At least what used to be an issues, was DC currents plating out of 
materials, the LM1830 and other water sensing designs I have seen lately 
use AC currents to minimize the plating action.

Alan B. Pearce wrote:
>> Let's say there are 16 such sensors.  Running a bundle of 16 pairs
>> of wires around seems... awkward.  Certainly they could share a common
>> ground, so that's down to 17 wires at the controller, down to two at
>> the last sensor...  16 comparators into a multiplexer, and have the
>> controller scan them every pass through the event loop.  Something
>> like that.  Is that the best we can do?  Maybe an analog multiplexer
>> and one comparator?
>>     
>
> Right, for sensors, especially drip sensors, my initial reaction is to make 
> a PCB with fine tracks and fine spacing, without solder resist. With the 
> RoHS fanaticism that is currently around, one of the finishes you can get is 
> a gold flash, which would seem to me, to be a suitable method to protect the 
> copper. Then a small micro can sense moisture bridging the tracks by the 
> conductivity of a drop of water, and with salt water this shouldn't be a 
> problem.
>
> A similar sensor could well be used as a flood sensor.
>
> However another technology that could be worth looking at is capacitive 
> sensing. I don't know how well this would do for drip sensors, but would 
> work well for flood sensing.
>
> As to how to have a heap of sensors spread around the place, I would look at 
> using a 10F200 series PIC at each sensor, as the sense driving element, and 
> communications to whatever host you use. It would be possible to use a 
> 1-wire style protocol with these, but it could equally be a software 
> transmit only UART driving an open collector transistor to allow multiple 
> units on a single line (see the current thread under [EE] on RS485 comms 
> protocol). I would not worry about a two way protocol (although it wouldn't 
> be too hard to implement), but rather just check the line to see if a 
> message is being sent, and if idle send a message. The message would include 
> a sensor ID, so the host knows where the message comes from.
>
> A 10F200 should be able to drive and sense a multifinger PCB as described 
> above, and Microchip also have an application note on using one for 
> capacitive touch sensing. The latter should be able to be converted to any 
> form of capacitive sensing.
>
> As an aside, a suitable capacitive sensor could well be used as an immersion 
> sensor to check the level of water in a tank. This has been discussed on 
> this list before. 
>
>   
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