How to design a basic Capacitor Sensor

[chadcy]

How to design a basic Conductivity Sensor (Capacitor-like)

Hey,

I want to design a basic Capacitor like sensor to detect Conductivity in some test material. I believe that it only entails two parallel plates in very close proximity with some AC current going through it. Will this produce some magnetic field? I believe this device will act as a sensor once it is placed near some test material, in which the amplitude of the Ac signal through the capacitor will be changed. Am I wrong about these deductions? If someone can give me some advice, that would be great.

chadcy

 

[berkeman]

Welcome to PF, chadcy. If you want to make a non-contact measurement, then parallel plates around your test material is a good start. Then you should figure out what the impedance of that arrangement is across frequency, and use that information to help you design your detector circuit.

You could use a simple capacitance meter at first, and connect it to your test plates. Observe how much the capacitance decreases when you place a conducting material between the plates. Quiz question -- why would the capacitance decrease? The capacitance meter is likely using 100kHz as a test frequency, or something similar. A better test would be to connect an HP4194 Impedance Analyzer or equivalent instrument to the plates, and observe the change in Z(f) with your test material between the plates (versus just air). By looking at both Z and loss across frequency, you may be able to find some things with your conductive material that would let you simplify the detecting circuit that you design. For example, maybe the conductivity is very lossy at 10MHz or something.

BTW, driving an AC voltage into the parallel plates generates an electric field, not a magnetic field. The presence or absence of a conductive material in the volume between the plates will alter how much charge flows on and off the plates, and this gives you a change in the AC current that is flowing compared to an empty test volume.