That's what I like about solving real world applications - things in practice that jump out at you. In this scenario a flow sensor with no flow outputs a logic high or low depending on where the vane magnet(s) is sitting in relation to the Hall sensor. When flow starts slowly, the input square wave increases in frequency from zero Hz. The 31.6 Hz low figure is the manufacturers starting point for linear operation. But the totaliser expects to see the low flow rate pulses and add them to total flow. This is what happens with a gasolene engine which can run with low flow at idle, then fuel flow increases to the level of maximum horsepower with up and down flow fluctuations due to throttle settings in between.
I'm focussed on this old obsolete flow meter because it is a common type type owned by many small boat users who have dead flow sensors. It is a unique product and no longer manufactured. Paddle wheel Flow sensors do not share a common standard for flow versus frequency/pulse count, some have more vanes, some with more than one magnet per vane and the vanes vary in diameter changing the pulse period. The maximum oprating frequency is usually around 1Khz, athough some for low speed high precision flow measurements can be higher. Arduino code offers a universal solution if I can get it to work and the library routines are on my side to help me. Thanks
P.S Even using interrupt routines can I assume the time spent executing code might only introduce a small error, compared to the maximum input pulse duration (e.g 1mS?).