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I need to find a very cheap way to detect water ad specific levels. No needs to have a linear measurement of its height.

I'm thinking about a simple FR4 PCB, with several couples of tracks. Each couple has a different length and ends with exposed pads. All the tracks are of course protected with the soldermask, but the exposed pads.

My question is about the rusting of the pads: what surface finishing should I use (if any) ? Please note the water is for irrigation and is rich of mineral salts and nutrients.

EDIT

The range of measurement is 2 cm to 20 cm and the neck at the top of the container is less than 15 mm.

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Can you use a float on an arm and detect the arm position with limit switches?

Ultrasonic sensors are also used for surface level measurements.

Using a PCB will give you trouble.

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  • \$\begingroup\$ I want to avoid any mechanical stuff because it's not so cheap and it's subject to wear. Ultrasonic sensors can be used but to measure such small distances (I'm editing the question with further details) need two transducers and the neck of the container is too small. \$\endgroup\$
    – Mark
    Commented Nov 3, 2020 at 8:38
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    \$\begingroup\$ Ok. Is the neck plastic? Can you use a capacitive sensor? Roll an aluminium foil around the neck, electrode in the liquid and measure capacitance... maybe a bit wild. \$\endgroup\$
    – Ralph
    Commented Nov 3, 2020 at 8:47
  • \$\begingroup\$ Yes, the whole enclosure is in plastic material. The capacitive approach is good. \$\endgroup\$
    – Mark
    Commented Nov 3, 2020 at 8:53
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    \$\begingroup\$ It is not as complicated as it might sound, but needs more effort than just a conductive test. There are ICs for the purpose and the accuracy will depend on the enclosure wall thickness. Impurities in the water will be beneficial for the measurement, but if the composition of the liquid changes a lot, it might affect the reading. \$\endgroup\$
    – Ralph
    Commented Nov 3, 2020 at 9:45

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