Calibration of Gaussmeters at Axial DC Magnetic Field Direction up to 1 Tesla at SCL

1Standards and Calibration Laboratory, SCL

Calibration of Gaussmeters at Axial DC Magnetic Field Direction up to 1 Tesla at SCL

Speaker: Chung Yin, PoonStandards and Calibration Laboratory (SCL)The Government of the Hong Kong Special

Administrative Region

2Standards and Calibration Laboratory, SCL

Measurement on DC magnetic flux density

• Measurement instruments– Gaussmeter with Hall Effect transverse / axial test

probe– Nuclear Magnetic Resonance (NMR) gaussmeter

• Sources of magnetic flux density

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Sources of Magnetic Flux Density

Helmholtz Coils

Electromagnet

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Calibration on Axial Direction

Test probe of the DUT(located at the z-axis of the Helmholtz coil)

Test probe of the Reference Meter (located at the x-axis of the Helmholtz coil)

B field direction

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Calibration on Axial Direction

B field direction

for axial measurement x

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Calibration Needs on Axial Field

• Toy Safety Standard• ASTM F963-08 Section 8.24 Magnet Test Method

– Measurement parameter : magnetic flux density ( gauss / Tesla )

» Equipment—dc field gauss meter with a resolution of 5 gauss (G) and an axial type probe.

» An active area diameter of 0.76 +/- 0.13 mm A distance between the active area and probe tip of 0.38 +/- 0.13 mm.

• Medical electrical equipment Safety (e.g. IEC 60601-2-33)

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Automatic calibration system on DC magnetic flux density for working-grade gaussmeter

in SCL

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An automatic calibration system in SCL

The major components of the system contain• A modified electromagnet• The electromagnet’s power supply• A water cooling system for the modified

electromagnet• A reference gaussmeter• A software controlled program

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A modified electromagnet system in SCL

About 10 mm

About 20 mm

Measuring Probe

Yoke

Small opening at the yoke with size and shape suitable for

positioning a common test probe in an axial

magnetic field

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Photos of the electromagnet

Filler rod

Opening at the yoke

test probe of the tested unit

test probe of reference meter

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Characterization of the Modified Electromagnet

• Field Plot with the opening at the RHS yoke

-80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 800.09500

0.09700

0.09900

0.10100

0.10300

0.10500

Z=-5 mm

Z=-3 mm

Z=-1 mm

Z= 0 mm

Z=+1 mm

Z=+3 mm

z=+5 mm

X (mm)

Fie

ld (

T)

+Y

Left Pole Cap

+Z

+X+Z

0

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

toward the RHS yoke

with opening

At the center of the yoke

12Standards and Calibration Laboratory, SCL

Characterization of the Modified Electromagnet

• Field Plot with filler rod inserted in the RHS yoke

-80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 800.09500

0.09600

0.09700

0.09800

0.09900

0.10000

0.10100

Z=-5 mm

Z=-3 mm

Z=-1 mm

Z= 0 mm

Z=+1 mm

Z=+3 mm

z=+5 mm

X (mm)

Fie

ld (

T)

+Y

Left Pole Cap

+Z

+X+Z

0

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

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Setup of Calibration

Reference Transverse

Probe

Pole Cap of the LHS Yoke

Axial Probe Under Test

The measurement will be taken at the location adjacent to the LHS yoke ( i.e. as far away from the opening (i.e. RHS yoke ) )

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Field Uniformity

0.0 2.0 4.0 6.0 8.0 10.0 12.0 -2600.00

-2400.00

-2200.00

-2000.00

-1800.00

-1600.00

-1400.00

-1200.00

-1000.00

-800.00

-600.00

-400.00

-200.00

0.00

200.00 Field Plot along Z-Axis

Rod Inserted, Gap = 20 mmPolynomial (Rod Inserted, Gap = 20 mm)

Z-Axis Deviation from Reference Centre in mm

Mag

net F

lux

Dens

ity D

evia

ted

from

Cen

ter R

efer

ence

in u

T/T

+YLeft Pole Cap

+Z

+X

+Z

On the surface of the LHS yoke

Z = 4 mm ; field uniformity

~ 500 x 10-6

Z = 7 mm ; field uniformity

~ 200 x 10-6

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Start

Operator input the test points to be measured

Initialize the equipment

Ramp up the magnet power supply from zero to an initial value 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

End

N

Y

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

Automation of the calibration

The software tool using Visual Basic for the automatic calibration of working-grade gaussmter which have either IEEE 488, RS232 or USB interface.

number of measurements; magnetic field levels for test; polarity of the test field ;settings of the reference meter ;settings of the UUT

Test field levels are sorted in ascending order by the program

Initial value = (Requested test level )/(electromagnet coefficient)

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Measurement Uncertainty

Uncertainty contributions due to• Field non-uniformity• Calibration setup• Performance of the test unit• Performance of the reference meter• Environmental effects

The best calibration and measurement capability of the working grade gaussmeter is 0.1 % from 50 mT to 1.0 T for axial direction (k=2, 95% CL)

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Thank you