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### Magnetic Interference to Compass from Tachometer and Voltmeter

Posted: Sat Mar 30, 2019 9:04 pm
Will a in-dash tachometer or voltmeter interfere with the operation of a nearby in-dash compass like a Ritchie X-21?

### Re: Magnetic Interference to Compass from Tachometer and Voltmeter

Posted: Mon Apr 01, 2019 4:45 pm
If you were looking for an answer that was either "yes" or "no" you may be disappointed.

A compass on a boat can be affected by any local magnetic field created on the boat that is sufficiently strong to cause the effect of the Earth's magnetic field to be altered and for the compass to swing off the magnetic field of the Earth or "magnetic North."

Electrical currents flowing near a magnetic compass may affect the compass indication of magnetic north. To test, observe the compass indication with all electrical power shut off to the electrical devices you wish to test for interference with the compass. Then apply electrical power to the devices under test, one at a time. Observe the compass for any change in reading when electrical power is applied.

To predict the outcome is difficult to do with certainty.

There is a prior discussion on "Magnetic field from DC current" which can give you some guidance. See

Magnetic Field From DC Current
http://continuouswave.com/ubb/Forum6/HTML/003731.html

Test before drilling a big hole in the dashboard.

### Re: Magnetic Interference to Compass from Tachometer and Voltmeter

Posted: Fri Apr 05, 2019 10:25 am
I don't think the particular model of compass will affect how well it tolerates interference from local magnetic fields. A compass has to be able to react to the Earth's magnetic field. Most boat compasses are designed with a gimbal mechanism that allows the compass to remain in a stable horizontal plane, in order that it can intercept the Earth's magnetic field. Compasses are even adjusted for "dip" or the angle from perfectly horizontal at which the Earth's magnetic field is anticipated to be received by the compass based on where on Earth the compass is located. I don't think this varies much from brand to brand or model to model.

Magnetic Dip is explained quite nicely in a Wikipedia article.

Compensation for Magnetic Dip is also explained in a separate article.

### Re: Magnetic Interference to Compass from Tachometer and Voltmeter

Posted: Fri Apr 05, 2019 12:08 pm
The type of "tachometer" and "voltmeter" are not well described. If the devices employ a WESTON meter movement, you should be on guard for possible magnetic field sources. A Weston meter movement contains a PERMANENT MAGNET. The magnetic field that exists around a Weston meter movement could affect the compass to a much greater degree than any magnetic field generated by the flow of a DC current in a wire.

Since the field is permanent, it will not disappear when the electrical power is removed. Therefore you will not be able to test for this influence as I described earlier in my test for fields created by DC currents. You will have to look for deflection of the compass as the compass is moved near to the Weston meter.

The Weston meter movement is almost universally used modern voltmeters with a dial pointer indicator.