Temparature compensation for capacitance

[seyyah]

I am thinking to design a capacitance sensor for level measurement. The capacitance that will be measured is formed by the probe that is connected to the measurement system and the substance at the environment as you know. What i wonder is, how temperature affects this capacitance? How can i compensate for the temperature change? Can you give me an idea? If there are other tips also, i will appreciate it. Thanks.

 

[FvM]

Hello,

I think the basic requirement for an accurate capacitive level sensor is a homogene and well defined medium to be sensed. As far as I know, capacitive level sensors are commonly used for liquids and sometimes for bulk material. Accuracy could be expected mainly for liquids.

A ideal medium is probably water due to its high er. You can try to manage, that you measure mainly the well defined capacitance of your sensor electrode isolation, e. g. a PTFE tube. With nonconductive liquids, dielectric properties contribute considerably to measurent results and should be expected temperature dependant.

I don't know if there is a principal behaviour in this regard, but probably polar and non-polar substances behave different. For pure compunds, you can hope to find some dielectric data in the internet, e. g. from the NIST chemistry webbook. https://webbook.nist.gov/chemistry/

You could try to find an electrode configuration, that allows to measure medium properties separately, or at best, eleminates its influence by measuring a ratio of two signals.

Precision industrial level measurement has turned to use radar like techniques with open transmission lines because the limitations of capacitive measurement.

Regards,
Frank