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Green-Trust.Org | Cistern Water Level Monitoring -
"I designed a very inexpensive and simple to use water level monitoring project. Using an inexpensive microcontroller and ultrasonic sensor, there are no moving parts. Installed in the top of a tank, it monitors the distance to the water, and knowing the height and radius of the tank, displays the number of liters (or gallons) left in the tank. You can display the output on a LCD, or transmit it over your home network to your PC." ‎- silpol