fluxmeter. They are constructed in two basic geometries: axial or transverse (Fig. 1). Transverse search coils are useful determine the magnetic flux when the magnetic field is perpendicular to the probe plane. Axial search coils determine the magnetic flux parallel to axis of the probe plane. Because of the potential small size of the coil one can measure the magnetic field strength or field gradient in a certain confined region.
		Fig. 1 Search coil configurations used with fluxmeters: transverse (top) axial (bottom).

Fluxmeter – principle of operation. The heart of any electronic fluxmeter is an operational amplifier. It uses a precision low drift amplifier with a high quality capacitor that integrates the voltage applied to the input terminals. The basic circuit of the electronic integrator is shown in Fig. 2. Is contains an input resistance, Ri, feedback capacitor, Cf, and an operational amplifier. The relationship between the input and output voltages, ei and eo, respectively is eo = 1/RC integral[ei dt]. By considering Faraday’s law the output voltage eo can be qritten as proportional to BAN and inversely proportional to RC, where B is the magnetic induction, A the area of the coil, N the number of turns of the coil, C the capacity and R the input resistance (Ri in the Figure).
Fig. 3 shows a typical commercial fluxmeter. The fluxmeter can be calibrated in Maxwell's, Gauss or Volts-seconds. Output terminals of a fluxmeter provide an analogical or digital signal that can be traced in a XY recorded or a computer.


Fig. 2 Block diagram of an electronic integrator.	Fig. 3 Typical commercial fluxmeter.

Important technical features to select a fluxmeter.

When choosing a fluxmeter the main technical features that costumers must take into account are the following: (a) sensitivity; (b) accuracy; (c) resolution and number of digits of the display; (d) range of output signal; (e) power supply, weight and dimensions (portable or bank instrument ?).

Fluxmeters – applications.

Fluxmeters are used to analyze magnetic circuits and for measuring flux in air gaps and magnetic materials sections. They provide accuracies better than 1% in the reading of AC and DC flux. So, fluxmeters constitute a building block for AC and DC hysteresigraphs or BH meter and permeameters.

The main applications of fluxmeters can be summarized as follows: measuring of magnetic fields, ferromagnetic detectors, instrumentation systems, hysteresis loop tracers, voltage integrator, production test systems, and stray flux detectors.