Capacitive watersensor

Water sensor to measure the level of water in a tank. It is based on the changing capacity of the capacitator that is submerged in water. the capacitator is composed from two concentric coils made from enameled copper wire. (To measure the capacity I used the QTouch system described at Atmel http://www.atmel.com/images/doc8207.pdf)
The obvious way to build something like this would be to use two concentric pipes that are isolated with paint. But it's tricky to ensure that the pipes are longter