Calibration of Axial DC Magnetic Field up to 1 Tesla at SCL D.W.K. Lee*, B.H.S. Lam† and Y.C. Chau#

*†#The Government of the Hong Kong Special Administrative Region Standards and Calibration Laboratory (SCL) *wklee@itc.gov.hk †hslam@itc.gov.hk

Abstract — This paper presents the method used at

SCL to calibrate magnetic field measurement instrument (i.e. teslameter or gaussmeter) for DC magnetic field in axial direction, 50 mT to 1 T, with expanded measurement uncertainty within 0.1 %. The equipment used is basically an electromagnet, which has modified and suitably characterized for such application.

Index Terms — DC magnetic field, axial direction, expanded measurement uncertainty.

I. INTRODUCTION

Measurement of DC magnetic field, for both transverse and axial direction, is usually performed using a Helmholtz Coil for generation of test field up to about 50 millitesla (mT), or below, due to physical size of the coil. Electromagnet can be used for generation of magnetic field at much higher levels. However, because of its structural limitation, electromagnet is restricted for measurement of transverse magnetic field only. SCL has developed an automatic system to calibrate gaussmeter for DC magnetic field in axial direction based on a modified electromagnet system, with measurement range from 50 mT to 1 T and expanded uncertainty within 0.1%.

Fig. 1. Calibration of DC magnetic field in axial direction

II. THE MODIFIED ELECTROMAGNET

At SCL, the electromagnet is modified that, at one of the magnet poles, there is an opening at the yoke’s centre. The opening is of size and shape suitable for positioning the measuring probe of the unit under test (UUT), such that the generated magnetic field can be measured by the UUT in axial direction. Also, a filler rod, of the same material as the magnet yoke, is fabricated by precision engineering for size and shape well fitted with the said opening. The system is so designed that when the filler rod is plugged into the magnet yoke

opening, the modified electromagnet is still suitable for UUT calibration in transverse direction with no excessive adverse effects.

Fig. 2A. The Electromagnet Fig. 2B. The Modified Yoke

III. CHARACTERIZATION OF THE MODIFIED ELECTROMAGNET

It is anticipated that the opening at the magnet yoke will affect the strength and uniformity of the field generated. Hence, comprehensive studies are necessary to reveal the anomalies so that errors and uncertainties are appropriately dealt with.

Figure 3 shows that, at the centre of the magnet yoke, the generated field is weakened and field weakening is becoming more serious towards the hole. Hence, in order to avoid the areas of seriously weakened fields, calibration of UUT should be performed in a position as far away from the opened yoke as possible. It is observed that field weakening follows a regular and possibly predictable pattern. Experimental results revealed that the generated field’s strength and uniformity can be optimized with the pole gap.

pole diameter = 150 mm pole gap = 20 mm z-axis along the pole axis

Along the z-axis, higher value for z indicates that the position is nearer the yoke with a hole in it.

Fig. 3. Magnetic Field Plot (along x-axis with no filler rod)

Electromagnet System(GMW 3473-70)

Pole gap separation around 30 mm

Gaussmeter Under Test

Reference Gaussmeter

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Measuring Probe

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312978-1-4673-0442-9/12/$31.00 ©2012 IEEE

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IV. CALIBRATION OF UUTS

During calibration, measuring probes of the reference and UUT meters are perpendicular to each other, sensing the magnetic field in transverse and axial directions respectively.

Fig. 4. Calibration of UUT against the reference

A. Consideration of Uncertainty due to Field Non-Uniformity

With pole gap at 30 mm, the generated field at 1.0 T, in axial direction, and uniformity within 500 × 10-6 in a spherical space of 4 mm radius can be achieved.

If the reference and UUT probes are within a close proximity, not larger than 4 mm, readings of the reference and UUT meters can be compared directly, with uncertainty due to field non-uniformity within 500 × 10-6.

pole diameter = 150 mm pole gap = 25 mm and 30 mm z-axis along the pole axis

Along the z-axis, higher value for z indicates that the position is nearer the yoke with a hole in it.

Fig. 5. Magnetic Field Plot (along z-axis with no filler rod)

B. Other Considerations

At SCL, magnetic measurement is traceable to a nuclear magnetic resonance teslameter, which is calibrated by the Laboratory’s frequency standard and in turn traceable to SCL’s Caesium Beam Frequency Standard. The calibration process is automated so that operator errors are minimized and repeatable results, with realistic uncertainty estimates, are achieved.

Fig. 6. Flow chart for measuring the UUT meter

Apart from field non-uniformity, other major uncertainty contributions due to the calibration set up, UUT performance and environmental effects are dealt with in accordance with the GUM. Also, to avoid measurement uncertainty due to effects of temperature, humidity, noise etc, an area is dedicated to this measurement and the electromagnet’s water cooling system is housed separately.

Fig. 7. General Layout- SCL Magnetic Measurement System

IV. CONCLUSION

A novel electromagnet system was developed at SCL to calibrate magnetic field measurement instruments for DC magnetic flux density in axial direction, ranging from 50 mT to 1.0 T, with expanded measurement uncertainty within 0.1%.

REFERENCES

[1] “Evaluation of measurement data - Guide to the expression of uncertainty in measurement,” JCGM 100:2008.

[2] “GMW 3473-70 Electromagnet User’s Manual,” July 2009.

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Axial Probe Under Test

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Start

Operator input the test points to be measured

Initialize the equipment

Ramp up the magnet power supply to energize the electromagnet.

(1) Simultaneously taking the readings on the reference and UUT meters.

(2) Determine the correction value of the UUT meter(3) Record the results into the data file.

All test points finished?

Ramp down the output of the magnet power supply

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Fine tune the supply’s output current to set the electromagnet to the target field.

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