Comments on: Shielding calculation formula http://mumetal.co.uk/2009/05/27/shielding-calculation-formula/ Magnetic shielding with mu metal Wed, 21 Jul 2010 14:46:36 +0000 hourly 1 http://wordpress.org/?v=3.0 By: nickmsl http://mumetal.co.uk/2009/05/27/shielding-calculation-formula/comment-page-1/#comment-285 nickmsl Wed, 21 Jul 2010 14:46:36 +0000 http://mumetal.co.uk/?p=159#comment-285 Dear Hugh, At present there is no known formula for ellipsoidal mumetal shields so we need to base the calculations on a closed shielding cylinder and a cubic box. For the minor axis we would assume a diameter of 30mm, therefore to reduce the field by 99% a layer of 0.2mm mumetal would be required For the major axis you cannot use the formula for a closed shielding cylinder, you would need to use the formula for a cubic box with a side length of 100mm. On this basis, to give an attenuation of 99% the mumetal thickness would need to be 0.8mm thick. Dear Hugh,

At present there is no known formula for ellipsoidal mumetal shields so we need to base the calculations on a closed shielding cylinder and a cubic box.

For the minor axis we would assume a diameter of 30mm, therefore to reduce the field by 99% a layer of 0.2mm mumetal would be required

For the major axis you cannot use the formula for a closed shielding cylinder, you would need to use the formula for a cubic box with a side length of 100mm. On this basis, to give an attenuation of 99% the mumetal thickness would need to be 0.8mm thick.

]]> By: Hugh http://mumetal.co.uk/2009/05/27/shielding-calculation-formula/comment-page-1/#comment-284 Hugh Mon, 12 Jul 2010 17:31:00 +0000 http://mumetal.co.uk/?p=159#comment-284 If I want to shield a thick wire with maybe 200 Amps in it so that it's field (about a few mT, I think, in air) will not (to 99%) penetrate outside the ellipsoidal mu-metal shield, how thick must it be (d) if the ellipsoid's minor axis D is about 3 cm and major D is 10 cm (wire length). If too thick, then dor 90% attenuation what is d? Thanks, Hugh If I want to shield a thick wire with maybe 200 Amps in it so that it’s field (about a few mT, I think, in air) will not (to 99%) penetrate outside the ellipsoidal mu-metal shield, how thick must it be (d) if the ellipsoid’s minor axis D is about 3 cm and major D is 10 cm (wire length). If too thick, then dor 90% attenuation what is d?
Thanks,
Hugh

]]> By: nickmsl http://mumetal.co.uk/2009/05/27/shielding-calculation-formula/comment-page-1/#comment-279 nickmsl Mon, 14 Jun 2010 15:10:47 +0000 http://mumetal.co.uk/?p=159#comment-279 Dear Laurent, Thanks for the mail. Our shielding formula for shielding boxes can be found on our website : www.magneticshields.co.uk under frequently asked questions. For the second layer please use the box side length for all instead of using diameter measurements. Best Regards Steve Locker Dear Laurent,

Thanks for the mail. Our shielding formula for shielding boxes can be found on our website : http://www.magneticshields.co.uk under frequently asked questions.
For the second layer please use the box side length for all instead of using diameter measurements.

Best Regards

Steve Locker

]]> By: Laurent Ren http://mumetal.co.uk/2009/05/27/shielding-calculation-formula/comment-page-1/#comment-278 Laurent Ren Sat, 12 Jun 2010 22:03:40 +0000 http://mumetal.co.uk/?p=159#comment-278 What is the shielding factor S for a double layer cubic shielding box? What is the shielding factor S for a double layer cubic shielding box?

]]> By: Nick Murby http://mumetal.co.uk/2009/05/27/shielding-calculation-formula/comment-page-1/#comment-8 Nick Murby Tue, 01 Sep 2009 15:10:17 +0000 http://mumetal.co.uk/?p=159#comment-8 The shielding formulas work in exactly the same way whether the source of interference is on the inside of the shield or on the outside - this is assuming that the magnet to be shielded is within the saturation induction range of a single or multi layer shield (see properties of mu-metal, supra 50 and multi layer shielding) It is also worth noting that whilst a shield housing a magnet will negate the effect of the magnetic field produced, the magnet will still be attracted to the shield as magnetic shielding alloys are ferromagnetic. It would therefore need housing in a non-magnetic, isolative structure (such as polyurethane), within the shield to make this attraction impossible through means of restriction of movement for optimal shielding. The shielding formulas work in exactly the same way whether the source of interference is on the inside of the shield or on the outside – this is assuming that the magnet to be shielded is within the saturation induction range of a single or multi layer shield (see properties of mu-metal, supra 50 and multi layer shielding)

It is also worth noting that whilst a shield housing a magnet will negate the effect of the magnetic field produced, the magnet will still be attracted to the shield as magnetic shielding alloys are ferromagnetic. It would therefore need housing in a non-magnetic, isolative structure (such as polyurethane), within the shield to make this attraction impossible through means of restriction of movement for optimal shielding.

]]> By: pablo http://mumetal.co.uk/2009/05/27/shielding-calculation-formula/comment-page-1/#comment-7 pablo Tue, 01 Sep 2009 09:34:00 +0000 http://mumetal.co.uk/?p=159#comment-7 How would these formulas work if I wanted to shield a Magnet inside a box? E.g. the field strenght inside is large and I want the field strenght on the outside to be as small as possible. How would these formulas work if I wanted to shield a Magnet inside a box?

E.g. the field strenght inside is large and I want the field strenght on the outside to be as small as possible.

]]>