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I ever made a fuel consuption meter for my domestic burner. It has a counter, divider, and 7segment display.
When the fuel valve is activated, the divider count pulses from the main power (50Hz). When the number of pulses for 1dl is reached the counter is incremented.
I used 4.5V battery to keep counter and divider value during power failure.
Interested? I can provide you the schematic, your job is to read and understand the datasheets of divider and counter, figure out how to configure the meter for specific fuel consumption.