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MEASUREMENT


Analysis of static and time-variable magnetic fields


The new Narda HP-01 is an exciting piece of kit for measuring high electromagnetic fields. Andy Pye takes an early look.


❱ ❱ The Magnetometer is able to measure magnetic field strengths for both static and time-variable fields


from relatively low values (eg 0.5mT for effects on active implants) up to very large values of several Tesla. Typical areas where high fields are present are: • Magnetic resonance tomography (MRT)


• Magnetic resonance spectroscopy (MRS)


T


he Narda HP-01 Magnetometer is a device for measuring static magnetic fields and time-variable fields of frequencies up to


1000Hz. Its main application area is measuring high static fields with the aim of ensuring the safety of personnel who have access to areas where such fields are present. The possible risks are due to


indirect effects, such as the force exerted on ferromagnetic objects. As a consequence, hazards can result from flying objects (projectile effect), or from the force exerted on passive implants, or from functional impairments in active implants. These must be identified and avoided.


Risks can also occur due to direct


effects when the magnetic flux density is very high and movement of the body within the field results in stimulation of sensory cells or nerve cells. Exposure limit values are laid down in the ICNIRP Guidelines and in the European Directive 2013/35/EU among other regulations. The action levels depend on the source of the hazard, ranging


6 /// EMC Testing 2018 Vol 1 No. 1


• Use and production of strong permanent magnets such as lifting magnets


• DC generators, particularly superconducting


• DC motors, eg in railways • Applications involving electric vehicles • Large industrial magnetic stirrers • Electrolysis used in the production of chlorine and aluminium.


Particularly high static magnetic fields are generated by magnetic resonance tomography (MRT). Static magnetic fields are also present in DC powered railways. The exposure level limits and action


levels specified in guidelines and standards cover a wide range of values. This would usually mean that several different measurement probes would be needed in order to achieve the necessary measurement dynamic range. The HP- 01 easily covers all these applications with its built-in Hall effect sensors, which have an unusually wide overall dynamic range from 10 µT to more than 10 T. Additional measurement probes are not needed. The orthogonal arrangement of the sensors in the tip of the probe wand ensures that the three


spatial axes are captured completely, guaranteeing an isotropic (non- directional) measurement. The sensors are located approximately 7mm below the tip of the wand and are indicated by a clearly visible groove. The HP-01 also includes a very


powerful FFT analyser that allows signal analysis in both the time and the frequency domains. The fast signal processing allows detailed, highly accurate gap-free analysis of the frequency spectrum. The results can be displayed as a live


spectrum, as a spectrogram versus time, or as a three-dimensional waterfall diagram. The “DC – 1Hz” operating mode


is a further special feature that is particularly suitable for standard- compliant measurements according to human safety standards. The limit values are usually specified for static fields and variable fields with frequencies of up to 1Hz. A finite impulse response (FIR) filter used in the HP-01 captures just these frequency components. The measurement is made using a high sampling rate and delivers the corresponding wideband value for the 0 Hz to 1 Hz range, which is required in order to demonstrate compliance with the limit values. If required, more extensive analysis in the sub-Hertz region can be performed from the time domain display of the instantaneous values. This would not be possible in this way from FFT analysis. n


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