Magnetic Field Strength Test Coils - John Dunn, Consultant, Ambertec, P.E., P.C.

I was using a pair of these specially purchased test coils to make some AC magnetic field strength measurements and my results were not very repeatable. Ordinarily, I would blame that on my being somewhat clumsy in handling the test coils, but this was getting to be a bit more of a problem than I could account for by dint of personal ineptitude.

Each test coil came potted in an epoxy cylinder with two very thin wires emerging from one end. The calibration information for these two coils was that output signal versus field strength  was based on the central axis of that epoxy cylinder, the Cylinder Axis,  being in alignment with the field being measured.

  

Between these two test coils, my results were quite inconsistent.

After much trevail and much discussion with the manufacturer, it came to light that the coils inside these epoxy cylinders were not oriented for their maximum pickup at an angle in alignment with the Cylinder Axis. One coil direction of maximum pickup was at seven degrees of angular displacement error versus the Cylinder Axis and the other was at fifteen degrees of error.

I never did do anything about this although the idea did occur to me to place a properly oriented coil of my own on the outside of the epoxy cyclinder to detect proper orientation in order to then use the inner coil's calibration, but I never got around to it.   

 

I don't know if this would have worked or not, but be aware that the angular errors existed in both of the test coils I was using and such might lead you astray also.

Comments

[george storm]

Shouldn't a coil sold for this purpose specify the alignment?

[John Dunn]

I agree! However, my discussion with the manufacturer was fruitless in that regard.

[David Archer]

Putting a coil external to the coil will not provide information about the orientation of the field internal to the coil. Any pickup on the external coil will depend on the total flux internal to the coil, which will be very insensitive to orientation. If you want to measure the orientation of the field inside the coil, you need a pickup inside the coil.

If the coils were wound in the same sense, but a different angle depending on direction, this slight effect would cancel out, and the maximum field would be along the axis. The contamination will be the equivalent of a single turn with a current equal to the current component along the longitudinal axis of the cylinder. It may be that there is not an issue, it does depend on the details of how the coil was wound.

[Joe Leikhim]

So, assuming the manufacturer calibrated the coils relying on the assembly's central access, their own calibration data would be skewed in the direction of under reporting the magnetic flux created by the coil. Time to select a new vendor!