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

[Jonathan212]

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?

 

[Nik_2213]

https://en.wikipedia.org/wiki/Permeability_(electromagnetism)
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...

 

[Jonathan212]

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]

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.

http://www.magnetic-shield.com/index.html

 

[marcusl]

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.

 

[Jonathan212]

I was starting with the wikipedia values for mu that go like this:

Iron (99.95% pure Fe annealed in H)	200000
Mu-metal	50000
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]

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]

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.

 

[Jonathan212]

Oopsa. Didn't get a notification by email for these later posts. I want to shield a bedroom. Sources would be mains electricity to begin with. No trains nearby, only traffic. Anything else that could be an issue?

 

[berkeman]

I want to shield a bedroom. Sources would be mains electricity to begin with.

That's problematic. You would need to line the walls with steel plates, probably a couple mm thick.

 

[Jonathan212]

Is it ok that there is a window?

 

[berkeman]

Is it ok that there is a window?

No, I'm afraid not. Any openings will let in B-field...

 

[marcusl]

No gaps either. You need a door like a bank vault.

 

[Jonathan212]

How does this relate to Faraday cages having holes?

 

[berkeman]

What reading have you done about E-field shielding and Faraday cages?

 

[berkeman]

And you do understand that low-frequency E-field shielding is different from low-frequency B-field shielding, and those are different from higher-frequency EM shielding, right?

 

[marcusl]

I’m with Berkeman—you can’t expect us to give you a full education, you’re responsible for your own education and need to research this topic before asking for help. Read up on RF screened rooms (Faraday cages) and magnetic shielding. We can then help clarify a detail or answer a specific question.

 

[Jonathan212]

I have a degree in electronic engineering but magnetic shielding was never taught and I never researched the subject myself. I'd like to know some numbers for the B-field inside such an enclosure. A room completely encased in 2 mm steel sheet (99.8% iron, see above) where a 10 A, 60 Hz current flows through a single wire outside the enclosure, 5 cm from the sheet. What B-field amplitude do we expect inside? What DC B-field do we expect inside due to the Earth's magnetic field?

 

[berkeman]

I have a degree in electronic engineering but magnetic shielding was never taught and I never researched the subject myself.

How much E&M did you take? At least in my classes, we covered magnetic fields and how they interact with magnetic materials...

I'd like to know some numbers for the B-field inside such an enclosure.

Did you read through the application notes that I linked to in post #4 yet?

 

[Jonathan212]

Almost exclusively microwaves. I looked at that site after my last post (do you work for them by any chance?) and did find a claim that a very thin sheet of a certain material of theirs would reduce the Earth's magnetic field by a factor of 125 or so, but no math is shown. Anyway, a more interesting question is: would it be bad for you if you slept without the Earth's Schumann resonances?

 

[marcusl]

As an EE, you should be able to calculate the fields from a wire. The Wikipedia article on magnetic shielding shows a calculation for a spherical shield, which will get you in the ballpark. Literature from shielding companies (Amuneal, Vacuumshmelze, etc) will give you more info. I really think you can do your own research if you try.