What about this flow meter?
Low flow flowmeter, mini - Q= 0,025- 3,0 LPM, 6.800 pulses/Litre
There are no prices though, maybe somebody can contact them (i dont actually need flow meters)? 6800pulses/liter makes instant reading easier when counting pulses.
Anyway - for instant reading, we can use time between pulses, for cumulative reading we can count pulses.
Low flow flowmeter, FCH-m-POM -3 LPM, 97478350: B.I.O-TECH Flowmeter