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So as the title states, the question is:

Is there a way to measure moisture on a electrically conductive flat surface?

Optionally, it would be great if I could have an indication of the amount of moisture on the surface, preferably as an analog value, so I can read it via an Arduino (when I say how much, I do not need it in like mililiters, just a higher or lower value representing moisture).

Just to give some context: I am working on a project to cool down a laser module using Peltier elements (TEC), and I want to know when moisture starts to form around the laser diode as a preventive safety feature, because water is not good for lasers diodes :) I am aware that I could just monitor the dew point, still, I am interested if this is even possible or not.

Thank you.

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I agree that measuring dew point is likely easier (buy a humidity sensor and do a bit of math), but if you want to measure condensation, consider using capacitive sensing.

Water has a dielectric constant much higher than that of air, so the capacitance between two plates will be higher if there is some water between them rather than merely air. (If the water is impure and therefore conductive, it instead reduces the effective distance between plates, also increasing the capacitance.) This is the technique commonly used soil moisture sensors.

Measuring capacitance can be done in many ways: I might use a hysteresis oscillator.

Note that you will need two conductive plates for this technique, not merely one, but they can be coplanar rather than parallel if that is easier to arrange.

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You can measure condensation on a specular surface by optically measuring the scattering.

Measure the differential signal from two photosensors, one reflected to the mirror-like surface and back and the other from a dry gray surface. You can use two reflective photosensor/emitter units. The reflected light intensity from the mirror surface will drop as the condensation forms.

Preferably keep them dark but if you need to reject light interference consider modulating the LEDs and measuring the AC voltage.

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