When electrodes are immersed in water, device generates sound.
Frequency of sound depends on dipping length.
At length = 0 (only touching water surface), frequency is 2500++ Hz,
At length = 22 mm frequency is about 200 Hz.
Both electrodes are made from stainless steel 316, diameter 0.61mm.
Electrodes are mounted in parallel, at distance between axes = 6 mm.

 

So what is the question?

 

I see a circuit and a description of the functioning. This is an interesting oscillator circuit. The waveform indicates an RC time constant but I do not see the capacitor.
I am guessing that some battery internal resistance is part of the feedback loop.

 

The waveform indicates an RC time constant but I do not see the capacitor.

Good that you noticed it.
Water (or table salt solution) is perfect insulator at voltages up to 1.23 V.
Metal conductor, immersed in water, forms huge capacitor,
about 20 μF on every 1 sq. cm of metal surface.
Small needles, showing below, have surface area 42.16 sq. mm each.
So, one electrode has capacitance 8.4 μF, but two electrodes,
connected in series, have capacitance 4.2 μF.

https://en.wikipedia.org/wiki/Double_layer_(surface_science)
https://en.wikipedia.org/wiki/Supercapacitor

 

Hola @Danko
Could you show hoy are they are phisically connected to be in series?
Sometimes I am incredibly slow. Sorry.

 

So what is the question?

I'm with Dave. What's the question?

And since you're using hollow needles, calculating the surface area, have you taken into account the surface area within the needles? Since I don't know how capacitance is a function of your system I can only ask "Does the internal surface even matter?" These are things I don't know the answer to. I only ask them to spur on additional thinking.

 

Hola @Danko
Could you show hoy are they phisically connected to be in series?
Sometimes I am incredibly slow; sorry.

The connection to the two needle probes is very clearly shown in the circuit drawing by "red"and "yellow" "to electrodes". That was clear initially, based on the very first sentence.
What is missing is that there is also a resistance connection between those electrodes, unless the water is absolutely pure, which only exists in theory. Thus my comment about RC time constant.

 

Does it work if the electrodes are connected to a variable resistor in air, no water ?

A forum is for anyone to expose something, not just to ask questions -I think-

 

Given that the two scope traces show a classic RC slope, the capacitance is definitely present. I have not considered the capacitance when working with moisture detection systems.

 

Does it work if the electrodes are connected to a variable resistor in air, no water ?

A forum is for anyone to expose something, not just to ask questions -I think-

That is what the definition of the General Electronics Chat forum says.

 

When I first visited this thread, I looked at the scope trace and thecircuit and I was thinking that there had to be a capacitor someplace, did they just forget to show it??. So we have positive proof of water providing a capacitance. But as there is also a resistance element that varies a lot with whatever else is present, This means that water capacitors will probably not be precision devices, nor considered for low-leakage applications.

 

Could you show hoy are they are phisically connected to be in series?
Sometimes I am incredibly slow.

One end of electrode #1 connected to yellow node of circuit,
free end of electrode #1 connected to water, water connected
to free end of electrode #2, other end of electrode #2 connected
to red node of circuit.
So, electrodes are connected in series.
Capacitors, formed on free ends of electrodes, are connected in series too.
You can even separate capacitors each from other, using two vessels
and bridge (U-shaped glass or plastic tube, filled by water) in time of charging,
then take out bridge and store energy in two capacitors separately:

"Does the internal surface even matter?"

Electrostatic:
"The electric field inside the metal can be totally zero but outside surface charges will always form and convey the field outside"

Does it work if the electrodes are connected to a variable resistor in air, no water ?

It does not oscillates for resistor with value from 0 Ω to ∞.
For example, oscillogram for resistor with value 30 k:

ADDED: In this circuit electrodes, immersed in water, are working
as variable capacitor, not as variable resistor.

 

The reason is that there must be some capacitance for oscillation to be possible. The circuit IS an RC oscillator, so there must be some "C" for it to oscillate.