Electronic Alarms and Home Security Devices - Homemade water sensor - Will this work

I have a ADT Safewatch 3000 / Vista 20p. I would like to add a basic water sensor so the alarm will sound in the house if my sump pump fails. I know I can buy the wired sensor very cheap online but I am wondering if I even need to buy one of those. Is it possible to run line to the sump pit and leave the exposed ends of the wires mounted about 1/4 inch apart? It will be a normally open circuit and a rising water level will close it and trigger the alarm. Is this possible or a bad idea for some reason I haven't thought of?

Thanks for any input.

Buy the switch.

The resistance of the water will be too high to trigger the alarm circuit.
This is why they charge forty dollars or so for the proper sensor.

http://www.homesecuritystore.com/ezstore123/DTProductZoom.asp?productID=608&bViaAC=False

This is the sensor switch I was looking at, and for all the world it looked like it was a piece of plastic around two open terminals. I did not think there would be any electronics inside the plastic that would make it different from two mounted bare wires. Ill take your word for it if you say there is.

You are right in that there is no power supply in the sensor you show.

A way I can see them doing it is to have two very closely spaced electrodes.
The shorter the path through the water the higher the resistance would be.

If you wanted to experiment you might be able to make this work but you have to consider that you are doing this to prevent expensive damage.
Whatever home made solution you come up with you need to make sure it will be reliable.

Fresh water, even reasonably dirty, fresh water is an insulator (some very large old-school capacitors used pure water as the dielectric). So, as counterintuitive as it seems, you will not get reliable results from simply putting bare zone input wires where they can get wet. On more sensitive panels, the most likely result will simply be a ground fault.

The guts of one of these things is really more akin to the inner works of a thermocouple, and it takes a very tiny amount of leak voltage to make it close it's internal relay.

http://www.grisk.com/specialty/2600water_sensor.htm

Is the actual maker's site with more detailed specs.

Also, there is an ever so slight voltage put out by the zone (that's how it senses resistance for supervision, zone doubling, etc.), which "powers" that water sensor. So technically it is powered, and the bit of circuitry in that sensor use that power.