In order to use a differential pressure sensor to measure flow, you have to induce an obstacle of a known pressure drop per flow range and measure the differential pressure across this obstacle. For example, If you induce an obstacle within a pipe that creates a ΔP of 5mbar when 14 l/min pass...
I'm Dimi and I hope I can add value to this forum with my answers. I've studied Electrical Engineering at Ohio University and I've worked in the field for decades, specializing in sensors.