Magnetic shield with holes

[jhiker]

Hi,
I have to shield a scientific instrument from an external magnetic field of 500G with a Supra50 tube of about 45mm OD and which is effectively closed at either end. I also have to put some 6mm radial holes in the shield tube to pump out the air. I can calculate the required tube wall thickness OK but I don't know what effect the 6mm holes will have on the shielding of the instrument and I can't find a reference on the web. Can anybody point me in the direction of a good source on shield design (with holes) or enlighten me regarding the significance of the 6mm holes.
Thanks in advance.

 

[MJR2]

The whole trick to magnetic shielding is to make the flux lines go where they don't want to go. You do that with ferromagnetic materials. Flux will follow ferromagnetic material in preference to air. Either carbon steel or 400 series stainless steel. Other metals such as 300 series SS and alumnium look like air to a DC magnetic field. There are others for shielding but they are generally more expensive. Holes generally will not reduce the effectiveness of the shield until the flux begins to exceed 15000 gauss in steel and 11000 gauss in 400 series SS.

 

[Dynatron]

Dear Sir: Regarding holes in a tube you need to shield a magnetic field, one poster on here said: so long as the diameter of the holes do not exceed the thickness of the metal, you will probably be okay, although I can't verify this information. Sincerely, Don Casey, Dynatron Company

 

[BrianR]

The leakage from a break in a shield is a function of the longest linear dimension of the break, the frequency of the field and the field impedance. The secret is to ensure that the 6mm holes you need do not force the magnetic current flow to detour around the holes because then you'll have leakage trouble.

Solution is to have more small holes if you can rather than larger ones, and provide a flux shunt(s) to minimise the flux density near the holes.