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Old 09-05-2017
Krikkitman's Avatar
Krikkitman Krikkitman is offline
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Join Date: Oct 2009
Location: Vancouver
Posts: 1,333
Default Saltwater Flood Sensor

So this is somewhat random, but if anybody needs (or just wants) a saltwater flood sensor, I have a few leftover custom-designed circuitboards available for just that purpose, and can sell 'em fully assembled and working for basically the cost of the components and my time soldering them together. So, like, $40 per sensor.

The backstory is my boat was retrofitted from a single-bellows Bukh saildrive to a double-bellows Yanmar saildrive. The Yanmar has a sensor between the two sets of bellows that lets you know if the primary outer bellows has failed (which would flood the dead space between the two bellows). The sensor itself is nothing more than two metal prongs mounted in a screw-in plastic plug.

Because it was a retrofit, there was no circuitry to monitor this sensor, so the installer just tied the wires off. I don't really like redundant wiring or disabled sensors, and my son is now taking electrical engineering and was looking for an interesting hardware project. So I gave requirements for a more sophisticated circuit than Yanmar's version (which my boat never had anyway).

This custom circuit self-monitors for the condition of the sensor wiring (the sensor wiring seems to me to be a likely failure point, and Yanmar's solution is to put a sticker on the saildrive saying "Test the wiring circuit annually by putting the sensor in a cup of water." Bah.)

The custom flood monitor has a 39K Ohm resistor across the sensor prongs, and constantly monitors the sensor circuit. If the resistance drops (saltwater detected!) it activates an LED/beeper solid on. If the resistance rises (poor connection or open circuit!) it flashes the LED/beeper.

So that you know everything is alive when first starting the engine (or turning on the circuit if you don't have it connected to the engine ignition), it also flashes the LED/beeper five times upon startup. Other than that, if everything is good, the LED/beeper stays off.

Overall, pretty simple. You need 12V power, a 12V LED (and/or beeper), and the sensor (this can be just a couple of tinned wire ends sticking into the space you want to monitor). The circuitboard itself is about 2-inches x 2-inches.

Its only real drawback it that it doesn't work in pure freshwater, although in practice while we found that tap water wouldn't activate the sensor, puddle water and lakewater both had enough salt ions to activate it. So it doesn't take many salt ions to do the trick, but if I was operating the boat in pure freshwater I might drop a pinch of salt into the space to be sure of getting an alarm in the event of flood.

Oh, and being my son's first-ever circuit board, it didn't occur to him to put mounting holes in it, so you need to mount it to wherever you want it with zap-straps (that's how I did it) or some other system.
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