Apparently measuring the flow of liquids can be difficult. This circuit displays the square wave from a SwissFlow sensor. This project was used this for monitoring the cooling system on a computer, but it could be useful for a lot of projects.
This schematic is meant to digitally display the output of an electronic Swissflow SF800 flow sensor. This sensor puts out an open collector square wave signal (like a fan RPM monitoring signal) between 50Hz (=0.5 liter/minute) and 2000Hz (=20 liter/minute).