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Will other wiring interfere with Magnetometer?

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nuckolls.bob(at)aeroelect
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Posted: Tue Jan 06, 2009 7:28 am Post subject: Will other wiring interfere with Magnetometer?

At 07:49 AM 1/6/2009, you wrote:

 	  Quote: 	
		  



Okay, so it would be okay to run all the wires and cables thorough the same

conduit but the current carrying ones "(such as navigation or landing light

wires)" would still need to pass at least 12" from the magnetometer.



With the magnetometer 18" forward of the rear spar (and thus the aileron

steel mass balance tube) the wires for Nav and strobes will have to be

routed 12" forward of that on their way to forward end of wingtip.  That

should be consistent with Bob Archer's antenna design.


	




   Magnetometer installation is a good example of

   a case where careful paralleling and perhaps

   twisting of outbound/return current paths as they

   pass by the magnetometer. I had a builder who

   reported a compass swing of several degrees when

   he turned on nav lights. Seems he grounded the lamp

   locally and had a single power lead running only

   a few inches from the remote compass sensor.



   He fixed the problem by running a twisted pair

   for power and ground past the compass sensor.

   Strobe and antenna wiring generally have no risk

   to magnetometer calibration.



   Only those bundles/wires with significant unidirectional

   DC currents flowing in them have a potential for

   problems. Of course, DISTANCE is also a strong

   attenuator of magnetic effects. I forget the

   exact relationship but I'm thinking that effects

   of a wire at 1" are reduced by a factor of more

   than 100 at 10".





        Bob . . .



        ----------------------------------------)

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

        ----------------------------------------

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Allen Fulmer

Joined: 10 Jan 2006
Posts: 79
Location: Alexander City, AL

Posted: Tue Jan 06, 2009 10:06 am Post subject: Will other wiring interfere with Magnetometer?

Thank you, Bob!



That will be easy to do now as I am just pulling wires.



Stein Air has a two conductor shielded cable: MIL-27500 Tefzel  AWG22 - 2

Conductor Shielded Cable



Would this be "the best" even if a little more expensive?  (LED Nav lights

only draw .3 amp or so per wing.)



And if shielded is fine then ground one end of the shield at forest of tabs

on firewall?



Thanks again for all you do for us.



Allen Fulmer



[quote]>>--

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_________________
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N808AF reserved
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peter(at)sportingaero.com
Guest

Posted: Tue Jan 06, 2009 10:40 am Post subject: Will other wiring interfere with Magnetometer?

Allen,



A few years ago I worked on an airplane that was the first of type to have a

magnetic anomaly detector in the fuselage. The thing could cope with

constant magnetic fields - that is circuits that were always on or off, but

not with variable ones. We learnt that shielded wires have no effect on the

magnetic influence of current carrier, and that a simple twisted pair is by

far better (I think the reason why was explained on the list a week or two

ago). So don't bother with the shielded wire, just twist up a pair of 22g

wires yourself - something like 5 to 10 twists per foot should be adequate.



Peter



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Allen Fulmer

Joined: 10 Jan 2006
Posts: 79
Location: Alexander City, AL

Posted: Tue Jan 06, 2009 7:09 pm Post subject: Will other wiring interfere with Magnetometer?

Thank you Peter.  Easier still!!



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