need some help about under-water capacitive sensors


Thread Starter



as you mentioned the choice of sensor depends on many things such as corrosive, temp, ph, volatility. I'm working on a device that should work under drinking water so the water level sensing is important, but it needs only sensing the existence of water (not the depth). My major problem is corrosion because the device's body is a metal such as brass coated by chrome.

I tried some capacitive circuits, but they didn't work properly.

the problem is that the surface of device must be connected to 40Volt-DC (maximum voltage on the device circuit) and all capacitive circuits I've found work with an AC voltage on the probes, and I want to use the metal surface as one of the probes. I'd like to know that can I use the metal surface and another conductive probe as the capacitor surfaces and water as dielectric (while one of probes is connected to 40Vdc)? Or I should choose the second probe a conductor with a thin nonconductive material on it as dielectric (to avoid corrosion of the probe)? Do you offer any useful circuit?

thank you in advance.
Your best bet is to use a stainless probe – it will work if you increase the sensitivity of the circuit.

You should also convert the signal to low voltage AC to prevent electrolysis. Do this first – or you may have major problems in the future.

Bob Hogg

Robert Scott

I think there are already some good off-the-shelf solutions to capacitive water sensing. But if you really want to "roll your own", you can do what I did a while ago with some success. I used a TLC555 RC-oscillator using a 56k resistor. The capacitor was formed by two side-by-side portions of a single circuit board. Each portion was about 6 cm2. This tiny board had the capacitor on the bottom and the surface-mount 555 on the top. Then I coated the bottom with two thin coats of epoxy, and potted the rest. The whole circuit board went under water. The presence or absence of water could be determined by the frequency of oscillation of the 555.

Robert Scott
Real-Time Specialties
Embedded Systems Consulting

Curt Wuollet

Why don't you simply use conductance rather than capacitance? If you must use capacitance with dc, you should switch the circuit and measure the time constant as a function of the capacitance. Generally only very pure water is a dielectric.


Dave Buckley

I'm also trying to measure the conductance of water, (containing varying amounts of salt) flowing through a PVC pipe. I'd like to use electromagnetic sounding similar to the geosciences. They measure salinity of the soil. Mine needs to know when salinity is increasing or decreasing. Any suggestions will be gratly appreciated.

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