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.

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

In order to assist you we would need some further information;

1. Is the magnet inside or outside of the tube?
2. Is it central or offset from the tube?
3. What is the proximity of the magnet to the tube?
4. What is the strength of the magnet?
5. What is the intended size, thickness and material of the tube?

With Kind Regards,

how can calculating flux/field distribution inside the tube
thanks

The 4:1 ratio mentioned is to screen the transverse component of the magnetic field.
If the end cap is placed just inside the tube it will perform as a magnetic shield in exactly the same manner as one placed on the outside of the tube. If you have a multi layer shield then you would need to have several end caps but for a single layer shield a single end cap would be satisfactory.

Mumetal is normally supplied in the”mechanically annealed” condition to allow for forming operations such as bending, and in this state it will mechanically react in a similar fashion to 316 grade stainless steel. This is quite different from a magnetic anneal which is a much higher temperature process (1100+degrees C instead of 900 for a mechanical anneal.)

Any mechanical deformation of mu metal after this final magnetic anneal will significantly reduce its properties, the exact amount is difficult to define as it depends on several factors:- the amount of deformation, the thickness of the mumetal, and the actual shape and direction of the field for which the shield is required, it is not unusual to find properties have been reduced to 30% of their original value by forming operations after the magnetic annealing process. In most shielding applications the rule is for optimum shielding and the magnetic anneal should be the last production process.

Regarding your second point , yes – as explained above, shield properties are fully recoverable by a further specialised magnetic annealing process (refer to my article on mumetal specifications for further information on this heat treatment process)