I’ve personally verified the flow sensing many times before on RPi and as far as my experiments went, it was quite accurate. I don’t have an RPi zero 2w so I cannot verify it for this particular version of RPi, but I’ve verified it on RPi 0 w, Rpi 2, Rpi 3 etc. What type of pulse generator are you using? Do you have a link? Also, what’s your version of raspbian?