Conductive level monitors are used in industrial systems sometimes. The industrial versions use low voltage AC on the electrodes to prevent catalytic action from eating the conduction nodes away. You could simulate this by alternating the connections:

Make the common node low and use pullups on the other nodes - covered nodes will read low.
Make the common node high and use pulldowns on the other nodes = covered nodes will read high.

Also note that the cleaner the water the less conductive it is! This could be got around by not using a single common electrode, but by scanning the common up the stack until no connection is detected to the next node. That way you are only ever testing between two adjacent nodes, not over the full length of the probe.

Combining AC on the probe and a scanning common starts to get complex, but it allows the system to work with very clean water and a very low voltage without catalytic action. I don't think you could do better with a simple system, and you have a Pico to do the hard work (once you've programmed it...). :)

There is a thread on thebackshed calles " rc fuel gauge" that measures the level of fuel using capacitive electrodes outside of a plastic tank.

Volhout

  Quote  

There is a thread on thebackshed calles " rc fuel gauge"

here

The buffer for the shield in above article was made with discrete components I had laying around. Today I would use a rail to rail opamp with something like 15 Mhz GBW. Much simpler.

If the PICAXE TOUCH16 command works well with a 20 liter caravan tank of water must be tested. Maybe small electrodes (vertcal copper strips) would be sufficient. And then wind bubble foam around the tank, and create the shield with household alu foil.

Anyway, the method is measuring capacitance, and with 2 electrodes at the sides, the relation is linear.
If you want to use a pico, there is a circuit in thebackshed for a capacitance and inductance meter with picomite. Also by me. You just need to add the shield. This circuit may even allow you to connect the shield to ground, since it is far more accurate, and thus can sense smaller capacitance changes in a large capacitor.

https://www.thebackshed.com/forum/ViewTopic_mobile.php?FID=16&TID=17092&PID=0&LastEntry=#223627

Volhout
Edited 2025-07-31 17:30 by Volhout

Depends on what's needed. :)  For industrial stuff we used to get an ultrasonic unit fitted into the top of the tank. That gives an analogue output for indication and, often, four relays: hi-hi, hi, lo and lo-lo (sometimes via a trip amp). These relay points are used by pumps for filling and/or emptying the tank. hi-hi and lo-lo are the end trip points that cut off the pump power and bring up alarms. The level will normally bounce between hi and lo. Ultrasonic are great and will work with anything, including granules. They don't have problems with "sticky" fluids or inflammable ones either (although the latter needs a special, very expensive, detector).

Another level sensor (used a lot in potable water systems, but can be used in aggressive environments too) is the "mercury" float switch. That is a completely sealed ball that floats on the fluid and is tethered by a chain. It is usually pear-shaped. When the tank is empty it hangs vertically and as the fluid level rises it floats at varying angles because of the weight of the chain. There can be several contacts operating at varying levels. Usually very expensive, but last forever!

We've even used a calibrated pressure sensor on tanks that have the output at the bottom. They can be very good indeed for water but have to be calibrated for the particular tank. Once set up they are remarkably linear and pretty cheap. Also, easily fitted as it's a case of fitting a T piece after the shut-off valve, with the sensor on the T, with it's own valve so that it can be removed without disrupting the flow.

I wonder if you could glue TTP223 touch sensor modules to the outside of the tank? :)