Once I had an Idea for making a device that could measure the water-level in our pond, but it turned into a device to measure the level of a liquid in a container. Because it was my very first project, I couldn't finish it right away. Some time later I entered the first edition of Arduino-jam. The place I learned a lot, but it was on the second edition, I worked on this project as an entry.
Now, it seems some people have found there way to this project and started using it, having all kinds of trouble with my sketch. Good thing, because I had to solve some problems and I made it better. Now we have version 2.4 and I now know for sure, it will work for other people too.
You're looking for some device that can measure the level of a liquid without having to put something into the liquid? DUAL TANK LEVEL MEASURING is your project.