HHS 5202-9

Description:

Helmholtz-Coils are especially designed to generate precisely defined magnetic fields from DC to the upper end of the audio frequency range and beyond. The generated fields are in a strongly linear relation to the coil current. The fieldstrength can be calculated exactly by analytical (or numerical) methods, based on the coils' geometry, the number of turns and the coil current. Therefore the HHS 5202-9 is ideally suited for the calibration of magnetic field probes or sensors. Due to the high temperature proof Teflon-coated wire packet it is possible to generate magnetic fields up to approx. 3000 A/m (for a short time). Typical applications are magnetic immunity testing according to automotive standards or MIL-461F. When generating magnetic fields with Helmholtz coils the coil current is directly proportional to the magnetic field strength.

The calibration of the magnetic field is finally traceable to a current measurement (or to a voltage drop at a known resistor).

The Helmholtz Coil itself is usually considered as primary standard due to the easily calculable relation between current and field strength. If this relation should be controlled, a loop sensor or monitoring loop can be used to determine the actual field strength.

Installation:

The Helmholtz-Coils should be installed on a desk in a sufficiently large separation from sources of unintentional magnetic fields, e.g. transformers in power supplies, conductors carrying high currents, computer monitors, loudspeakers, cathode ray tubes (CRT) and so on. All kinds of magnetic material (e.g. steel, nickel, cobalt) should be removed from the near surrounding of the coil. The wires which are used to connect the current source with the Helmholtz-Coil should be twisted to avoid an unwanted injection of magnetic flux.

The coil terminals are assigned with the characters A, B, C and D. The generator (current source, audio-amplifier...) is connected to the terminals A and C, the terminals B and D are connected with the short cable supplied with the coil.

An additional verification can be done by measuring the magnetic field strength between the coils. Assuming a wrong connection, the field strength decays very sharply in the center between the coils, because the fields compensate each other.

Field strength determination:

There are two methods to determine the actual magnetic field strength:

1. Determination of the coil current

• Current transformer clamp
• Measuring the voltage drop across a well known resistor
• Direct current measurement

2. Determination of the field strength using a field monitoring loop

The direct current measurement has the disadvantage that the measurement equipment itself heats up, which leads to increased measurement uncertainty or even destruction. The use of a calibrated current transformer clamp has two advantages: it is floating (potential isolation between measuring circuitry and amplifier output circuitry) and without thermal stress. In cases where the voltage drop across a known resistor (e.g. 100 mW / 20 W up to 14 A or 10 mW for highest currents) is measured, it is essential to provide sufficient cooling and potential isolation of mains driven voltmeters. Using small shunt resistor values causes less heat dissipation may however cause higher measurement uncertainties because the wanted shunt resistance is hardly higher than unwanted contact resistances. Beyond several kHz the inductance of the shunt resistor may become dominant. For that reason a low inductive shunt resistor with well known impedance Z like the SHUNT 9571 should be used.

The determination of the magnetic field strength using a sensor loop (field monitoring loop) allows also potential isolation without temperature stress. The FESP 5133-7/41 is suitable for this purpose.

HHS 5202-81

Description:

Helmholtz-Coils are especially designed to generate precisely defined magnetic fields from DC to the upper end of the audio frequency range and beyond. The generated fields are in a strongly linear relation to the coil current. The field-strength can be calculated exactly by analytical (or numerical) methods, based on the coils' geometry, the number of turns and the coil current. Therefore, the HHS 5202-81 is ideally suited for the calibration of magnetic field probes or sensors. Due to the high temperature proof Teflon-coated wire packet it is possible to generate magnetic fields up to approx. 3000 A/m (for a short time). Typical applications are magnetic immunity testing according to automotive standards or MIL-461F. When generating magnetic fields with Helmholtz coils, the coil current is directly proportional to the magnetic field strength.

The calibration of the magnetic field is finally traceable to a current measurement (or to a voltage drop at a known resistor).

The Helmholtz Coil itself is usually considered as primary standard due to the easily calculable relation between current and field strength. If this relation should be controlled, a loop sensor or monitoring loop can be used to determine the actual field strength.

Installation:

The Helmholtz-Coils should be installed on a desk in a sufficiently large separation from sources of unintentional magnetic fields, e.g. transformers in power supplies, conductors carrying high currents, computer monitors, loudspeakers, cathode ray tubes (CRT) and so on. All kinds of magnetic material (e.g. steel, nickel, cobalt) should be removed from the near surrounding of the coil. The wires which are used to connect the current source with the Helmholtz-Coil should be twisted to avoid an unwanted injection of magnetic flux.

The coil terminals are assigned with the characters A, B, C and D. The generator (current source, audio-amplifier...) is connected to the terminals A and C, the terminals B and D are connected with the short cable supplied with the coil.

  An additional verification can be done by measuring the magnetic fieldstrength between the coils. Assuming a wrong connection, the fieldstrength decays very sharply in the center between the coils, because the fields compensate each other.

Field strength determination:

There are two methods to determine the actual magnetic field strength:

1. Determination of the coil current

• Current transformer clamp

• Measuring the voltage drop across a well known resistor

• Direct current measurement

2. Determination of the field strength using a field monitoring loop

The direct current measurement has the disadvantage that the measurement equipment itself heats up, which leads to increased measurement uncertainty or even destruction. The use of a calibrated current transformer clamp has two advantages: it is floating (potential isolation between measuring circuitry and amplifier output circuitry) and without thermal stress. In cases where the voltage drop across a known resistor (e.g. 100 mW / 20 W up to 14 A or 10 mW for highest currents) is measured, it is essential to provide sufficient cooling and potential isolation of mains driven voltmeters. Using small shunt resistor values causes less heat dissipation may however cause higher measurement uncertainties because the wanted shunt resistance is hardly higher than unwanted contact resistances. Beyond several kHz the inductance of the shunt resistor may become dominant. For that reason a low inductive shunt resistor with well known impedance Z like the SHUNT 9571 should be used.

The determination of the magnetic field strength using a sensor loop (field monitoring loop) allows also potential isolation without temperature stress. The FESP 5133-7/41 is suitable for this purpose.

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