How about these flow sensors: $11
450 output pulses/liter
http://www.amazon.com/gp/product/B00...A1EX7LU0M6LM21
Product Description
DESCRIPTION:
Waterproof, heat resistance, pressure resistance, cold resistance
Easy to install
Frequency: F=4.8 * Q (L / Min) error: ± 2% voltage: 3.5-24VDC, current can not exceed 10mA, 450 output pulses/liters,
F=constant * units of flow (L / min) * time (seconds).
Flow range: 1-60L/min
Maximum current: 15 mA (DC 5V)
Working voltage range: DC 5～24 V
Load capacity:≤10 mA(DC 5V)
Operating Temp:≤80℃,Operating humidity:35%～90%RH
Allow compression: Water pressure >1.75Mpa
Insulation resistance: >100M OHM
Storage Temperature:-25～+80℃
Storage humidity:25%～95%RH
Output Waveform: Square Wave, output pulse signal
ROHS Compliant
Material: ABS Plastic
Color: Black
Cable length: approx. 15cm
INCLUDED:
1 x Flow Meter
Arduino - PulseIn
I have simple solid Arduino code using timed pulse for my CO2 sensor:
see 1st post in thread:
http://ecorenovator.org/forum/conser...no-sketch.html