[Ken9]

I have seen postings related to shielding a compass with the material sold by AC Spruce but have never heard if it works. I assume it is MEW metal.
I have a PAI-700 compass mounted in my panel, about 1" away from my EFIS and it's useless due to interference. Has anyone had any luck with this material?

RV-9 flying since July. 65 hours. Smile still there.

 

[Mike S]

http://en.wikipedia.org/wiki/Mu-metal

Is this what they are selling??

 

[Ken9]

Yes, it looks like it has the same nickel content as described here.
Sorry about the spelling error.

 

[David Paule]

It's been a long time since I've worked with the stuff, but as I remember, thin is better than thick (the article alludes to that and thin is what ACS sells) and the post-forming heat-treatment is essential.

Both factors make it difficult to use successfully. But when used properly, it does work.

Any chance you can relocate the compass?

 

[Ken9]

The material is .005" so should meet that requirement. Not sure how to heat treat but may give it a try. Sounds like no one has actually tried it successfully I guess.
Relocation would be difficult.

 

[GOEZ]

I've used this material from ACS but due to a double change I'm not sure if it's the uMetal or a new compass that fixed the problem. Certainly the uMetal doesn't seem to be causing any issues.

I had a Falcon vertical card compass that was mounted less than 6" from a dual electric tach that had a plastic rear case. Less than five minutes after first startup the compass went Tango Uniform and continually spun. After removal, the compass was still bad. Although I also have a different metal-cased tach, I was confident in the calibration of the current one and wanted to delay changing it during initial testing. It was also a real pain to get to the area of the console under my compass. I made several changes at once so I don't know which one was the root cure for my problem:

- The compass was replaced with a PAI-700
- The back of the tach case was wrapped in uMetal
- A strip of uMetal (~ 4" x 5") was placed under the console just below the compass.

Fifty hours later I can report that the PAI-700 is still working fine and there appears to be no problems. However, I can't say whether this was due to the uMetal or the compass upgrade. I am aware of several other people who also had problems with the original Falcon compass.

 

[elippse]

I'm curious how you would shield a magnetic compass from stray magnetic fields in wiring without, in turn, shielding or distorting the earth's magnetic field? 'Just askin'!

 

[Ironflight]

I'm curious how you would shield a magnetic compass from stray magnetic fields in wiring without, in turn, shielding or distorting the earth's magnetic field? 'Just askin'!

I had the same question....isn't it sorta like protecting a solar array by keeping it in the dark?

 

[David Paule]

You don't wrap this stuff around the compass, you wrap it around the other things that create a magnetic field which would disturb the compass.

Incidentally, check out the magnetism of a Duracell battery. How about that flashlight?

 

[apkp777]

Here's the quote from ACS about the material

"Magnetic Shielding is an alloy containing approximately 80% nickel (Ni). Nickel has magnetic and magnetostrictive properties. In the soft or annealed state, the molecules in nickel will absorb magnetism much like a sponge absorb water --- restricting the flow and allowing no specific pattern to emerge. Use this .005” x 5” x 12” metal strip to shield the precision PAI-700 compass from magnetic interference in the instrument panel."

Ken, I am sorry my friend. It sounds like "snake oil" to me. If you go back and search a while ago I posted a question about the accuracy of Vertical Card Compasses and the results weren't too encouraging. I remember one reply was something like this. "perfectly accurate, as long as I am heading 281 degrees."

For the price it may be worth ordering it. At this point your compass is TU without something.

 

[elippse]

You don't wrap this stuff around the compass, you wrap it around the other things that create a magnetic field which would disturb the compass.QUOTE]

I've worked with mu-metal shielding, and I have some in my possession. I've used it where I want to shield a device from an unwanted magnetic field. Basically it's a high magnetic conductance material which shunts a magnetic field around any chassis or device it encloses, but there will still be the incident magnetic field at its edges. In other words, if you place a sheet of this material over a conductor which has a current flowing, it will reduce the field normal to the conductor but not at its edges, so to totally enclose the field, the conductor would have to be enclosed for a sufficient length to reduce the field strength at the protected device. This is one of the advantages of using twisted-pair wiring; it's current-induced magnetic field is greatly attenuated due to the cancellation effect of the opposite direction fields.
The +14V lead from the battery to the buss on my plane went across the bottom of the instrument panel about 14" below the compass. It would deflect the compass about 5-8 degrees when the battery was being charged with 2-3A. I had to run it down across the floor to get it to less than 2 degrees. Fortunately when I installed my Dynon I put the magnetometer half-way between the canopy and the stab in the top of the fuselage, and then I had to get rid of the speaker in the bulkhead behind the baggage compartment.

 

[pczar3]

At last

now you're in my sandbox! I work for Fermi National Accelerator Laboratory outside Chicago. One of our machines, about three and half miles in circumference, uses electromagnets to steer the particle beam. They ramp up to 6800 amps and back down every 2.2 seconds. To the point, in the same tunnel about 4 feet above the accelerator we have a storage machine with permanent magnets. The ramping field could be seen outside of about 22 feet of dirt and prevented the storage machine from working. In order to make the storage machine work we installed mu-metal shields about half way between the two machines. It eliminated the stray fields. You need very thin material as elippse said above but it can be layered to achieve the shielding level needed. The ACS description almost made me snort my coffee out my nose! As elippse said it allows the magnetic field lines to travel through the mu-metal easier than through air. So, not snake oil.

Paul N694BP reserved

 

[Ken9]

Well, I must say, this has been an interesting discussion. I also was questioning the claim by ACS that wraping the compass in this material would shield it without also shielding the wanted magnetic field.
Considering the number of these compasses out there, it would seem that, if the mumetal works, someone would have reported success or failure.
Guess i'll have to spend the $19 and give it a try.
Will report any success or failure here.
Thanks to all who have responded.
Ken

 

[walkman]

Well, I must say, this has been an interesting discussion. I also was questioning the claim by ACS that wraping the compass in this material would shield it without also shielding the wanted magnetic field.
Considering the number of these compasses out there, it would seem that, if the mumetal works, someone would have reported success or failure.
Guess i'll have to spend the $19 and give it a try.
Will report any success or failure here.
Thanks to all who have responded.
Ken

and? how did it go?

 

[minute2]

Mu-metal will interfere with a magnetic compass

I used to design mu-metal shields and I have some understanding of the material. I think I can answer a few of these questions. Mu-metal re-directs magnetic fields, it does not attenuate or trap them. A magnetic field cannot be shunted to ground like an electrostatic (voltage) field can be. A magnetic field likes being in mu-metal better than almost any other material. It enters the mu-metal and can be directed by the mu-metal shield, but it will eventually come back out of the shield somewhere and end up going in approximately the same direction it was going before it was intercepted by the shield. The exact behavior will be dictated by the various magnetic and voltage fields in the local area of the field. In your situation, your compass should be being shielded from any voltage fields, generated by the EFIS and other equipment, by the metal glare shield and the housing of the EFIS (both grounded I assume). If you wrap mu-metal directly around your compass you will get erroneous results from it if it works at all. You could try to shield the EFIS's magnetic shield with a mu-metal sheet or box over/around the EFIS, but that magnetic field IS going to come back out of that shield somewhere and will likely still disturb the compass reading. The distance of 1 inch is way to close to any high current carrying device. Distance is about your only solution with magnetic fields, for a magnetic compass anyway. I am sorry for your trouble, but I am going to learn from it. I was going to do the same thing: place a big EFIS tucked up tight to the glare shield and place my magnetic compass right over the top of it. I no doubt would have had the same problems you are having. Can people comment on where they have successfully mounted the magnetic compass relative to EFIS's ? (and other gear)??. Especially for the RV-8.

Other fun facts about mu-metal. It must be fully annealed for it to work well. Cold-work: bending, folding, cutting, forming, even banging, disrupts mu-metals annealing and dramatically lowers performance. It must be annealed in a hydrogen, or inert gas atmosphere furnace. Mere mortal DIY'rs are unlikely to have one of these. If you do get mu-metal mail ordered from someplace like ACS, it is likely to have been fully annealed prior to shipping. You should minimize the amount of bending and banging it receives during your fabrication in order to maintain as much of its "annealing" as possible. Also, thin layers are not "better", it is just that thicker doesn't work any better than thin. A layer of mu-metal can only absorb so much magnetic field density and this is independent of thickness. Another layer can carry any field that could not be absorbed by the first layer(s).

Kerry Stevens