Coil Field Examiner.

[Burnit]

I would like to setup a coil field examiner to test the strength of magnetic fields from metal detector coils.

First up
Is there a difference between AC & DC magnetic field detection equipment?

What is the best cost effective method to do this simply, is there a simple circuit i can use to get the field strength to a data acquision card in a PC.

Thanks

 

[logbook]

Detecting an AC field is easy. You just use a small coil of wire. The smaller the coil the less the induced voltage but the better the spatial resolution. You will be using Faraday's Law of Induction.

E= N * d(phi)/dt

N turns.
phi is the total magnetic flux through the coil
d(phi)/dt is the calculus expression for the rate of change of magnetic flux.

phi = flux density * area of coil

None of this works for a DC field. For a DC field you need either a Hall effect device of a GMR (Giant Magneto Resistive) device.

Hopefully these points can be googled to get you more data.

 

[electricpete]

Cheapo version of gaussmeter for detecting dc fields:

http://my.execpc.com/~rhoadley/magmeter.htm

As was mentioned, to detect ac what you need is a coil connected to a high impedance voltmeter (draws virtually no current).
V = d/dt (Phi) = d/dt (N*A*B(t)*cos(theta)

where N is number of turns, A is area, B is flux, theta is angle of coil plane compared to flux... we will from this point forward assume the coil is tilted until max measurement -> cos(theta)=1

V = N*A*d/dt(B)

Assuming B (and hence V) are a single frequency sinusoid with B and V representing peak values, at frequency f, then Vrms = V/sqrt(2) and ||d/dt(B)|| = ||2*Pi*f* B||

Vrms = N * A * 2 * Pi * f * B / sqrt(2)
Vrms = Pi*sqrt(2) * N * A * f * B
Vrms = 4.44 * N * A * f * B
B= Vrms/(4.44 * N * A * f)
Everything in SI units. Else add appropriate unit conversions.


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[electricpete]

Maybe it goes without saying, but.... the sense coil has no iron. Otherwise it changes what it's trying to measure (flux density in air).

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