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 longterm isolated due to imperfections of the coating.