Understanding electromagnetic water flow meters

[lalbatros]

You may know that water flow can be measured by an electromagnetic flowmeter.
A strong magnetic field is applied perpendicular to the flow.
An electric field is measured in the third direction.

It seems to work just like a dynamo, which I understand, at least I believed that.

But why does the fluid need to be a conductor, like water?
Does the measurement depend on the conductivity of the fluid?
And finally is there no electric field induced in a moving but not conducting fluid? What happens in a isolating fluid?

Could some of you help me to clarify my understanding, staring from the fundamentals?
Some numbers put on this topic could be useful too.

Thanks,

Michel

 

[FredGarvin]

They operate on the principal that if you move a conductor through a magnetic field, a voltage is produced. The flow will be proportional to the flow rate. That is why the fluid needs to be conductive.

What do you mean by an isolating flow?

 

[berkeman]

What do you mean by an isolating flow?

I think he meant to ask about what happens in an insulating (non-conducting) liquid. This flow measurement technique would not work with a non-conducting liquid.

They operate on the principal that if you move a conductor through a magnetic field, a voltage is produced. The flow will be proportional to the flow rate. That is why the fluid needs to be conductive.

At first I didn't think this was right, but if I think about it in terms of eddy currents getting induced in the conducting liquid as it flows past, that makes more sense. So you would put a B field through the liquid perpendicular to the flow direction, and measure the delta-V that you get between two points along one side of the conduit, slightly above and below the spot where you focus the B-field... Does that sound right? Does anybody have a pointer to a drawing of one of these things?

 

[lalbatros]

Indeed, by "insulating fluid" I meant "non conducting".
The emf produced within the fluid is given by the famous vxB law.
I guess the voltmeter probes should be aligned perpendicular to B and v.

My problem is that I don't see why the fluid needs to be conducting, since the law doesn't involve the conductivity.
Would it be only because a current is measured?
And could the electric field not be measured in another way?

But indeed, understanding how a dynamo works should help me.

Any suggestion?

 

[Astronuc]

My problem is that I don't see why the fluid needs to be conducting, since the law doesn't involve the conductivity.

In order for a potential difference to be established, the charges (electrons) have to be mobile so that the accumulate in one direction while leaving postive ions in the other direction. If the charges cannot move, i.e. if the fluid is non-conducting, there would be no potential difference, but perhaps the molecules or atoms would align like dipoles.

This is the principle of magnetohydrodynamics (MHD).
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/maspec.html#c5

 

[lalbatros]

Astronuc,

You are right: this electric field occurs in the frame attached to the fluid, and results in a force on charged particles.

For this effect to be observed in the 'pipe's frame of reference', the impact on the charged particles has to be observed. And this needs the fluid to be conducting, probably there is no other practical way.

Still, in a non-conducting fluid there will be a polarisation as you mention.
I guess this polarisation could -in principle- also be observed in the 'fixed frame of reference'. If this is right, the obstacle is only technical.

 

[Haydendiego]

Electromagnetic flowmeters are used to measure flow speed. Electromagnet is fitted outside the housing when flow passes the electromagnet it generates results in the form of pulses. Total number of pulses indicates the total flow.