Mu metal foil not working.

[50AE]

I got my hands on a foil whom vendor claims it's mu metal scavenged from a bankrupt electronic devices factory. It looks exactly like an original mu-metal foil, shiny, resilient, not oxidizing. I bought a very little bit to test it, but it doesn't seem to do any 50Hz shielding, judging from my oscilloscope, phone and permanent magnets tests. I heard somewhere that it's shielding properties expire with time or handling, could it be the case?

 

[tomchr]

I was told many years ago that mu metal lost its properties when machined or bent. I doubt mu metal foil would be of much use - if it is indeed mu metal...

Tom

 

[jcx]

permanent magnets and mu metal are a poor combo - the mu metal is easily saturated

once saturated its mag perm is compromised - needs degaussing to recover



practical mag shielding uses air space and cheap iron - low carbon hot rolled steel is the most common, effective - in part because you can cheaply use so much more thickness to handle even large mag fields

 

[50AE]

So the conclusions are that it must be cut into the practical lenghts, installed, and then degaussed on place? Maybe it was the way they did it in the factory?

 

[bear]

What are you trying to do with the foil? As in how are you using it??

_-_-

 

[Sjef]

Mu-metal does not actualy shield anything, it kind of bends magnetics fields to put it simply. Just putting a plain sheet between a permanent magnet or ac magnetic field does nothing at all and is totally waste of material this way. That's why you don't measure anything different with or without the mu-metal in place. It's not a miracle shielding stuff, not at all. It only works if you build a cage or shorting ring with this stuff. This way it can bend the stray field of a transformer for example in a way that most of stays inside the mu-metal ring/cage.

 

[bwaslo]

Mumetal loses some perm when cut or bent sharply and should be annealed after. It also should enclose what it is trying to shield - don't expect a flat sheet with a magnet on the other side to do much. For instance on a crt, wrap it around the neck or back. It also needs thickness, a thin foil won't help with strong fields.

 

[50AE]

Wel, aren't HDD magnet plates made from mu metal? How do they shield the magnet's field. Of course, they are at least 30 times thicker than this foil.

I am trying to shield parts from 50/100Hz fields. I also tried to make a loop from the foil, but it didn't help much either

 

[jcx]

lots of bad info on the web - especially about mu metal

I doubt that expensive high Ni alloy is used when low carbon steel works

if you want a more compact and higher flux density motor you use high Co alloy pole pieces - which is more even more expensive than Ni


mu metal saturates at a fraction of the field of Fe, common low carbon steel - its special property is to have high initial permeability at very low mag field strength

 

[rjm]

Indeed, the typical application is shielding something (Low energy electrons in our case.) from what are otherwise already very low fields. Often a Faraday cage is used to deal with the worst of it, the mu metal shield just mops up.

 

[DF96]

I am trying to shield parts from 50/100Hz fields.

"Parts" themselves should be bought with a mu-metal can already around them, if necessary. 'Circuits', on the other hand, need to be built with loops as small as possible and perhaps twisted too. Remember, only loops can respond to a magnetic field.