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I have been told that passing DC current through two probes in water will lead to corrosion on the terminals.

Apparently AC does not cause corrosion. Preventing corrosion is of high importance to me.

I am using a microcontroller in this design. What is the best way to generate and measure the AC signal?

Or otherwise, how do say dive watches detect water?

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    \$\begingroup\$ Why do dive watches detect water? \$\endgroup\$ – Solar Mike Dec 15 '18 at 13:03
  • \$\begingroup\$ I'm not sure. I just remember seeing one once and it had these two metal circles on it. I was told that they were used to detect water. I assume that they are designed not to corrode. \$\endgroup\$ – MMmmm Dec 15 '18 at 13:09
  • \$\begingroup\$ Dive watches detect pressure, not water. \$\endgroup\$ – JRE Dec 15 '18 at 13:13
  • \$\begingroup\$ Dive watches and dive computers detect both pressure AND the presence of water. And yes, they use fixed electrodes both for detecting water and as user interface buttons that only work for outside of the water functionality. \$\endgroup\$ – Edgar Brown Dec 15 '18 at 14:21
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    \$\begingroup\$ In what application are you trying to detect water ? different approach might be possible depending the needs. \$\endgroup\$ – Damien Dec 15 '18 at 17:16
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You can generate a square wave and capacitively couple it to the electrodes. Since water has a very high dielectric constant you can even insulate the electrodes from the water.

The details depend on your circuit ground relative to the water potential.

I suspect you could even use DC and exposed electrodes if you kept the voltage low enough, say 50 or 100mV.

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