"Fig.3 is shows the typical sensitivity characteristics of the MQ-136 for several gases. in their:
Temp: 20℃、
Humidity: 65%、
O2 concentration 21%
RL=20kΩ
Ro: sensor resistance at 10ppm of H2Sin the clean air.
Rs: sensor resistance at various concentrations of gases."
Unfortunately their description of Ro doesn't match the chart. The chart shows that Ro/Rs at 10ppm is no 1.0. Looks more like 1.3
The official way to measure Ro is to measure the sensor resistance in a 10ppm concentration of the gas in air at 65% humidity and 20°C. Fortunately they provide a level for clean air. Looks like Rs/Ro is about 3.4 for clear air so take Rs in clean air and multiply by 3.4 to get Ro. Then you can use Rs/Ro to get rough approximations of ppm:
3.4 = 0 ppm
1.3 = 10 ppm
0.6 = 110 ppm
Looks like linear interpolation would be close enough.
Please note the pre-heat time: 24 hours. The heater circuit must be running for a full day before you can expect to get accurate results. The heater circuit draws 160 mA (800 mW at 5.0 V).
To calculate Rs you can take an analog reading:
const float Rl = 20000.0; // Load resistor is 20k Ohms.
float Rs = Rl * ((1023.0 / analogRead(A0)) - 1.0);
const float Ro = <value you calculated based on Rs in clean air>;
float ratio = Rs/Ro;