Thanks you have given me some ideas. This flowmeter is designed as a 'Log' for marine use. It doesn't just read accumulated flow which as you say would be easy to do by replacing the flowmeter with an led display. It also takes starting fuel tank volume in NVRAM and counts down as fuel is used. This is probably the most important function for a boat. I know I could probably do all this from scratch, but the flowmeter I have is physically sized and sealed for boat dash mounting.
If I count pulses from any flow sensor I've still got to correct the result in the calculation to fit the curve whilst allowing for the ability to re-calibrate. This flow meter does that: You allow a measured amount of fuel to flow through, then apply a digital correction factor. Unfortunately it only works for the original sensor for a +-1-2% correction.
I'll go offline now and back to measuring, then come back when I have some substantive results.