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Will other wiring interfere with Magnetometer?

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

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

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

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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peter(at)sportingaero.com
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PostPosted: Tue Jan 06, 2009 10:40 am    Post subject: Will other wiring interfere with Magnetometer? Reply with quote

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

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

Thank you Peter. Easier still!!

[quote]>>--


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