Hartley oscillator idea

I have a question regarding the application of a Hartley oscillator to a problem and am interested in knowing if there might be a better way. I need to simply know if there is the presence of a liquid in a plastic tube or not. My thought was to use a Hartley oscillator with closely matched inductors, and run the tube as a core through one of them. My thought is that when a liquid is present, the coils will be imbalanced and the result could be detected. Possible?

You probably want something similar to a metal detector, where two coils close together have high frequency oscillations going through them.
An approaching object causes interference in the oscillations. This creates a difference frequency.

From my experience using a metal detector, the object does not necessarily have to be metallic. If you press the reset button while holding it high off the ground, then bring it down near the ground, it will cause a signal.

However this might be because the soil is slightly conductive. So we cannot predict results with all substances. To get your liquid to trigger a metal detector setup, you will need to adjust everything so it is very sensitive.

I don't believe the Hartley oscillator will do the job for you. The inductors are configured to reinforce oscillations, so they are less sensitive to interference, not more sensitive.

what characteristics does the liquid have = conductivity? dielectric constant? To get the maximum effect you need to make a very good LC circuit with a high Q and high impedance (use a low value capacitor). this so the presence of any liquid will lower the Q and /or change the frequency by the largest factor. In the extreme if the feed back is fiddled with it so the oscillator just oscillates when dry, the presence of the liquid may stop it completely. This will change the DC operating conditions so the results can be checked with a DVM.
Frank

Re: Dielectric detector idea

What you want is a capacitance meter with foil electrodes that measures at a high frequency such as 1MHz or perhaps use the dielectric of the medium to shift the frequency of an LC Pierce oscillator using a ground reference near the middle of the object for common mode noise rejection.

- water has a high permittivity (60) relative to air(1)
- plastic may be low , PVC=3

The setup seems to describe an AC magnetical sensor which would primarly work for high conductive material with eddy current losses, typically metals. For liquid detection, you better refer to a capacitive proximity switch principle.

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I see, SunnySkyguy suggested the same.