Why UltraSound?

I was looking for a transducer

1. That would not suffer from wear and tear (e.g. mechanical switches).
2. Would return stable values, no complex calibration required.
3. Cheap and reliable

The PIC microcontroller would pluse the transducer for 300 microseconds using a Pulse Width Modulation signal at 40KHz that it would generate.  The ultrasound reflected on the water surface would be turned back into an electrical signal by the same transducer.  The microcontroller would use the time elapsed between which the ultrasound was generated and the echo it received to calculate the distance.  Since the transducer would be in fixed a position, the level of water in the tank would be easily derived by subtracting the distance obtained from the space between the transducer and bottom of the tank.  Below is the setup of the transducer/sensor on the air pipe of my 1500L tank.

Next the transducer removed from the air pipe and the PCB containing the microcontroller with its analog circuit.

Finally the display showing in percentage the amount of water in the tanks.  In fact I am measuring the level of water in 2 different tanks (1500L & 300L).  I have also 2 displays one on the ground floor and the second upstairs.  Each display comprises a 4x20 character LCD.  The 1st line shows the temperature of my solar water heater.  The last 2 lines displays the percentage of water in the (1500L & 300L) tanks as well as a bar graph per tank.