I've used these sensors to detect liquid (I use that word loosely) level in a black water tank in an RV tank. The sensor is attached to the outside wall and provides a signal when the inside level reaches that of the sensor. They've been working well. The thank I've been using it with has walls about 1/4" thick. And the sensors are adjustable.

Amazon https://a.co/d/gQXe3ke

 

I've used these sensors to detect liquid (I use that word loosely) level in a black water tank in an RV tank. The sensor is attached to the outside wall and provides a signal when the inside level reaches that of the sensor. They've been working well. The thank I've been using it with has walls about 1/4" thick. And the sensors are adjustable.

Amazon https://a.co/d/gQXe3ke

Hi,

Looks like the liquid to be detected has to be electrically conductive with that sensor.

 

I can think of two options:
1) Have something float that will block a ray of light or small laser.
2) Place two plates of metal on either side of the container, outside of it, and measure the capacitance between them. The capacitance will change with the height of the liquid and proportionally with the dielectric constant of said liquid.

 

2) Place two plates of metal on either side of the container, outside of it, and measure the capacitance between them. The capacitance will change with the height of the liquid and proportionally with the dielectric constant of said liquid.

In theory, yes, in practice the small dimensions of the plates and the 0.5cm of the poly-whatever of the container either side, swamp any change due to the liquid content - though it does depend on the, as yet unknown, liquid.. A rough 'flat plate' calculation based on water gives, effectively, 8.5pF for each wall in series with between 0.4pF (empty) and 4pF (full), a total capacitance of between 0.37pF and 2.1pF. That's a very small, hard to measure accurately, change - the connecting wires flapping around are likely to produce just a big a change! Maybe some VHF resonance technique might work, but that's way too complex for this application.

 

Are you closing a valve when the sensor detects water? Then you will have to take the solenoid current into account if it is just an LED then why?

Could you use a current sensor instead? 555's are kind of a thing of mine.

 

After some basic testing, I have found that an ifared sensor/receiver module considers ANY LIQUID, as a solid. Desnt matter if the liquid is black or clear. As long as the transmitter/sensor is in contact with the liquid, and not looking thru a transparent window (it will "see" the transparent window as a solid) this seems to work.

 

Ok, so just to wrap this all up, for people who have been generous enough to add to this question.
After much experimentation with this, I have found that infared emitters and receivers are the way to go. I dont know why I didnt realize this, but infared of course considers all liquids a solid, no matter the compositions color, or density. It can be transparent, absolute black, or anything in-between, so s long as there is a small peep hole in the bottom for the egress of the beam, and the entrance of the beam, (sealing around both diodes so no liquid can pour thru, of course), any liquid (withing reason) that I seem to put into it is read as a solid.

 

Are you closing a valve when the sensor detects water? Then you will have to take the solenoid current into account if it is just an LED then why?

Could you use a current sensor instead? 555's are kind of a thing of mine.

555s are my love, Italian food, and women being a close second and third