The sensor of finding the device in water?

Question

Electronics

It is given - some hermetic device that can be immersed in water.
It is necessary to use the cheapest possible means to simply detect the fact that the device is in the water (under water) - whether it is in the water or not ... The level, depth, etc. - do not matter.
Water is ordinary tap water.
In addition to resistive ones (the device can, let's say, just wet it - you need to not react to it if possible, possible corrosion and all that - in general, avoid using sensors on the resistive effect) - how else can this be done?

Answer 1

The resistive sensor is the simplest. To avoid corrosion, you can use two stainless steel screws with caps sticking out and passing through the body hermetically. If you place them far enough or even diametrically on the body, then wetting will not help. Although ... What is the thickness of the water film in question? You can also put the device in a bucket of water.
To avoid corrosion, it is possible, as suggested above, to use alternating current or just a meander from the midpoint through a voltage divider.
Of course, you can make a capacitive sensor. From the inside, glue two separate foil plates on the body. If there is no water outside, then the capacitance of such a capacitor will be very small, but when water is added, it will become, as it were, two more plates and together, as it were, two series capacitors will be formed. Include it in the resonant circuit and select its parameters so that the resonance is observed in the presence of a large amount of water as external plates.
By the presence of resonance, you determine the immersion.
You can try the microwave sensor. Here is one, for example:
https://aliexpress.ru/item/32880755669.html?spm=a2...
Surely it will also react to water. The question is how to choose and adjust the sensitivity for stable operation and ignore the walls of the container.
You can also collect by analogy with one of the options above. you can take out the coil of the oscillating circuit as close as possible to the external environment. Obviously. that its inductance will vary depending on the magnetic permeability of the medium, and they are different for water with ions and for air. You can detect the resonant frequency of the circuit.
If the device will try to deceive and for this the attackers will have many unpunished or unrecorded attempts, then nothing will help you. Still deceived. People. when he is interested, he masters TRIZ on an instinctive level. For example, you can hang a bucket in a water tank at the level of your device, when there is water in the tank - everything is normal, when not - water will remain in the bucket and will simulate the level for your device.
In such hostile conditions, you can fight like this:
- Collect more control factors, keep them secret.
- Do not give intruders a clear and unambiguous signal about the protection operation, signal secretly, or introduce a randomizer. It is better to ignore some of the attacks, but report the rest, than to react to each one and thus give attackers the opportunity to easily and quickly pick up the bypass parameters.
- Introduce an alarm delay, better randomized, even better, so that the alarm did not work every time. This will complicate the selection of bypass parameters, make it long and unprofitable, and allow you to catch the villains red-handed.