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.

 

[ravenprp]

,

Designing a basic Capacitor Sensor is a relatively simple process. First, you will need to determine the desired capacitance for your sensor based on the conductivity range you want to detect. This can be calculated using the formula C = εA/d, where C is the capacitance, ε is the permittivity of the material between the plates, A is the area of the plates, and d is the distance between them. Once you have this value, you can start building your sensor.

To construct the sensor, you will need two parallel plates made of a conductive material such as metal. The plates should be as flat and smooth as possible to ensure a consistent and accurate reading. The distance between the plates should be very small, ideally in the micrometer range, to increase the sensitivity of the sensor. You can achieve this by using spacers or by coating the plates with a thin layer of insulating material.

Next, you will need to connect the plates to a circuit that can measure the capacitance. This can be done using a simple AC circuit with a source of AC current, such as a function generator, and a capacitance meter. The AC current will create an electric field between the plates, and any changes in the conductivity of the material placed near the sensor will cause a change in the capacitance, which can be measured by the capacitance meter.

To design a Conductivity Sensor using a capacitor-like setup, you can follow a similar process. Instead of using two parallel plates made of conductive material, you will need to use one plate made of a conductive material and one made of a non-conductive material, such as glass. The non-conductive plate will act as a dielectric, and the conductivity of the material placed near the sensor will affect the capacitance between the two plates. This change in capacitance can then be measured and used to determine the conductivity of the material.

In conclusion, designing a basic Capacitor Sensor and Conductivity Sensor using a capacitor-like setup involves determining the desired capacitance, constructing the sensor with parallel plates, and connecting it to a circuit for measurement. With the right materials and setup, these sensors can effectively detect changes in conductivity and provide valuable data for various applications. I hope this helps guide you in designing your sensor. Good luck!