MQ-9 gas sensor module has high sensitivity to Carbon Monoxide, Methane and LPG. Sensitive material of MQ-9 gas sensor is SnO2, which with lower conductivity in clean air.
 
It makes detection by cycling high and low temperature, and detects CO when low temperature (heated by 1.5V). When temperature rises (heated by 5.0V), it detects Methane, Propane, combustible gas, etc. and cleans the other gases as it adsorbed under low temperature.
 
MQ-9 usually applied in domestic gas leakage detector, industrial gas detector, portable gas detector, etc.