Magnetic strength behind non magnetic enclosure

[josson]

I am looking at new enclosures and encapsulants for some magnetic based motion sensors. The materials used are nonmagnetic. I am not quite sure how much of an affect, if at all, changing these materials would have on the sensors ability to pick up pulses generated outside the enclosure (ie what would the affect be on the range). Is there any relationships, equations, or reference material that I could use to quickly evaluate this based on know parameters such as density, material, or thickness.

 

[nbucska]

There are many ways to skin a cat but it has to be an
actual cat -- there is no way to skin a generalized cat.

If you want help, describe the actual problem .


<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[josson]

My question is a general one and my knowledge of magnetics is somewhat limited but I will try to be more specific.

I have a magnet inside a plastic material of a known thickness. If I switch to another material, be it plastic, rubber, or cardboard, will the strength of the magnetic field outside of the enclosure be affected? If so, is there a way to compare two materials based on commonly specified properties from a data sheet without having to run comparision testing for all materials being considered?

 

[MagMike]

All the materials you mentioned are not ferromagnetic (i.e., magnetic permeability of 1), and they will not interact with the magnet. As far as the magnet is concerned, they all look like air. The magnetic field from the magnet passes through them with no interaction.

If you had switched to a ferromagnetic material (iron, nickel, cobalt or one of their alloys), then things get very tricky and there is no general relationship for determining the effect. One has to resort to FEA/BEA.

 

[EdStainless]

If both materials are actually nonmagnetic and the distance doesn't change then the field strength won't change.
The distance is the big factor.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[josson]

thanks for your replies MagMike & EdStainless

 

[josson]

For my application the sensor is stationary and ferromagnetic targets pass in front of the sensor at relatively consant rate which may range anywhere from 1 to 50000 pulses per minute.

Would I be correct in inferring from the above posts, that for my specific application that materials with low conductivity are better?

 

[MagMike]

Hello Josson,

You are correct in that a non-ferromagnetic, low conductivity material would be best.

 

[gphatch]

To split hairs for a moment... there is no such thing as a "non-magnetic" material. For example, all materials exhibit soem degree of diamagnetism. Other materials also exhibit paramagnetism or antiferromagnetism. Such transient state effects are very small, but are perceptible. It's just that magnetized ferromagnetic or ferrimagnetic materials will completely "drown out" any paramagnetic / diamagnetic / antiferromagnetic effects of the housing material, since they are orders of magnitude larger.

Gareth P. Hatch, Ph.D.
Director of Technology
Dexter Magnetic Technologies

http://www.dextermag.com