Helmholtz coil phenomenon 2

[MAC3382]

Hello, All:

We have started using a Helmholtz coil and a flux meter to measure the magnets used in our products. We typically measure in Weber Centimeters and this works well for a relative comparison to a known good magnet sample. (We compare the magnetic moments between a magnets that works in our application to magnets in a batch.)

Sometimes we wnat to make a direct measurment for documentation on a print, but no one trusts the method. It seems that the (+) and (-) side of the magnet returns different values. This is probalbly from the fact the coils are not perfect. Do you agree?

Also, we have always see a difference between the flip and extraction methods. I would think that the returned values would be closer, but the flip method is always a few % higher.

The differences in returned values for different methods when measuring the same magnet is challenging. I would appreciate any input you may have to offer.

Thank you,

mac

 

[EdStainless]

We saw some issues also. If you disconnect the two coils and test in each on separately you will probably find that they do not have the exact same response. Minor differences in coil wind patterns and fill will do this.
Is everything within a few feet of the test area non-metallic? What about the legs of the work bench? A few % isn't bad, I would settle for that.

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Plymouth Tube

 

[MagMike]

One does not need to do a perfect job when winding a Helmholz coil, but a certain amount of care is necessary. My company makes/uses them extensively and we've found them to be very consistent, even at different facilities.

EdStainless raised a very important point about keeping ferromagnetic materials away. I inadvertently embarassed a colleague once who was consistently getting 10% higher readings, but only because there was a steel support bar underneath the test bench.

It is very important that the Helmholz coil is sized appropriately for the parts being tested. One coil may not work for every part. They are most accurate when the part is approximately 1/3 of the volume between the coils. Larger parts will cause problems if they aren't perfectly centered. Very small parts (relative to the coil) will give extremely small readings and fluxmeter precision will be important.

It should also go without saying that the parts need to be centered in the coil prior to taking measurements.

I like the flip method over the extraction method because it minimizes the +/- effect and it doubles the reading, providing a larger signal to noise ratio. Either method is perfectly valid, but you want to make sure your magnet supplier and your customer are doing the same.

Mike

 

[MAC3382]

Thank you Ed and Mag Mike:

You were correct. Our wood table top has a horizontal steel support. When we moved testing loactions the extract /flip methods were much closer. Still not exact, but much better.

I always value your responses and posts.

Thanks again,

mac