1 mm magnetic shield of pure iron same as 1 cm of mu-metal?

  • Thread starter Jonathan212
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When materials with high permeability are used to shield against low EMF frequencies of the order of 10 Hz, is the effectiveness linearly dependent on thickness and permeability?

In other words, pure iron (mu=200,000) can be 10 times thinner than mu-metal (mu=20,000) for the same result with a complete enclosure like a sphere or cube?
 
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Had me worried there, as I would have said the relationship was reversed...
Your 'pure iron' figure is for 99.95% iron, hydrogen annealed. Mere 99.8% iron is but~5k.

As I understand it, mu-metal and its kin are easier to shape and hence use. But, a hefty steel box as an outer container is a cost-effective way to mitigate magnetic interference before reaching inner mu-metal layer(s).
Much depends on field strength and frequency, detail outside my experience...
 
What about the math of it? Material A has permeability a, material B has permeability b, is a spherical enclosure made of A the same as one made of B if the thickness of B is a/b times the thickness of A? What value of magnetic field would you measure inside each exposed to a 10 Hz e/m wave?
 

berkeman

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What about the math of it? Material A has permeability a, material B has permeability b, is a spherical enclosure made of A the same as one made of B if the thickness of B is a/b times the thickness of A? What value of magnetic field would you measure inside each exposed to a 10 Hz e/m wave?
I'm not familiar with the "pure iron" example you gave earlier. In our products, we generally use mu-metal shields, sometimes with the netic/co-netic combination to optimize the shielding properties.

Check out the Shielding Tutorials at this website below (we use them for most of our magnetic shielding needs). Click the "ALL ABOUT SHIELDING" link at the top of that web page to get to the long list of their tutorial pages. :smile:


241919
 

marcusl

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You’ve reversed mu for iron and mu-metal. They are not equivalent because mu-metal saturates at low ambient fields. Iron is less effective at low fields but remains effective at much higher fields.
 
I was starting with the wikipedia values for mu that go like this:
Iron (99.95% pure Fe annealed in H)200000
Mu-metal50000
Iron (99.8% pure)5000

So, nothing reversed. The lack of equivalence sort of answers the question.

All options are open though, the above are just examples. Now what sort of frequencies and intensities are we up against as civilians in cities, where magnetic shielding is relevant?
 

marcusl

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Well I learned something, I never saw that value before. The answer is yes, you can use a thinner sheet of higher mu material so long as it doesn't saturate in the applied field. If it does, then thicker lower mu material will be more effective.
 

berkeman

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Now what sort of frequencies and intensities are we up against as civilians in cities, where magnetic shielding is relevant?
Can you clarify this? What are you wanting to shield from magnetic fields, and what are the sources of the fields.
 

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