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I have only rudimentary knowledge of electronics so I ask for your forbearance.

There are many water level indicator circuits, of the type shown below, listed online and demonstrated on YouTube.

enter image description here

The circuit clearly works when demonstrated with a bucket of water or a plastic bottle but my question is, is it a practical solution for a 5,000 litre tank?

I have taken note of the learned comments regarding corrosion and that gold contacts would be the best for this kind of circuit, etc. My intention is to use a series of stainless steel screws arrayed along the outside of a PVC tube, with the wires kept dry inside the pipe and only the stainless screws making contact with the water.

As for corrosion, would pulling the probes out of the water every now and then and cleaning them make this a bit more viable?

Am I correct in assuming that the current going through a few litres in a bucket, is not the same as current going through a lot more water, especially if the tank, which is 1.8 metres high, is full?

Should it be viable, what modifications would one need to make to the circuit? Up the voltage perhaps?

Thanks in advance for any and all comments and advice.

Regards

Edit: -

Thank you to all for the very comprehensive answers. You have convinced me that the project is likely to create more problems than I will be able to deal with at my level of knowledge. It is also a reminder that when something seems to be too good to be true, it usually is not true.

I think I may attempt an electro-mechanical solution consisting of floating magnets and reed switches. I will have questions, so there is bound to be a new post on that subject in the not too distant future.

As a matter of interest, partially in response to Phil C's answer: All other conditions being ideal, if locating a single negative electrode at the bottom of the tank would possibly be too a long path with increased resistance in such a big volume of water, would there be any advantage if one placed a second tube running parallel to the pipe with the sensors, with a negative electrode directly opposite and within a few centimetres of each sensor? Would the current follow the shortest path?

Just curious.

Thanks again to everybody

Regards

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I can see how your circuit can appear to work in a bucket, but it is not well scaleable, nor good for long term use. There are a number of problems:

  1. The current required thru the water is relatively high. There is one transistor gain between the water current and that thru the LED, but that's not a lot. There will be problems with high-resistivity water and long distances.

  2. The high current will cause corrosion more quickly than a low current.

  3. The current is always in the same direction. This causes assymetric corrosion, which leads to a battery effect. Depending on details, that battery effect may eventually cause readings to be missed or cause false readings.

  4. There is no thresholding to indicate definite on and definite off. The LEDs can be partially lit.

  5. There is not even a attempt to limit susceptibility to common mode noise.

  6. You didn't say anything about the 6 V supply being isolated. That means there can be unintended current paths to elsewhere. This is a bad idea.

For more details on sensing water resitively see my answer https://electronics.stackexchange.com/a/33938/4512.

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You would be correct in thinking a longer path between contacts through a liquid will lead to an increase in resistance. Whether or not this is going to be an issue will largely come down to the specific purity and temperature of the water, and will probably require experimentation. Warm salt water will have dramatically greater conductivity and corrosion rates vs cold distilled water, for example.

You can overcome the distance with higher voltage, however you may also have to adjust the biasing resistors accordingly. You'll also want to be quite sure you are using an isolated voltage source and never touching the water or sensor when it's charged.

Your solution to corrosion prevention seems a reasonable place to start. A periodic cleaning and inspection can never be a bad idea as well. If corrosion becomes a real issue, you may want to look into a sacrificial galvanic anode.

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I assume the use of AC is to reduce the corrosion on the probes A to F. However, the circuit won't work properly on the negative half-cycles of the AC supply. The transistors will have a negative base voltage, and the LEDs will be reverse biased. The LEDs will probably protect the transistors from damage, but the result will be rather flickery lamps.

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My first instinct is to use small FETs to reduce electrode current. You would want gate pulldowns to reduce suseptability to noise. They should have a fairly large value to (again) keep electrode current low. Sealing the screws might be trickier than you think. Many adhesives do not have good water resistence long term. Titanium screws (or rods) might be better than SS (Amazon). A problem is the PVC tube would get coated with slime over time. This would probably leave all the lights turned on. A Teflon tube might work better but would have other problems. You might want a different electrode configuration.

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